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

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Date:   Thu, 11 Jan 2018 16:10:35 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <ak@...ux.intel.com>, Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, tglx@...utronix.de,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk,
        x86@...nel.org, bp@...en8.de, rga@...zon.de,
        thomas.lendacky@....com
Subject: Re: [PATCH v2.1] x86/retpoline: Fill return stack buffer on vmexit

On Thu, 2018-01-11 at 09:04 -0600, Josh Poimboeuf wrote:
> 
> > How about this one then (with ANNOTATE_NOSPEC_ALTERNATIVE):
> > 
> > -       asm volatile (ALTERNATIVE("",
> > +       asm volatile (ALTERNATIVE("jmp " alt_end_marker "f",
> >                                   __stringify(__FILL_RETURN_BUFFER(%0, %1, _%=)),
> >                                   X86_FEATURE_RETPOLINE)
> >                       : "=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 @@
   */
 .macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
 #ifdef CONFIG_RETPOLINE
-	ALTERNATIVE "jmp .Lskip_rsb_\@",				\
+	ANNOTATE_NOSPEC_ALTERNATIVE
+	ALTERNATIVE "jmp .Lskip_rsb_\@",				\
 		__stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP,\@))	\
 		\ftr
 .Lskip_rsb_\@:
@@ -149,6 +150,7 @@
 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
 
 #elif defined(CONFIG_X86_32) && defined(CONFIG_RETPOLINE)
+# define ANNOTATE_NOSPEC_ALTERNATIVE
 /*
  * 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)
 #ifdef CONFIG_RETPOLINE
 	unsigned long loops = RSB_CLEAR_LOOPS / 2;
 
-	asm volatile (ALTERNATIVE("jmp .Lskip_rsb_%=",
+	asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
+		      ALTERNATIVE("jmp .Lskip_rsb_%=",
 				  __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1, _%=)),
 				  X86_FEATURE_RETPOLINE)
 		      ".Lskip_rsb_%=:"
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