Re: [PATCHv2 1/2] arch: Move CONFIG_DEBUG_RODATA and CONFIG_SET_MODULE_RONX to be common

[Pavel Machek]

On Mon 2017-02-06 10:47:45, Laura Abbott wrote:
> On 02/03/2017 01:08 PM, Kees Cook wrote:
> > On Fri, Feb 3, 2017 at 12:29 PM, Russell King - ARM Linux
> >  wrote:
> >> On Fri, Feb 03, 2017 at 11:45:56AM -0800, Kees Cook wrote:
> >>> On Fri, Feb 3, 2017 at 9:52 AM, Laura Abbott  wrote:
>  diff --git a/arch/Kconfig b/arch/Kconfig
>  index 99839c2..22ee01e 100644
>  --- a/arch/Kconfig
>  +++ b/arch/Kconfig
>  @@ -781,4 +781,32 @@ config VMAP_STACK
>    the stack to map directly to the KASAN shadow map using a 
>  formula
>    that is incorrect if the stack is in vmalloc space.
> 
>  +config ARCH_NO_STRICT_RWX_DEFAULTS
>  +   def_bool n
>  +
>  +config ARCH_HAS_STRICT_KERNEL_RWX
>  +   def_bool n
>  +
>  +config DEBUG_RODATA
>  +   def_bool y if !ARCH_NO_STRICT_RWX_DEFAULTS
>  +   prompt "Make kernel text and rodata read-only" if 
>  ARCH_NO_STRICT_RWX_DEFAULTS
> >>>
> >>> Ah! Yes, perfect. I totally forgot about using conditional "prompt"
> >>> lines. Nice!
> >>
> >> It's no different from the more usual:
> >>
> >> bool "Make kernel text and rodata read-only" if 
> >> ARCH_NO_STRICT_RWX_DEFAULTS
> >> default y if !ARCH_NO_STRICT_RWX_DEFAULTS
> >> depends on ARCH_HAS_STRICT_KERNEL_RWX
> >>
> >> But... I really don't like this - way too many negations and negatives
> >> which make it difficult to figure out what's going on here.
> >>
> >> The situation we have today is:
> >>
> >> -config DEBUG_RODATA
> >> -   bool "Make kernel text and rodata read-only"
> >> -   depends on MMU && !XIP_KERNEL
> >> -   default y if CPU_V7
> >>
> >> which is "allow the user to select DEBUG_RODATA if building a MMU non-XIP
> >> kernel", suggesting that the user turns it on for ARMv7 CPUs.
> >>
> >> That changes with this and the above:
> >>
> >> +   select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
> >> +   select ARCH_HAS_STRICT_MODULE_RWX if MMU
> >> +   select ARCH_NO_STRICT_RWX_DEFAULTS if !CPU_V7
> >>
> >> This means that ARCH_HAS_STRICT_KERNEL_RWX is set for a MMU non-XIP
> >> kernel, which carries the same pre-condition for DEBUG_RODATA - no
> >> problem there.
> >>
> >> However, ARCH_NO_STRICT_RWX_DEFAULTS is set for non-ARMv7 CPUs, which
> >> means the "Make kernel text and rodata read-only" prompt _is_ provided
> >> for those.  However, for all ARMv7 systems, we go from "suggesting that
> >> the user enables the option" to "you don't have a choice, you get this
> >> whether you want it or not."
> >>
> >> I'd prefer to keep it off for my development systems, where I don't
> >> care about kernel security.  If we don't wish to do that as a general
> >> rule, can we make it dependent on EMBEDDED?
> >>
> >> Given that on ARM it can add up to 4MB to the kernel image - there
> >> _will_ be about 1MB before the .text section, the padding on between
> >> __modver and __ex_table which for me is around 626k, the padding
> >> between .notes and the init sections start with .vectors (the space
> >> between __ex_table and end of .notes is only 4124, which gets padded
> >> up to 1MB) and lastly the padding between the .init section and the
> >> data section (for me around 593k).  This all adds up to an increase
> >> in kernel image size of 3.2MB on 14.2MB - an increase of 22%.
> >>
> >> So no, I'm really not happy with that.
> > 
> > Ah yeah, good point. We have three cases: unsupported, mandatory,
> > optional, but we have the case of setting the default for the optional
> > case. Maybe something like this?
> > 
> > config STRICT_KERNEL_RWX
> >   bool "Make kernel text and rodata read-only" if ARCH_OPTIONAL_KERNEL_RWX
> >   depends on ARCH_HAS_STRICT_KERNEL_RWX
> >   default ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
> > 
> > unsupported:
> > !ARCH_HAS_STRICT_KERNEL_RWX
> > 
> > mandatory:
> > ARCH_HAS_STRICT_KERNEL_RWX
> > !ARCH_OPTIONAL_KERNEL_RWX
> > 
> > optional:
> > ARCH_HAS_STRICT_KERNEL_RWX
> > ARCH_OPTIONAL_KERNEL_RWX
> > with default controlled by ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
> > 
> > Then arm is:
> >   select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
> >   select ARCH_HAS_STRICT_MODULE_RWX if MMU
> >   select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
> >   select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7
> > 
> > x86 and arm64 are:
> >   select ARCH_HAS_STRICT_KERNEL_RWX
> >   select ARCH_HAS_STRICT_MODULE_RWX
> > 
> > ?
> > 
> > -Kees
> > 
> 
> Yes, that looks good. I wanted it to be mandatory to avoid the
> mindset of "optional means we don't need it" but I see there
> are some cases where it's better to turn it off. I'll see if
> I can emphasize this properly in the help text ("Say Y here
> unless you love security exploits running in production")

What about fixing the memory wastage, instead? If you want something
almost-always-on, it should not waste megabytes of memory.

