linux-kernel - Re: [PATCH 1/8] kasan,x86: Frob kasan

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Message-ID: <20190306173715.GA32534@hirez.programming.kicks-ass.net>
Date:   Wed, 6 Mar 2019 18:37:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>, valentin.schneider@....com,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>, jgross@...e.com
Subject: Re: [PATCH 1/8] kasan,x86: Frob kasan_report() in an exception

On Wed, Mar 06, 2019 at 06:14:51PM +0100, Peter Zijlstra wrote:
> On Wed, Mar 06, 2019 at 02:13:47PM +0100, Peter Zijlstra wrote:
> > +static __always_inline unsigned long smap_save(void)
> > +{
> > +	unsigned long flags = arch_local_save_flags();
> > +	clac();
> > +	return flags;
> > +}
> > +
> > +static __always_inline void smap_restore(unsigned long flags)
> > +{
> > +	arch_local_irq_restore(flags);
> > +}
> 
> ARGH; the bloody paravirt me harder nonsense makes that pvops calls.
> 
> And that (obviously) explodes.. Anybody got any clue why that Xen
> trainwreck wants to paravirt: "PUSHF;POP" and "PUSH;POPF" !?
> 
> Also; I suppose I can ALTERNATIVE the whole thing, because Xen will not
> be having SMAP in the first place I suppose.

The below seems to 'work'.

--- a/arch/x86/include/asm/smap.h
+++ b/arch/x86/include/asm/smap.h
@@ -46,8 +46,6 @@
 
 #ifdef CONFIG_X86_SMAP
 
-#include <asm/irqflags.h>
-
 static __always_inline void clac(void)
 {
 	/* Note: a barrier is implicit in alternative() */
@@ -62,14 +60,19 @@ static __always_inline void stac(void)
 
 static __always_inline unsigned long smap_save(void)
 {
-	unsigned long flags = arch_local_save_flags();
-	clac();
+	unsigned long flags;
+
+	asm volatile (ALTERNATIVE("", "pushf; pop %0; " __stringify(__ASM_CLAC),
+				  X86_FEATURE_SMAP)
+		      : "=rm" (flags) : : "memory", "cc");
+
 	return flags;
 }
 
 static __always_inline void smap_restore(unsigned long flags)
 {
-	arch_local_irq_restore(flags);
+	asm volatile (ALTERNATIVE("", "push %0; popf", X86_FEATURE_SMAP)
+		      : : "g" (flags) : "memory", "cc");
 }
 
 /* These macros can be used in asm() statements */