Re: Save a WRMSR GS.base?

[Borislav Petkov]

On Thu, Jun 04, 2026 at 07:36:01PM -0700, H. Peter Anvin wrote:
> >diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> >index b85e715ebb30..248c39da9ba0 100644
> >--- a/arch/x86/kernel/process_64.c
> >+++ b/arch/x86/kernel/process_64.c
> >@@ -391,16 +391,23 @@ static __always_inline void x86_pkru_load(struct thread_struct *prev,
> > static __always_inline void x86_fsgsbase_load(struct thread_struct *prev,
> > 					      struct thread_struct *next)
> > {
> >+	bool loaded_gs = false;
> >+
> > 	if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
> > 		/* Update the FS and GS selectors if they could have changed. */
> > 		if (unlikely(prev->fsindex || next->fsindex))
> > 			loadseg(FS, next->fsindex);
> >-		if (unlikely(prev->gsindex || next->gsindex))
> >+
> >+		if (unlikely(prev->gsindex || next->gsindex)) {
> > 			loadseg(GS, next->gsindex);
> >+			loaded_gs = true;
> >+		}
> > 
> > 		/* Update the bases. */
> > 		wrfsbase(next->fsbase);
> >-		__wrgsbase_inactive(next->gsbase);
> >+
> >+		if (!(cpu_feature_enabled(X86_FEATURE_LKGS) && loaded_gs))
> >+			__wrgsbase_inactive(next->gsbase);
> > 	} else {
> > 		load_seg_legacy(prev->fsindex, prev->fsbase,
> > 				next->fsindex, next->fsbase, FS);
> >
> >
> >Thx.
> >
> 
> Also consider that user space might have done: 
> 
>     mov gs,...
>     wrgsbase ...
> 
> So gs.selector > 3 doesn't necessarily mean that the base is consistent with the descriptor.

Right, I want to avoid the second write to KERNEL_GS_BASE iff we have done
LKGS before.

-- 
Regards/Gruss,
    Boris.

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