And BTW it is help text, not 

Re: [PATCHv2 1/2] arch: Move CONFIG_DEBUG_RODATA and CONFIG_SET_MODULE_RONX to be common

[Laura Abbott]

On 02/03/2017 01:08 PM, Kees Cook wrote:
> On Fri, Feb 3, 2017 at 12:29 PM, Russell King - ARM Linux
>  wrote:
>> On Fri, Feb 03, 2017 at 11:45:56AM -0800, Kees Cook wrote:
>>> On Fri, Feb 3, 2017 at 9:52 AM, Laura Abbott  wrote:
 diff --git a/arch/Kconfig b/arch/Kconfig
 index 99839c2..22ee01e 100644
 --- a/arch/Kconfig
 +++ b/arch/Kconfig
 @@ -781,4 +781,32 @@ config VMAP_STACK
   the stack to map directly to the KASAN shadow map using a formula
   that is incorrect if the stack is in vmalloc space.

 +config ARCH_NO_STRICT_RWX_DEFAULTS
 +   def_bool n
 +
 +config ARCH_HAS_STRICT_KERNEL_RWX
 +   def_bool n
 +
 +config DEBUG_RODATA
 +   def_bool y if !ARCH_NO_STRICT_RWX_DEFAULTS
 +   prompt "Make kernel text and rodata read-only" if 
 ARCH_NO_STRICT_RWX_DEFAULTS
>>>
>>> Ah! Yes, perfect. I totally forgot about using conditional "prompt"
>>> lines. Nice!
>>
>> It's no different from the more usual:
>>
>> bool "Make kernel text and rodata read-only" if 
>> ARCH_NO_STRICT_RWX_DEFAULTS
>> default y if !ARCH_NO_STRICT_RWX_DEFAULTS
>> depends on ARCH_HAS_STRICT_KERNEL_RWX
>>
>> But... I really don't like this - way too many negations and negatives
>> which make it difficult to figure out what's going on here.
>>
>> The situation we have today is:
>>
>> -config DEBUG_RODATA
>> -   bool "Make kernel text and rodata read-only"
>> -   depends on MMU && !XIP_KERNEL
>> -   default y if CPU_V7
>>
>> which is "allow the user to select DEBUG_RODATA if building a MMU non-XIP
>> kernel", suggesting that the user turns it on for ARMv7 CPUs.
>>
>> That changes with this and the above:
>>
>> +   select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
>> +   select ARCH_HAS_STRICT_MODULE_RWX if MMU
>> +   select ARCH_NO_STRICT_RWX_DEFAULTS if !CPU_V7
>>
>> This means that ARCH_HAS_STRICT_KERNEL_RWX is set for a MMU non-XIP
>> kernel, which carries the same pre-condition for DEBUG_RODATA - no
>> problem there.
>>
>> However, ARCH_NO_STRICT_RWX_DEFAULTS is set for non-ARMv7 CPUs, which
>> means the "Make kernel text and rodata read-only" prompt _is_ provided
>> for those.  However, for all ARMv7 systems, we go from "suggesting that
>> the user enables the option" to "you don't have a choice, you get this
>> whether you want it or not."
>>
>> I'd prefer to keep it off for my development systems, where I don't
>> care about kernel security.  If we don't wish to do that as a general
>> rule, can we make it dependent on EMBEDDED?
>>
>> Given that on ARM it can add up to 4MB to the kernel image - there
>> _will_ be about 1MB before the .text section, the padding on between
>> __modver and __ex_table which for me is around 626k, the padding
>> between .notes and the init sections start with .vectors (the space
>> between __ex_table and end of .notes is only 4124, which gets padded
>> up to 1MB) and lastly the padding between the .init section and the
>> data section (for me around 593k).  This all adds up to an increase
>> in kernel image size of 3.2MB on 14.2MB - an increase of 22%.
>>
>> So no, I'm really not happy with that.
> 
> Ah yeah, good point. We have three cases: unsupported, mandatory,
> optional, but we have the case of setting the default for the optional
> case. Maybe something like this?
> 
> config STRICT_KERNEL_RWX
>   bool "Make kernel text and rodata read-only" if ARCH_OPTIONAL_KERNEL_RWX
>   depends on ARCH_HAS_STRICT_KERNEL_RWX
>   default ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
> 
> unsupported:
> !ARCH_HAS_STRICT_KERNEL_RWX
> 
> mandatory:
> ARCH_HAS_STRICT_KERNEL_RWX
> !ARCH_OPTIONAL_KERNEL_RWX
> 
> optional:
> ARCH_HAS_STRICT_KERNEL_RWX
> ARCH_OPTIONAL_KERNEL_RWX
> with default controlled by ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
> 
> Then arm is:
>   select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
>   select ARCH_HAS_STRICT_MODULE_RWX if MMU
>   select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
>   select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7
> 
> x86 and arm64 are:
>   select ARCH_HAS_STRICT_KERNEL_RWX
>   select ARCH_HAS_STRICT_MODULE_RWX
> 
> ?
> 
> -Kees
> 

Yes, that looks good. I wanted it to be mandatory to avoid the
mindset of "optional means we don't need it" but I see there
are some cases where it's better to turn it off. I'll see if
I can emphasize this properly in the help text ("Say Y here
unless you love security exploits running in production")

