linux-kernel - Re: [PATCH 2/2] x86/retpoline: Fix return buffer filling

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Date:   Mon, 12 Feb 2018 19:06:02 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org,
        bp@...en8.de, dwmw2@...radead.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 2/2] x86/retpoline: Fix return buffer filling

On Mon, Feb 12, 2018 at 04:04:22PM -0800, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> An earlier patch moved the RSB filling out of line, ending
> it with a return. This results in the return buffer filling
> only giving 15 instead of 16 usable returns because
> the return from fill_rsb already uses one up.
> 
> Since the kernel call chains can be quite deep that's
> somewhat dangerous and better avoided.
> 
> Always do one extra return buffer fill in the caller.
> I added a STUFF_ONE_RSB macro for this, and fixed
> up all the caller
> 
> An alternative would be to revert 1dde7415e999
> and just go back to the inline version.
> 
> Needs the earlier patch to avoid lots of bogus warnings
> from objtool, but even with that there is one left.
> 
> Fixes: 1dde7415e999 ("x86/retpoline: Simplify vmexit...")
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>

Instead of patch 1/2, the alternatives need to be annotated like:

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index af0ca73abb23..4deeb869ab83 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -89,6 +89,7 @@
 .macro FILL_RETURN_BUFFER nr:req ftr:req
 #ifdef CONFIG_RETPOLINE
 	ALTERNATIVE "", "call __clear_rsb" , \ftr
+	ANNOTATE_NOSPEC_ALTERNATIVE
 	ALTERNATIVE "", "STUFF_ONE_RSB", \ftr
 #endif
 .endm
@@ -178,7 +179,7 @@ extern char __indirect_thunk_end[];
 static inline void vmexit_fill_RSB(void)
 {
 #ifdef CONFIG_RETPOLINE
-	alternative_input("",
+	alternative_input(ANNOTATE_NOSPEC_ALTERNATIVE,
 			  "call __fill_rsb;" STUFF_ONE_RSB,
 			  X86_FEATURE_RETPOLINE,
 			  ASM_NO_INPUT_CLOBBER(_ASM_BX, "memory"));