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

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?

> +	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)
--