Thanks,
Laura
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Re: [PATCHv2 1/2] arch: Move CONFIG_DEBUG_RODATA and CONFIG_SET_MODULE_RONX to be common

[Kees Cook]

On Fri, Feb 3, 2017 at 2:28 PM, Russell King - ARM Linux
 wrote:
> On Fri, Feb 03, 2017 at 01:08:40PM -0800, Kees Cook wrote:
>> On Fri, Feb 3, 2017 at 12:29 PM, Russell King - ARM Linux
>>  wrote:
>> > On Fri, Feb 03, 2017 at 11:45:56AM -0800, Kees Cook wrote:
>> >> On Fri, Feb 3, 2017 at 9:52 AM, Laura Abbott  wrote:
>> >> > diff --git a/arch/Kconfig b/arch/Kconfig
>> >> > index 99839c2..22ee01e 100644
>> >> > --- a/arch/Kconfig
>> >> > +++ b/arch/Kconfig
>> >> > @@ -781,4 +781,32 @@ config VMAP_STACK
>> >> >   the stack to map directly to the KASAN shadow map using a 
>> >> > formula
>> >> >   that is incorrect if the stack is in vmalloc space.
>> >> >
>> >> > +config ARCH_NO_STRICT_RWX_DEFAULTS
>> >> > +   def_bool n
>> >> > +
>> >> > +config ARCH_HAS_STRICT_KERNEL_RWX
>> >> > +   def_bool n
>> >> > +
>> >> > +config DEBUG_RODATA
>> >> > +   def_bool y if !ARCH_NO_STRICT_RWX_DEFAULTS
>> >> > +   prompt "Make kernel text and rodata read-only" if 
>> >> > ARCH_NO_STRICT_RWX_DEFAULTS
>> >>
>> >> Ah! Yes, perfect. I totally forgot about using conditional "prompt"
>> >> lines. Nice!
>> >
>> > It's no different from the more usual:
>> >
>> > bool "Make kernel text and rodata read-only" if 
>> > ARCH_NO_STRICT_RWX_DEFAULTS
>> > default y if !ARCH_NO_STRICT_RWX_DEFAULTS
>> > depends on ARCH_HAS_STRICT_KERNEL_RWX
>> >
>> > But... I really don't like this - way too many negations and negatives
>> > which make it difficult to figure out what's going on here.
>> >
>> > The situation we have today is:
>> >
>> > -config DEBUG_RODATA
>> > -   bool "Make kernel text and rodata read-only"
>> > -   depends on MMU && !XIP_KERNEL
>> > -   default y if CPU_V7
>> >
>> > which is "allow the user to select DEBUG_RODATA if building a MMU non-XIP
>> > kernel", suggesting that the user turns it on for ARMv7 CPUs.
>> >
>> > That changes with this and the above:
>> >
>> > +   select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
>> > +   select ARCH_HAS_STRICT_MODULE_RWX if MMU
>> > +   select ARCH_NO_STRICT_RWX_DEFAULTS if !CPU_V7
>> >
>> > This means that ARCH_HAS_STRICT_KERNEL_RWX is set for a MMU non-XIP
>> > kernel, which carries the same pre-condition for DEBUG_RODATA - no
>> > problem there.
>> >
>> > However, ARCH_NO_STRICT_RWX_DEFAULTS is set for non-ARMv7 CPUs, which
>> > means the "Make kernel text and rodata read-only" prompt _is_ provided
>> > for those.  However, for all ARMv7 systems, we go from "suggesting that
>> > the user enables the option" to "you don't have a choice, you get this
>> > whether you want it or not."
>> >
>> > I'd prefer to keep it off for my development systems, where I don't
>> > care about kernel security.  If we don't wish to do that as a general
>> > rule, can we make it dependent on EMBEDDED?
>> >
>> > Given that on ARM it can add up to 4MB to the kernel image - there
>> > _will_ be about 1MB before the .text section, the padding on between
>> > __modver and __ex_table which for me is around 626k, the padding
>> > between .notes and the init sections start with .vectors (the space
>> > between __ex_table and end of .notes is only 4124, which gets padded
>> > up to 1MB) and lastly the padding between the .init section and the
>> > data section (for me around 593k).  This all adds up to an increase
>> > in kernel image size of 3.2MB on 14.2MB - an increase of 22%.
>> >
>> > So no, I'm really not happy with that.
>>
>> Ah yeah, good point. We have three cases: unsupported, mandatory,
>> optional, but we have the case of setting the default for the optional
>> case. Maybe something like this?
>>
>> config STRICT_KERNEL_RWX
>>   bool "Make kernel text and rodata read-only" if ARCH_OPTIONAL_KERNEL_RWX
>>   depends on ARCH_HAS_STRICT_KERNEL_RWX
>>   default ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
>>
>> unsupported:
>> !ARCH_HAS_STRICT_KERNEL_RWX
>>
>> mandatory:
>> ARCH_HAS_STRICT_KERNEL_RWX
>> !ARCH_OPTIONAL_KERNEL_RWX
>>
>> optional:
>> ARCH_HAS_STRICT_KERNEL_RWX
>> ARCH_OPTIONAL_KERNEL_RWX
>> with default controlled by ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
>>
>> Then arm is:
>>   select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
>>   select ARCH_HAS_STRICT_MODULE_RWX if MMU
>>   select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
>>   select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7
>>
>> x86 and arm64 are:
>>   select ARCH_HAS_STRICT_KERNEL_RWX
>>   select ARCH_HAS_STRICT_MODULE_RWX
>
> Looks to me like it will do the job.
>
> In passing, I noticed that, on ARM:
>
>   3 .rodata   002212b4  c0703000  c0703000  00703000  2**6
>   CONTENTS, ALLOC, LOAD, DATA
>
> a lack of READONLY there - which suggests something in this lot isn't
> actually read-only data:
>
> .rodata   : AT(ADDR(.rodata) - LOAD_OFFSET) {   \
> VMLINUX_SYMBOL(__start_rodata) = .; \
> 

