Re: [PATCH 15/29] x86: Disable IBT around firmware
From: Peter Zijlstra
Date: Mon Feb 21 2022 - 05:46:10 EST
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;