Re: [PATCH v7 24/26] x86: Enable User-Mode Instruction Prevention

[Borislav Petkov]

On Tue, Jul 25, 2017 at 05:44:08PM -0700, Ricardo Neri wrote:
> On Fri, 2017-06-09 at 18:10 +0200, Borislav Petkov wrote:
> > On Fri, May 05, 2017 at 11:17:22AM -0700, Ricardo Neri wrote:
> > > User_mode Instruction Prevention (UMIP) is enabled by setting/clearing a
> > > bit in %cr4.
> > > 
> > > It makes sense to enable UMIP at some point while booting, before user
> > > spaces come up. Like SMAP and SMEP, is not critical to have it enabled
> > > very early during boot. This is because UMIP is relevant only when there 
> > > is
> > > a userspace to be protected from. Given the similarities in relevance, it
> > > makes sense to enable UMIP along with SMAP and SMEP.
> > > 
> > > UMIP is enabled by default. It can be disabled by adding clearcpuid=514
> > > to the kernel parameters.

...

> So would this become a y when more machines have UMIP?

I guess. Stuff which proves reliable and widespread gets automatically
enabled with time, in most cases. IMHO, of course.

> Why would static_cpu_has() reply wrong if alternatives are not in place?
> Because it uses the boot CPU data? When it calls _static_cpu_has() it
> would do something equivalent to

Nevermind - I forgot that static_cpu_has() now drops to dynamic check
before alternatives application.

-- 
Regards/Gruss,
Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 
(AG Nürnberg)
-- 
--
To unsubscribe from this list: send the line "unsubscribe linux-msdos" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: [PATCH v7 24/26] x86: Enable User-Mode Instruction Prevention

[Ricardo Neri]

On Fri, 2017-06-09 at 18:10 +0200, Borislav Petkov wrote:
> On Fri, May 05, 2017 at 11:17:22AM -0700, Ricardo Neri wrote:
> > User_mode Instruction Prevention (UMIP) is enabled by setting/clearing a
> > bit in %cr4.
> > 
> > It makes sense to enable UMIP at some point while booting, before user
> > spaces come up. Like SMAP and SMEP, is not critical to have it enabled
> > very early during boot. This is because UMIP is relevant only when there is
> > a userspace to be protected from. Given the similarities in relevance, it
> > makes sense to enable UMIP along with SMAP and SMEP.
> > 
> > UMIP is enabled by default. It can be disabled by adding clearcpuid=514
> > to the kernel parameters.
> > 
> > Cc: Andy Lutomirski 
> > Cc: Andrew Morton 
> > Cc: H. Peter Anvin 
> > Cc: Borislav Petkov 
> > Cc: Brian Gerst 
> > Cc: Chen Yucong 
> > Cc: Chris Metcalf 
> > Cc: Dave Hansen 
> > Cc: Fenghua Yu 
> > Cc: Huang Rui 
> > Cc: Jiri Slaby 
> > Cc: Jonathan Corbet 
> > Cc: Michael S. Tsirkin 
> > Cc: Paul Gortmaker 
> > Cc: Peter Zijlstra 
> > Cc: Ravi V. Shankar 
> > Cc: Shuah Khan 
> > Cc: Vlastimil Babka 
> > Cc: Tony Luck 
> > Cc: Paolo Bonzini 
> > Cc: Liang Z. Li 
> > Cc: Alexandre Julliard 
> > Cc: Stas Sergeev 
> > Cc: x...@kernel.org
> > Cc: linux-msdos@vger.kernel.org
> > Signed-off-by: Ricardo Neri 
> > ---
> >  arch/x86/Kconfig | 10 ++
> >  arch/x86/kernel/cpu/common.c | 16 +++-
> >  2 files changed, 25 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index 702002b..1b1bbeb 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -1745,6 +1745,16 @@ config X86_SMAP
> >  
> >   If unsure, say Y.
> >  
> > +config X86_INTEL_UMIP
> > +   def_bool y
> 
> That's a bit too much. It makes sense on distro kernels but how many
> machines out there actually have UMIP?

So would this become a y when more machines have UMIP?
> 
> > +   depends on CPU_SUP_INTEL
> > +   prompt "Intel User Mode Instruction Prevention" if EXPERT
> > +   ---help---
> > + The User Mode Instruction Prevention (UMIP) is a security
> > + feature in newer Intel processors. If enabled, a general
> > + protection fault is issued if the instructions SGDT, SLDT,
> > + SIDT, SMSW and STR are executed in user mode.
> > +
> >  config X86_INTEL_MPX
> > prompt "Intel MPX (Memory Protection Extensions)"
> > def_bool n
> > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> > index 8ee3211..66ebded 100644
> > --- a/arch/x86/kernel/cpu/common.c
> > +++ b/arch/x86/kernel/cpu/common.c
> > @@ -311,6 +311,19 @@ static __always_inline void setup_smap(struct 
> > cpuinfo_x86 *c)
> > }
> >  }
> >  
> > +static __always_inline void setup_umip(struct cpuinfo_x86 *c)
> > +{
> > +   if (cpu_feature_enabled(X86_FEATURE_UMIP) &&
> > +   cpu_has(c, X86_FEATURE_UMIP))
> 
> Hmm, so if UMIP is not build-time disabled, the cpu_feature_enabled()
> will call static_cpu_has().
> 
> Looks like you want to call cpu_has() too because alternatives haven't
> run yet and static_cpu_has() will reply wrong. Please state that in a
> comment.