Re: [PATCHv2 1/2] arch: Move CONFIG_DEBUG_RODATA and CONFIG_SET_MODULE_RONX to be common

[Russell King - ARM Linux]

On Fri, Feb 03, 2017 at 01:08:40PM -0800, Kees Cook wrote:
> On Fri, Feb 3, 2017 at 12:29 PM, Russell King - ARM Linux
>  wrote:
> > On Fri, Feb 03, 2017 at 11:45:56AM -0800, Kees Cook wrote:
> >> On Fri, Feb 3, 2017 at 9:52 AM, Laura Abbott  wrote:
> >> > diff --git a/arch/Kconfig b/arch/Kconfig
> >> > index 99839c2..22ee01e 100644
> >> > --- a/arch/Kconfig
> >> > +++ b/arch/Kconfig
> >> > @@ -781,4 +781,32 @@ config VMAP_STACK
> >> >   the stack to map directly to the KASAN shadow map using a 
> >> > formula
> >> >   that is incorrect if the stack is in vmalloc space.
> >> >
> >> > +config ARCH_NO_STRICT_RWX_DEFAULTS
> >> > +   def_bool n
> >> > +
> >> > +config ARCH_HAS_STRICT_KERNEL_RWX
> >> > +   def_bool n
> >> > +
> >> > +config DEBUG_RODATA
> >> > +   def_bool y if !ARCH_NO_STRICT_RWX_DEFAULTS
> >> > +   prompt "Make kernel text and rodata read-only" if 
> >> > ARCH_NO_STRICT_RWX_DEFAULTS
> >>
> >> Ah! Yes, perfect. I totally forgot about using conditional "prompt"
> >> lines. Nice!
> >
> > It's no different from the more usual:
> >
> > bool "Make kernel text and rodata read-only" if 
> > ARCH_NO_STRICT_RWX_DEFAULTS
> > default y if !ARCH_NO_STRICT_RWX_DEFAULTS
> > depends on ARCH_HAS_STRICT_KERNEL_RWX
> >
> > But... I really don't like this - way too many negations and negatives
> > which make it difficult to figure out what's going on here.
> >
> > The situation we have today is:
> >
> > -config DEBUG_RODATA
> > -   bool "Make kernel text and rodata read-only"
> > -   depends on MMU && !XIP_KERNEL
> > -   default y if CPU_V7
> >
> > which is "allow the user to select DEBUG_RODATA if building a MMU non-XIP
> > kernel", suggesting that the user turns it on for ARMv7 CPUs.
> >
> > That changes with this and the above:
> >
> > +   select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
> > +   select ARCH_HAS_STRICT_MODULE_RWX if MMU
> > +   select ARCH_NO_STRICT_RWX_DEFAULTS if !CPU_V7
> >
> > This means that ARCH_HAS_STRICT_KERNEL_RWX is set for a MMU non-XIP
> > kernel, which carries the same pre-condition for DEBUG_RODATA - no
> > problem there.
> >
> > However, ARCH_NO_STRICT_RWX_DEFAULTS is set for non-ARMv7 CPUs, which
> > means the "Make kernel text and rodata read-only" prompt _is_ provided
> > for those.  However, for all ARMv7 systems, we go from "suggesting that
> > the user enables the option" to "you don't have a choice, you get this
> > whether you want it or not."
> >
> > I'd prefer to keep it off for my development systems, where I don't
> > care about kernel security.  If we don't wish to do that as a general
> > rule, can we make it dependent on EMBEDDED?
> >
> > Given that on ARM it can add up to 4MB to the kernel image - there
> > _will_ be about 1MB before the .text section, the padding on between
> > __modver and __ex_table which for me is around 626k, the padding
> > between .notes and the init sections start with .vectors (the space
> > between __ex_table and end of .notes is only 4124, which gets padded
> > up to 1MB) and lastly the padding between the .init section and the
> > data section (for me around 593k).  This all adds up to an increase
> > in kernel image size of 3.2MB on 14.2MB - an increase of 22%.
> >
> > So no, I'm really not happy with that.
> 
> Ah yeah, good point. We have three cases: unsupported, mandatory,
> optional, but we have the case of setting the default for the optional
> case. Maybe something like this?
> 
> config STRICT_KERNEL_RWX
>   bool "Make kernel text and rodata read-only" if ARCH_OPTIONAL_KERNEL_RWX
>   depends on ARCH_HAS_STRICT_KERNEL_RWX
>   default ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
> 
> unsupported:
> !ARCH_HAS_STRICT_KERNEL_RWX
> 
> mandatory:
> ARCH_HAS_STRICT_KERNEL_RWX
> !ARCH_OPTIONAL_KERNEL_RWX
> 
> optional:
> ARCH_HAS_STRICT_KERNEL_RWX
> ARCH_OPTIONAL_KERNEL_RWX
> with default controlled by ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
> 
> Then arm is:
>   select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
>   select ARCH_HAS_STRICT_MODULE_RWX if MMU
>   select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
>   select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7
> 
> x86 and arm64 are:
>   select ARCH_HAS_STRICT_KERNEL_RWX
>   select ARCH_HAS_STRICT_MODULE_RWX

Looks to me like it will do the job.

