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

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 <[email protected]>
> > Cc: Andrew Morton <[email protected]>
> > Cc: H. Peter Anvin <[email protected]>
> > Cc: Borislav Petkov <[email protected]>
> > Cc: Brian Gerst <[email protected]>
> > Cc: Chen Yucong <[email protected]>
> > Cc: Chris Metcalf <[email protected]>
> > Cc: Dave Hansen <[email protected]>
> > Cc: Fenghua Yu <[email protected]>
> > Cc: Huang Rui <[email protected]>
> > Cc: Jiri Slaby <[email protected]>
> > Cc: Jonathan Corbet <[email protected]>
> > Cc: Michael S. Tsirkin <[email protected]>
> > Cc: Paul Gortmaker <[email protected]>
> > Cc: Peter Zijlstra <[email protected]>
> > Cc: Ravi V. Shankar <[email protected]>
> > Cc: Shuah Khan <[email protected]>
> > Cc: Vlastimil Babka <[email protected]>
> > Cc: Tony Luck <[email protected]>
> > Cc: Paolo Bonzini <[email protected]>
> > Cc: Liang Z. Li <[email protected]>
> > Cc: Alexandre Julliard <[email protected]>
> > Cc: Stas Sergeev <[email protected]>
> > Cc: [email protected]
> > Cc: [email protected]
> > Signed-off-by: Ricardo Neri <[email protected]>
> > ---
> >  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