Re: [PATCH v2.1] x86/retpoline: Fill return stack buffer on vmexit

[David Woodhouse]

On Thu, 2018-01-11 at 09:04 -0600, Josh Poimboeuf wrote:
> 
> > How about this one then (with ANNOTATE_NOSPEC_ALTERNATIVE):
> >Â
> > -ÂÂÂÂÂÂÂasm volatile (ALTERNATIVE("",
> > +ÂÂÂÂÂÂÂasm volatile (ALTERNATIVE("jmp " alt_end_marker "f",
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ__stringify(__FILL_RETURN_BUFFER(%0, %1, _%=)),
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂX86_FEATURE_RETPOLINE)
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ: "=r" (dummy), ASM_CALL_CONSTRAINT : : "memory" );
> 
> Looks good to me.
> 
> Another IRC discussion was that Boris may eventually add a feature to
> the alternatives code to automatically insert such a jump if there are a
> lot of nops.

OK, v3 sent out with that. I've just manually put in a jump round it
(less hackishly than the alt_end_marker one) in the oldinstr for now.

This wants rolling into your objtool fixes:

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -119,7 +119,8 @@
ÂÂÂ*/
Â.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
Â#ifdef CONFIG_RETPOLINE
-	ALTERNATIVE "jmp .Lskip_rsb_\@",				\
+	ANNOTATE_NOSPEC_ALTERNATIVE
+	ALTERNATIVE "jmp .Lskip_rsb_\@",				\
Â		__stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP,\@))	\
Â		\ftr
Â.Lskip_rsb_\@:
@@ -149,6 +150,7 @@
Â# define THUNK_TARGET(addr) [thunk_target] "r" (addr)
Â
Â#elif defined(CONFIG_X86_32) && defined(CONFIG_RETPOLINE)
+# define ANNOTATE_NOSPEC_ALTERNATIVE
Â/*
 * For i386 we use the original ret-equivalent retpoline, because
 * otherwise we'll run out of registers. We don't care about CET
@@ -201,7 +203,8 @@ static inline void vmexit_fill_RSB(void)
Â#ifdef CONFIG_RETPOLINE
Â	unsigned long loops = RSB_CLEAR_LOOPS / 2;
Â
-	asm volatile (ALTERNATIVE("jmp .Lskip_rsb_%=",
+	asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
+		ÂÂÂÂÂÂALTERNATIVE("jmp .Lskip_rsb_%=",
Â				ÂÂ__stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1, _%=)),
Â				ÂÂX86_FEATURE_RETPOLINE)
Â		ÂÂÂÂÂÂ".Lskip_rsb_%=:"

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