In passing, I noticed that, on ARM:

  3 .rodata   002212b4  c0703000  c0703000  00703000  2**6
  CONTENTS, ALLOC, LOAD, DATA

a lack of READONLY there - which suggests something in this lot isn't
actually read-only data:

.rodata   : AT(ADDR(.rodata) - LOAD_OFFSET) {   \
VMLINUX_SYMBOL(__start_rodata) = .; \
*(.rodata) *(.rodata.*) \
RO_AFTER_INIT_DATA  /* Read only after init */  \
KEEP(*(__vermagic)) /* Kernel version magic */  \

Re: [PATCHv2 1/2] arch: Move CONFIG_DEBUG_RODATA and CONFIG_SET_MODULE_RONX to be common

[Kees Cook]

On Fri, Feb 3, 2017 at 12:29 PM, Russell King - ARM Linux
 wrote:
> On Fri, Feb 03, 2017 at 11:45:56AM -0800, Kees Cook wrote:
>> On Fri, Feb 3, 2017 at 9:52 AM, Laura Abbott  wrote:
>> > diff --git a/arch/Kconfig b/arch/Kconfig
>> > index 99839c2..22ee01e 100644
>> > --- a/arch/Kconfig
>> > +++ b/arch/Kconfig
>> > @@ -781,4 +781,32 @@ config VMAP_STACK
>> >   the stack to map directly to the KASAN shadow map using a formula
>> >   that is incorrect if the stack is in vmalloc space.
>> >
>> > +config ARCH_NO_STRICT_RWX_DEFAULTS
>> > +   def_bool n
>> > +
>> > +config ARCH_HAS_STRICT_KERNEL_RWX
>> > +   def_bool n
>> > +
>> > +config DEBUG_RODATA
>> > +   def_bool y if !ARCH_NO_STRICT_RWX_DEFAULTS
>> > +   prompt "Make kernel text and rodata read-only" if 
>> > ARCH_NO_STRICT_RWX_DEFAULTS
>>
>> Ah! Yes, perfect. I totally forgot about using conditional "prompt"
>> lines. Nice!
>
> It's no different from the more usual:
>
> bool "Make kernel text and rodata read-only" if 
> ARCH_NO_STRICT_RWX_DEFAULTS
> default y if !ARCH_NO_STRICT_RWX_DEFAULTS
> depends on ARCH_HAS_STRICT_KERNEL_RWX
>
> But... I really don't like this - way too many negations and negatives
> which make it difficult to figure out what's going on here.
>
> The situation we have today is:
>
> -config DEBUG_RODATA
> -   bool "Make kernel text and rodata read-only"
> -   depends on MMU && !XIP_KERNEL
> -   default y if CPU_V7
>
> which is "allow the user to select DEBUG_RODATA if building a MMU non-XIP
> kernel", suggesting that the user turns it on for ARMv7 CPUs.
>
> That changes with this and the above:
>
> +   select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
> +   select ARCH_HAS_STRICT_MODULE_RWX if MMU
> +   select ARCH_NO_STRICT_RWX_DEFAULTS if !CPU_V7
>
> This means that ARCH_HAS_STRICT_KERNEL_RWX is set for a MMU non-XIP
> kernel, which carries the same pre-condition for DEBUG_RODATA - no
> problem there.
>
> However, ARCH_NO_STRICT_RWX_DEFAULTS is set for non-ARMv7 CPUs, which
> means the "Make kernel text and rodata read-only" prompt _is_ provided
> for those.  However, for all ARMv7 systems, we go from "suggesting that
> the user enables the option" to "you don't have a choice, you get this
> whether you want it or not."
>
> I'd prefer to keep it off for my development systems, where I don't
> care about kernel security.  If we don't wish to do that as a general
> rule, can we make it dependent on EMBEDDED?
>
> Given that on ARM it can add up to 4MB to the kernel image - there
> _will_ be about 1MB before the .text section, the padding on between
> __modver and __ex_table which for me is around 626k, the padding
> between .notes and the init sections start with .vectors (the space
> between __ex_table and end of .notes is only 4124, which gets padded
> up to 1MB) and lastly the padding between the .init section and the
> data section (for me around 593k).  This all adds up to an increase
> in kernel image size of 3.2MB on 14.2MB - an increase of 22%.
>
> So no, I'm really not happy with that.

Ah yeah, good point. We have three cases: unsupported, mandatory,
optional, but we have the case of setting the default for the optional
case. Maybe something like this?

config STRICT_KERNEL_RWX
  bool "Make kernel text and rodata read-only" if ARCH_OPTIONAL_KERNEL_RWX
  depends on ARCH_HAS_STRICT_KERNEL_RWX
  default ARCH_OPTIONAL_KERNEL_RWX_DEFAULT

unsupported:
!ARCH_HAS_STRICT_KERNEL_RWX

mandatory:
ARCH_HAS_STRICT_KERNEL_RWX
!ARCH_OPTIONAL_KERNEL_RWX

optional:
ARCH_HAS_STRICT_KERNEL_RWX
ARCH_OPTIONAL_KERNEL_RWX
with default controlled by ARCH_OPTIONAL_KERNEL_RWX_DEFAULT

Then arm is:
  select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
  select ARCH_HAS_STRICT_MODULE_RWX if MMU
  select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
  select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7

x86 and arm64 are:
  select ARCH_HAS_STRICT_KERNEL_RWX
  select ARCH_HAS_STRICT_MODULE_RWX

?

