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SubjectRe: [PATCH 15/29] x86: Disable IBT around firmware

Could you trim replies so that I can actually find what you write?

On Mon, Feb 21, 2022 at 12:27:20AM -0800, Kees Cook wrote:

> >+#ifdef CONFIG_X86_IBT
> >+
> >+u64 ibt_save(void)
> >+{
> >+ u64 msr = 0;
> >+
> >+ if (cpu_feature_enabled(X86_FEATURE_IBT)) {
> >+ rdmsrl(MSR_IA32_S_CET, msr);
> >+ wrmsrl(MSR_IA32_S_CET, msr & ~CET_ENDBR_EN);
> >+ }
> >+
> >+ return msr;
> >+}
> >+
> >+void ibt_restore(u64 save)
>
> Please make these both __always_inline so there no risk of them ever gaining ENDBRs and being used by ROP to disable IBT...

Either that or mark them __noendbr. The below seems to work.

Do we have a preference?


--- a/arch/x86/include/asm/ibt.h
+++ b/arch/x86/include/asm/ibt.h
@@ -48,8 +48,8 @@ static inline bool is_endbr(const void *
return val == gen_endbr64();
}

-extern u64 ibt_save(void);
-extern void ibt_restore(u64 save);
+extern __noendbr u64 ibt_save(void);
+extern __noendbr void ibt_restore(u64 save);

#else /* __ASSEMBLY__ */

--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -596,7 +596,7 @@ __setup("nopku", setup_disable_pku);

#ifdef CONFIG_X86_IBT

-u64 ibt_save(void)
+__noendbr u64 ibt_save(void)
{
u64 msr = 0;

@@ -608,7 +608,7 @@ u64 ibt_save(void)
return msr;
}

-void ibt_restore(u64 save)
+__noendbr void ibt_restore(u64 save)
{
u64 msr;

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