Why would static_cpu_has() reply wrong if alternatives are not in place?
Because it uses the boot CPU data? When it calls _static_cpu_has() it
would do something equivalent to

   testb test_bit, boot_cpu_data.x86_capability[bit].

I am calling cpu_has because cpu_feature_enabled(), via
static_cpu_has(), will use the boot CPU data while cpu_has would use the
local CPU data. Is this what you meant?

I can definitely add a comment with this explanation, if it makes sense.

Thanks and BR,
Ricardo

--
To unsubscribe from this list: send the line "unsubscribe linux-msdos" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: [PATCH v7 24/26] x86: Enable User-Mode Instruction Prevention

[Borislav Petkov]

On Fri, May 05, 2017 at 11:17:22AM -0700, Ricardo Neri wrote:
> User_mode Instruction Prevention (UMIP) is enabled by setting/clearing a
> bit in %cr4.
> 
> It makes sense to enable UMIP at some point while booting, before user
> spaces come up. Like SMAP and SMEP, is not critical to have it enabled
> very early during boot. This is because UMIP is relevant only when there is
> a userspace to be protected from. Given the similarities in relevance, it
> makes sense to enable UMIP along with SMAP and SMEP.
> 
> UMIP is enabled by default. It can be disabled by adding clearcpuid=514
> to the kernel parameters.
> 
> Cc: Andy Lutomirski 
> Cc: Andrew Morton 
> Cc: H. Peter Anvin 
> Cc: Borislav Petkov 
> Cc: Brian Gerst 
> Cc: Chen Yucong 
> Cc: Chris Metcalf 
> Cc: Dave Hansen 
> Cc: Fenghua Yu 
> Cc: Huang Rui 
> Cc: Jiri Slaby 
> Cc: Jonathan Corbet 
> Cc: Michael S. Tsirkin 
> Cc: Paul Gortmaker 
> Cc: Peter Zijlstra 
> Cc: Ravi V. Shankar 
> Cc: Shuah Khan 
> Cc: Vlastimil Babka 
> Cc: Tony Luck 
> Cc: Paolo Bonzini 
> Cc: Liang Z. Li 
> Cc: Alexandre Julliard 
> Cc: Stas Sergeev 
> Cc: x...@kernel.org
> Cc: linux-msdos@vger.kernel.org
> Signed-off-by: Ricardo Neri 
> ---
>  arch/x86/Kconfig | 10 ++
>  arch/x86/kernel/cpu/common.c | 16 +++-
>  2 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 702002b..1b1bbeb 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1745,6 +1745,16 @@ config X86_SMAP
>  
> If unsure, say Y.
>  
> +config X86_INTEL_UMIP
> + def_bool y

That's a bit too much. It makes sense on distro kernels but how many
machines out there actually have UMIP?

> + depends on CPU_SUP_INTEL
> + prompt "Intel User Mode Instruction Prevention" if EXPERT
> + ---help---
> +   The User Mode Instruction Prevention (UMIP) is a security
> +   feature in newer Intel processors. If enabled, a general
> +   protection fault is issued if the instructions SGDT, SLDT,
> +   SIDT, SMSW and STR are executed in user mode.
> +
>  config X86_INTEL_MPX
>   prompt "Intel MPX (Memory Protection Extensions)"
>   def_bool n
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 8ee3211..66ebded 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -311,6 +311,19 @@ static __always_inline void setup_smap(struct 
> cpuinfo_x86 *c)
>   }
>  }
>  
> +static __always_inline void setup_umip(struct cpuinfo_x86 *c)
> +{
> + if (cpu_feature_enabled(X86_FEATURE_UMIP) &&
> + cpu_has(c, X86_FEATURE_UMIP))

Hmm, so if UMIP is not build-time disabled, the cpu_feature_enabled()
will call static_cpu_has().

Looks like you want to call cpu_has() too because alternatives haven't
run yet and static_cpu_has() will reply wrong. Please state that in a
comment.

-- 
Regards/Gruss,
Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 
(AG Nürnberg)
-- 
--
To unsubscribe from this list: send the line "unsubscribe linux-msdos" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html