-Kees

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Re: [PATCHv2 1/2] arch: Move CONFIG_DEBUG_RODATA and CONFIG_SET_MODULE_RONX to be common

[Russell King - ARM Linux]

On Fri, Feb 03, 2017 at 11:45:56AM -0800, Kees Cook wrote:
> On Fri, Feb 3, 2017 at 9:52 AM, Laura Abbott  wrote:
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index 99839c2..22ee01e 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -781,4 +781,32 @@ config VMAP_STACK
> >   the stack to map directly to the KASAN shadow map using a formula
> >   that is incorrect if the stack is in vmalloc space.
> >
> > +config ARCH_NO_STRICT_RWX_DEFAULTS
> > +   def_bool n
> > +
> > +config ARCH_HAS_STRICT_KERNEL_RWX
> > +   def_bool n
> > +
> > +config DEBUG_RODATA
> > +   def_bool y if !ARCH_NO_STRICT_RWX_DEFAULTS
> > +   prompt "Make kernel text and rodata read-only" if 
> > ARCH_NO_STRICT_RWX_DEFAULTS
> 
> Ah! Yes, perfect. I totally forgot about using conditional "prompt"
> lines. Nice!

It's no different from the more usual:

bool "Make kernel text and rodata read-only" if 
ARCH_NO_STRICT_RWX_DEFAULTS
default y if !ARCH_NO_STRICT_RWX_DEFAULTS
depends on ARCH_HAS_STRICT_KERNEL_RWX

But... I really don't like this - way too many negations and negatives
which make it difficult to figure out what's going on here.

The situation we have today is:

-config DEBUG_RODATA
-   bool "Make kernel text and rodata read-only"
-   depends on MMU && !XIP_KERNEL
-   default y if CPU_V7

which is "allow the user to select DEBUG_RODATA if building a MMU non-XIP
kernel", suggesting that the user turns it on for ARMv7 CPUs.

That changes with this and the above:

+   select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
+   select ARCH_HAS_STRICT_MODULE_RWX if MMU
+   select ARCH_NO_STRICT_RWX_DEFAULTS if !CPU_V7

This means that ARCH_HAS_STRICT_KERNEL_RWX is set for a MMU non-XIP
kernel, which carries the same pre-condition for DEBUG_RODATA - no
problem there.

However, ARCH_NO_STRICT_RWX_DEFAULTS is set for non-ARMv7 CPUs, which
means the "Make kernel text and rodata read-only" prompt _is_ provided
for those.  However, for all ARMv7 systems, we go from "suggesting that
the user enables the option" to "you don't have a choice, you get this
whether you want it or not."

I'd prefer to keep it off for my development systems, where I don't
care about kernel security.  If we don't wish to do that as a general
rule, can we make it dependent on EMBEDDED?

Given that on ARM it can add up to 4MB to the kernel image - there
_will_ be about 1MB before the .text section, the padding on between
__modver and __ex_table which for me is around 626k, the padding
between .notes and the init sections start with .vectors (the space
between __ex_table and end of .notes is only 4124, which gets padded
up to 1MB) and lastly the padding between the .init section and the
data section (for me around 593k).  This all adds up to an increase
in kernel image size of 3.2MB on 14.2MB - an increase of 22%.

So no, I'm really not happy with that.

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Re: [PATCHv2 1/2] arch: Move CONFIG_DEBUG_RODATA and CONFIG_SET_MODULE_RONX to be common

[Kees Cook]

On Fri, Feb 3, 2017 at 9:52 AM, Laura Abbott  wrote:
> There are multiple architectures that support CONFIG_DEBUG_RODATA and
> CONFIG_SET_MODULE_RONX. These options also now have the ability to be
> turned off at runtime. Move these to an architecture independent
> location and make these options def_bool y for almost all of those
> arches.
>
> Signed-off-by: Laura Abbott 
> ---
> v2: This patch is now doing just the refactor of the existing config options.
> ---
>  arch/Kconfig  | 28 
>  arch/arm/Kconfig  |  3 +++
>  arch/arm/Kconfig.debug| 11 ---
>  arch/arm/mm/Kconfig   | 12 
>  arch/arm64/Kconfig|  5 ++---
>  arch/arm64/Kconfig.debug  | 11 ---
>  arch/parisc/Kconfig   |  1 +
>  arch/parisc/Kconfig.debug | 11 ---
>  arch/s390/Kconfig |  5 ++---
>  arch/s390/Kconfig.debug   |  3 ---
>  arch/x86/Kconfig  |  5 ++---
>  arch/x86/Kconfig.debug| 11 ---
>  12 files changed, 38 insertions(+), 68 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 99839c2..22ee01e 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -781,4 +781,32 @@ config VMAP_STACK
>   the stack to map directly to the KASAN shadow map using a formula
>   that is incorrect if the stack is in vmalloc space.
>
> +config ARCH_NO_STRICT_RWX_DEFAULTS
> +   def_bool n
> +
> +config ARCH_HAS_STRICT_KERNEL_RWX
> +   def_bool n
> +
> +config DEBUG_RODATA
> +   def_bool y if !ARCH_NO_STRICT_RWX_DEFAULTS
> +   prompt "Make kernel text and rodata read-only" if 
> ARCH_NO_STRICT_RWX_DEFAULTS

Ah! Yes, perfect. I totally forgot about using conditional "prompt" lines. Nice!

