Messages in this thread | | | Date | Wed, 11 Jan 2023 16:27:49 -0800 | Subject | Re: [PATCH 3/7] x86/cpu: Disable kernel LASS when patching kernel alternatives | From | "Chen, Yian" <> |
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On 1/10/2023 2:41 PM, Sohil Mehta wrote: > On 1/9/2023 9:52 PM, Yian Chen wrote: > >> LASS stops access to a lower half address in kernel, >> and this can be deactivated if AC bit in EFLAGS >> register is set. Hence use stac and clac instructions >> around access to the address to avoid triggering a >> LASS #GP fault. >> > > It seems we are implicitly relying on the on stac() and clac() calls > that are added for SMAP. Have you tried running with SMAP disabled i.e > "clearcpuid=smap"? > Yes, I tested with clearcpuid=smap.
> I believe there needs to be a dependency between LASS and SMAP. > Yes, In kernel mode, LASS depends on SMAP to work. And in user mode, it doesn't, so the dependency description in following may miss user space effect.
> diff --git a/arch/x86/kernel/cpu/cpuid-deps.c > b/arch/x86/kernel/cpu/cpuid-deps.c > index d95221117129..00bc7e4a65d2 100644 > --- a/arch/x86/kernel/cpu/cpuid-deps.c > +++ b/arch/x86/kernel/cpu/cpuid-deps.c > @@ -79,6 +79,7 @@ static const struct cpuid_dep cpuid_deps[] = { > { X86_FEATURE_XFD, X86_FEATURE_XSAVES }, > { X86_FEATURE_XFD, X86_FEATURE_XGETBV1 }, > { X86_FEATURE_AMX_TILE, X86_FEATURE_XFD }, > + { X86_FEATURE_LASS, X86_FEATURE_SMAP }, > {} > };
Thanks, Yian
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