Re: Save a WRMSR GS.base?

[Borislav Petkov]

On Thu, Jun 04, 2026 at 10:17:57AM +0100, Andrew Cooper wrote:
> Yes, but it took me writing a "no" email to spot it.

Oh, I know those situations.

> If the LKGS (in load seg) was called unconditionally, then yes it would
> be safe to drop the __wrgsbase_inactive(), but it's not.
> 
> Consider a prev and next which both have the same ->gsindex (so skips
> loadseg()), but have different ->gsbase (still need to update KERN_GS_BASE).

Gah, ofc.

So we'll have to do something like this which is ugly as hell:

---

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.

-- 
Regards/Gruss,
    Boris.

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