Acked-by: Kees Cook 

-Kees

-- 
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Pixel Security
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Re: [PATCHv2 1/2] arch: Move CONFIG_DEBUG_RODATA and CONFIG_SET_MODULE_RONX to be common

[Mark Rutland]

On Fri, Feb 03, 2017 at 09:52:21AM -0800, Laura Abbott wrote:
> There are multiple architectures that support CONFIG_DEBUG_RODATA and
> CONFIG_SET_MODULE_RONX. These options also now have the ability to be
> turned off at runtime. Move these to an architecture independent
> location and make these options def_bool y for almost all of those
> arches.
> 
> Signed-off-by: Laura Abbott 

>From my POV this looks good. FWIW:

Acked-by: Mark Rutland 

Mark.

> ---
> v2: This patch is now doing just the refactor of the existing config options.
> ---
>  arch/Kconfig  | 28 
>  arch/arm/Kconfig  |  3 +++
>  arch/arm/Kconfig.debug| 11 ---
>  arch/arm/mm/Kconfig   | 12 
>  arch/arm64/Kconfig|  5 ++---
>  arch/arm64/Kconfig.debug  | 11 ---
>  arch/parisc/Kconfig   |  1 +
>  arch/parisc/Kconfig.debug | 11 ---
>  arch/s390/Kconfig |  5 ++---
>  arch/s390/Kconfig.debug   |  3 ---
>  arch/x86/Kconfig  |  5 ++---
>  arch/x86/Kconfig.debug| 11 ---
>  12 files changed, 38 insertions(+), 68 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 99839c2..22ee01e 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -781,4 +781,32 @@ config VMAP_STACK
> the stack to map directly to the KASAN shadow map using a formula
> that is incorrect if the stack is in vmalloc space.
>  
> +config ARCH_NO_STRICT_RWX_DEFAULTS
> + def_bool n
> +
> +config ARCH_HAS_STRICT_KERNEL_RWX
> + def_bool n
> +
> +config DEBUG_RODATA
> + def_bool y if !ARCH_NO_STRICT_RWX_DEFAULTS
> + prompt "Make kernel text and rodata read-only" if 
> ARCH_NO_STRICT_RWX_DEFAULTS
> + depends on ARCH_HAS_STRICT_KERNEL_RWX
> + help
> +   If this is set, kernel text and rodata memory will be made read-only,
> +   and non-text memory will be made non-executable. This provides
> +   protection against certain security exploits (e.g. executing the heap
> +   or modifying text)
> +
> +config ARCH_HAS_STRICT_MODULE_RWX
> + def_bool n
> +
> +config DEBUG_SET_MODULE_RONX
> + def_bool y if !ARCH_NO_STRICT_RWX_DEFAULTS
> + prompt "Set loadable kenrel module data as NX and text as RO" if 
> ARCH_NO_STRICT_RWX_DEFAULTS
> + depends on ARCH_HAS_STRICT_MODULE_RWX && MODULES
> + help
> +   If this is set, module text and rodata memory will be made read-only,
> +   and non-text memory will be made non-executable. This provides
> +   protection against certain security exploits (e.g. writing to text)
> +
>  source "kernel/gcov/Kconfig"
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 186c4c2..aa73ca8 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -4,10 +4,13 @@ config ARM
>   select ARCH_CLOCKSOURCE_DATA
>   select ARCH_HAS_DEVMEM_IS_ALLOWED
>   select ARCH_HAS_ELF_RANDOMIZE
> + select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
> + select ARCH_HAS_STRICT_MODULE_RWX if MMU
>   select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
>   select ARCH_HAVE_CUSTOM_GPIO_H
>   select ARCH_HAS_GCOV_PROFILE_ALL
>   select ARCH_MIGHT_HAVE_PC_PARPORT
> + select ARCH_NO_STRICT_RWX_DEFAULTS if !CPU_V7
>   select ARCH_SUPPORTS_ATOMIC_RMW
>   select ARCH_USE_BUILTIN_BSWAP
>   select ARCH_USE_CMPXCHG_LOCKREF
> diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
> index d83f7c3..426d271 100644
> --- a/arch/arm/Kconfig.debug
> +++ b/arch/arm/Kconfig.debug
> @@ -1738,17 +1738,6 @@ config PID_IN_CONTEXTIDR
> additional instructions during context switch. Say Y here only if you
> are planning to use hardware trace tools with this kernel.
>  
> -config DEBUG_SET_MODULE_RONX
> - bool "Set loadable kernel module data as NX and text as RO"
> - depends on MODULES && MMU
> - ---help---
> -   This option helps catch unintended modifications to loadable
> -   kernel module's text and read-only data. It also prevents execution
> -   of module data. Such protection may interfere with run-time code
> -   patching and dynamic kernel tracing - and they might also protect
> -   against certain classes of kernel exploits.
> -   If in doubt, say "N".
> -
>  source "drivers/hwtracing/coresight/Kconfig"
>  
>  endmenu
> diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
> index f68e8ec..419a035 100644
> --- a/arch/arm/mm/Kconfig
> +++ b/arch/arm/mm/Kconfig
> @@ -1051,18 +1051,6 @@ config ARCH_SUPPORTS_BIG_ENDIAN
> This option specifies the architecture can support big endian
> operation.
>  
> -config DEBUG_RODATA
> - bool "Make kernel text and rodata read-only"
> - depends on MMU && !XIP_KERNEL
> - default y if CPU_V7
> - help
> -   If this is set, kernel text and rodata memory will be made
> -   read-only, and non-text kernel memory will be made non-executable.
> -   

[PATCHv2 1/2] arch: Move CONFIG_DEBUG_RODATA and CONFIG_SET_MODULE_RONX to be common

[Laura Abbott]

There are multiple architectures that support CONFIG_DEBUG_RODATA and
CONFIG_SET_MODULE_RONX. These options also now have the ability to be
turned off at runtime. Move these to an architecture independent
location and make these options def_bool y for almost all of those
arches.

