Messages in this thread |  | | Date | Wed, 11 Jan 2023 16:37:17 -0800 | | Subject | Re: [PATCH 3/7] x86/cpu: Disable kernel LASS when patching kernel alternatives | | From | Dave Hansen <> |
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On 1/11/23 16:27, Chen, Yian wrote: >> 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. It works by accident, then.
clearcpuid=smap means that the kernel should be running as if CPUID.(EAX=07H,ECX=0H):EBX.SMAP[bit 20]==0. STAC/CLAC should #UD in that case.
The only reason that it happens to work is that STAC/CLAC apparently actually continue to work even if CR4.SMAP==0.
I'm actually a _bit_ surprised by this, but I bet there's a good reason for it.
In any case, please just make LASS dependent on SMAP. It's the right thing to do on several levels.
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