Re: [kernel-hardening] [PATCH 2/4] arm64: Reduce ELF_ET_DYN_BASE

[Ard Biesheuvel]

On 23 June 2017 at 14:02, Kees Cook  wrote:
> On Fri, Jun 23, 2017 at 6:52 AM, Kees Cook  wrote:
>> On Thu, Jun 22, 2017 at 11:57 PM, Ard Biesheuvel
>>  wrote:
>>> Hi Kees,
>>>
>>> On 22 June 2017 at 18:06, Kees Cook  wrote:
 Now that explicitly executed loaders are loaded in the mmap region,
 position PIE binaries lower in the address space to avoid possible
 collisions with mmap or stack regions. For 64-bit, align to 4GB to
 allow runtimes to use the entire 32-bit address space for 32-bit
 pointers.

 Signed-off-by: Kees Cook 
 ---
  arch/arm64/include/asm/elf.h | 13 ++---
  1 file changed, 6 insertions(+), 7 deletions(-)

 diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
 index 5d1700425efe..f742af8f7c42 100644
 --- a/arch/arm64/include/asm/elf.h
 +++ b/arch/arm64/include/asm/elf.h
 @@ -113,12 +113,13 @@
  #define ELF_EXEC_PAGESIZE  PAGE_SIZE

  /*
 - * This is the location that an ET_DYN program is loaded if exec'ed.  
 Typical
 - * use of this is to invoke "./ld.so someprog" to test out a new version 
 of
 - * the loader.  We need to make sure that it is out of the way of the 
 program
 - * that it will "exec", and that there is sufficient room for the brk.
 + * This is the base location for PIE (ET_DYN with INTERP) loads. On
 + * 64-bit, this is raised to 4GB to leave the entire 32-bit address
 + * space open for things that want to use the area for 32-bit pointers.
   */
 -#define ELF_ET_DYN_BASE(2 * TASK_SIZE_64 / 3)
 +#define ELF_ET_DYN_BASE(test_thread_flag(TIF_32BIT) ?  \
 +   0x00040UL : \
 +   0x1UL)

>>>
>>> Why are you merging this with the COMPAT definition?
>>
>> It seemed like the right thing to do since a single definition could
>> handle both cases. Is there something I'm overlooking in the arm64
>> case?
>
> And like 5 minutes later there was a loud head-slapping noise in my
> office. Durr, yeah, arm64 doesn't have to deal with a "native 32-bit"
> mode, so the merge isn't needed. Yes yes, I will split it back up and
> drop the thread flag test.
>

Oh, is that what I heard?

Re: [kernel-hardening] [PATCH 2/4] arm64: Reduce ELF_ET_DYN_BASE

[Kees Cook]

On Fri, Jun 23, 2017 at 6:52 AM, Kees Cook  wrote:
> On Thu, Jun 22, 2017 at 11:57 PM, Ard Biesheuvel
>  wrote:
>> Hi Kees,
>>
>> On 22 June 2017 at 18:06, Kees Cook  wrote:
>>> Now that explicitly executed loaders are loaded in the mmap region,
>>> position PIE binaries lower in the address space to avoid possible
>>> collisions with mmap or stack regions. For 64-bit, align to 4GB to
>>> allow runtimes to use the entire 32-bit address space for 32-bit
>>> pointers.
>>>
>>> Signed-off-by: Kees Cook 
>>> ---
>>>  arch/arm64/include/asm/elf.h | 13 ++---
>>>  1 file changed, 6 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
>>> index 5d1700425efe..f742af8f7c42 100644
>>> --- a/arch/arm64/include/asm/elf.h
>>> +++ b/arch/arm64/include/asm/elf.h
>>> @@ -113,12 +113,13 @@
>>>  #define ELF_EXEC_PAGESIZE  PAGE_SIZE
>>>
>>>  /*
>>> - * This is the location that an ET_DYN program is loaded if exec'ed.  
>>> Typical
>>> - * use of this is to invoke "./ld.so someprog" to test out a new version of
>>> - * the loader.  We need to make sure that it is out of the way of the 
>>> program
>>> - * that it will "exec", and that there is sufficient room for the brk.
>>> + * This is the base location for PIE (ET_DYN with INTERP) loads. On
>>> + * 64-bit, this is raised to 4GB to leave the entire 32-bit address
>>> + * space open for things that want to use the area for 32-bit pointers.
>>>   */
>>> -#define ELF_ET_DYN_BASE(2 * TASK_SIZE_64 / 3)
>>> +#define ELF_ET_DYN_BASE(test_thread_flag(TIF_32BIT) ?  \
>>> +   0x00040UL : \
>>> +   0x1UL)
>>>
>>
>> Why are you merging this with the COMPAT definition?
>
> It seemed like the right thing to do since a single definition could
> handle both cases. Is there something I'm overlooking in the arm64
> case?

And like 5 minutes later there was a loud head-slapping noise in my
office. Durr, yeah, arm64 doesn't have to deal with a "native 32-bit"
mode, so the merge isn't needed. Yes yes, I will split it back up and
drop the thread flag test.

Thanks!