Signed-off-by: Laura Abbott 
---
v2: This patch is now doing just the refactor of the existing config options.
---
 arch/Kconfig  | 28 
 arch/arm/Kconfig  |  3 +++
 arch/arm/Kconfig.debug| 11 ---
 arch/arm/mm/Kconfig   | 12 
 arch/arm64/Kconfig|  5 ++---
 arch/arm64/Kconfig.debug  | 11 ---
 arch/parisc/Kconfig   |  1 +
 arch/parisc/Kconfig.debug | 11 ---
 arch/s390/Kconfig |  5 ++---
 arch/s390/Kconfig.debug   |  3 ---
 arch/x86/Kconfig  |  5 ++---
 arch/x86/Kconfig.debug| 11 ---
 12 files changed, 38 insertions(+), 68 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 99839c2..22ee01e 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -781,4 +781,32 @@ config VMAP_STACK
  the stack to map directly to the KASAN shadow map using a formula
  that is incorrect if the stack is in vmalloc space.
 
+config ARCH_NO_STRICT_RWX_DEFAULTS
+   def_bool n
+
+config ARCH_HAS_STRICT_KERNEL_RWX
+   def_bool n
+
+config DEBUG_RODATA
+   def_bool y if !ARCH_NO_STRICT_RWX_DEFAULTS
+   prompt "Make kernel text and rodata read-only" if 
ARCH_NO_STRICT_RWX_DEFAULTS
+   depends on ARCH_HAS_STRICT_KERNEL_RWX
+   help
+ If this is set, kernel text and rodata memory will be made read-only,
+ and non-text memory will be made non-executable. This provides
+ protection against certain security exploits (e.g. executing the heap
+ or modifying text)
+
+config ARCH_HAS_STRICT_MODULE_RWX
+   def_bool n
+
+config DEBUG_SET_MODULE_RONX
+   def_bool y if !ARCH_NO_STRICT_RWX_DEFAULTS
+   prompt "Set loadable kenrel module data as NX and text as RO" if 
ARCH_NO_STRICT_RWX_DEFAULTS
+   depends on ARCH_HAS_STRICT_MODULE_RWX && MODULES
+   help
+ If this is set, module text and rodata memory will be made read-only,
+ and non-text memory will be made non-executable. This provides
+ protection against certain security exploits (e.g. writing to text)
+
 source "kernel/gcov/Kconfig"
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 186c4c2..aa73ca8 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -4,10 +4,13 @@ config ARM
select ARCH_CLOCKSOURCE_DATA
select ARCH_HAS_DEVMEM_IS_ALLOWED
select ARCH_HAS_ELF_RANDOMIZE
+   select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
+   select ARCH_HAS_STRICT_MODULE_RWX if MMU
select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
select ARCH_HAVE_CUSTOM_GPIO_H
select ARCH_HAS_GCOV_PROFILE_ALL
select ARCH_MIGHT_HAVE_PC_PARPORT
+   select ARCH_NO_STRICT_RWX_DEFAULTS if !CPU_V7
select ARCH_SUPPORTS_ATOMIC_RMW
select ARCH_USE_BUILTIN_BSWAP
select ARCH_USE_CMPXCHG_LOCKREF
diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index d83f7c3..426d271 100644
--- a/arch/arm/Kconfig.debug
+++ b/arch/arm/Kconfig.debug
@@ -1738,17 +1738,6 @@ config PID_IN_CONTEXTIDR
  additional instructions during context switch. Say Y here only if you
  are planning to use hardware trace tools with this kernel.
 
-config DEBUG_SET_MODULE_RONX
-   bool "Set loadable kernel module data as NX and text as RO"
-   depends on MODULES && MMU
-   ---help---
- This option helps catch unintended modifications to loadable
- kernel module's text and read-only data. It also prevents execution
- of module data. Such protection may interfere with run-time code
- patching and dynamic kernel tracing - and they might also protect
- against certain classes of kernel exploits.
- If in doubt, say "N".
-
 source "drivers/hwtracing/coresight/Kconfig"
 
 endmenu
diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index f68e8ec..419a035 100644
--- a/arch/arm/mm/Kconfig
+++ b/arch/arm/mm/Kconfig
@@ -1051,18 +1051,6 @@ config ARCH_SUPPORTS_BIG_ENDIAN
  This option specifies the architecture can support big endian
  operation.
 
-config DEBUG_RODATA
-   bool "Make kernel text and rodata read-only"
-   depends on MMU && !XIP_KERNEL
-   default y if CPU_V7
-   help
- If this is set, kernel text and rodata memory will be made
- read-only, and non-text kernel memory will be made non-executable.
- The tradeoff is that each region is padded to section-size (1MiB)
- boundaries (because their permissions are different and splitting
- the 1M pages into 4K ones causes TLB performance problems), which
- can waste memory.
-
 config DEBUG_ALIGN_RODATA