-Kees

>>>  #ifndef __ASSEMBLY__
>>>
>>> @@ -173,8 +174,6 @@ extern int arch_setup_additional_pages(struct 
>>> linux_binprm *bprm,
>>>
>>>  #ifdef CONFIG_COMPAT
>>>
>>> -#define COMPAT_ELF_ET_DYN_BASE (2 * TASK_SIZE_32 / 3)
>>> -
>>>  /* AArch32 registers. */
>>>  #define COMPAT_ELF_NGREG   18
>>>  typedef unsigned int   compat_elf_greg_t;
>>> --
>>> 2.7.4
>>>
>
>
>
> --
> Kees Cook
> Pixel Security



-- 
Kees Cook
Pixel Security

Re: [kernel-hardening] [PATCH 2/4] arm64: Reduce ELF_ET_DYN_BASE

[Kees Cook]

On Thu, Jun 22, 2017 at 11:57 PM, Ard Biesheuvel
 wrote:
> Hi Kees,
>
> On 22 June 2017 at 18:06, Kees Cook  wrote:
>> Now that explicitly executed loaders are loaded in the mmap region,
>> position PIE binaries lower in the address space to avoid possible
>> collisions with mmap or stack regions. For 64-bit, align to 4GB to
>> allow runtimes to use the entire 32-bit address space for 32-bit
>> pointers.
>>
>> Signed-off-by: Kees Cook 
>> ---
>>  arch/arm64/include/asm/elf.h | 13 ++---
>>  1 file changed, 6 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
>> index 5d1700425efe..f742af8f7c42 100644
>> --- a/arch/arm64/include/asm/elf.h
>> +++ b/arch/arm64/include/asm/elf.h
>> @@ -113,12 +113,13 @@
>>  #define ELF_EXEC_PAGESIZE  PAGE_SIZE
>>
>>  /*
>> - * This is the location that an ET_DYN program is loaded if exec'ed.  
>> Typical
>> - * use of this is to invoke "./ld.so someprog" to test out a new version of
>> - * the loader.  We need to make sure that it is out of the way of the 
>> program
>> - * that it will "exec", and that there is sufficient room for the brk.
>> + * This is the base location for PIE (ET_DYN with INTERP) loads. On
>> + * 64-bit, this is raised to 4GB to leave the entire 32-bit address
>> + * space open for things that want to use the area for 32-bit pointers.
>>   */
>> -#define ELF_ET_DYN_BASE(2 * TASK_SIZE_64 / 3)
>> +#define ELF_ET_DYN_BASE(test_thread_flag(TIF_32BIT) ?  \
>> +   0x00040UL : \
>> +   0x1UL)
>>
>
> Why are you merging this with the COMPAT definition?

It seemed like the right thing to do since a single definition could
handle both cases. Is there something I'm overlooking in the arm64
case?

-Kees

>
>>  #ifndef __ASSEMBLY__
>>
>> @@ -173,8 +174,6 @@ extern int arch_setup_additional_pages(struct 
>> linux_binprm *bprm,
>>
>>  #ifdef CONFIG_COMPAT
>>
>> -#define COMPAT_ELF_ET_DYN_BASE (2 * TASK_SIZE_32 / 3)
>> -
>>  /* AArch32 registers. */
>>  #define COMPAT_ELF_NGREG   18
>>  typedef unsigned int   compat_elf_greg_t;
>> --
>> 2.7.4
>>



-- 
Kees Cook
Pixel Security

Re: [kernel-hardening] [PATCH 2/4] arm64: Reduce ELF_ET_DYN_BASE

[Ard Biesheuvel]

Hi Kees,

On 22 June 2017 at 18:06, Kees Cook  wrote:
> Now that explicitly executed loaders are loaded in the mmap region,
> position PIE binaries lower in the address space to avoid possible
> collisions with mmap or stack regions. For 64-bit, align to 4GB to
> allow runtimes to use the entire 32-bit address space for 32-bit
> pointers.
>
> Signed-off-by: Kees Cook 
> ---
>  arch/arm64/include/asm/elf.h | 13 ++---
>  1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
> index 5d1700425efe..f742af8f7c42 100644
> --- a/arch/arm64/include/asm/elf.h
> +++ b/arch/arm64/include/asm/elf.h
> @@ -113,12 +113,13 @@
>  #define ELF_EXEC_PAGESIZE  PAGE_SIZE
>
>  /*
> - * This is the location that an ET_DYN program is loaded if exec'ed.  Typical
> - * use of this is to invoke "./ld.so someprog" to test out a new version of
> - * the loader.  We need to make sure that it is out of the way of the program
> - * that it will "exec", and that there is sufficient room for the brk.
> + * This is the base location for PIE (ET_DYN with INTERP) loads. On
> + * 64-bit, this is raised to 4GB to leave the entire 32-bit address
> + * space open for things that want to use the area for 32-bit pointers.
>   */
> -#define ELF_ET_DYN_BASE(2 * TASK_SIZE_64 / 3)
> +#define ELF_ET_DYN_BASE(test_thread_flag(TIF_32BIT) ?  \
> +   0x00040UL : \
> +   0x1UL)
>

Why are you merging this with the COMPAT definition?

>  #ifndef __ASSEMBLY__
>
> @@ -173,8 +174,6 @@ extern int arch_setup_additional_pages(struct 
> linux_binprm *bprm,
>
>  #ifdef CONFIG_COMPAT
>
> -#define COMPAT_ELF_ET_DYN_BASE (2 * TASK_SIZE_32 / 3)
> -
>  /* AArch32 registers. */
>  #define COMPAT_ELF_NGREG   18
>  typedef unsigned int   compat_elf_greg_t;
> --
> 2.7.4
>