linux-kernel - Re: [PATCH 06/24] x86,kvm: Fix indirect calls in emulator

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Date:   Tue, 23 Jan 2018 21:28:59 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Jason Baron <jbaron@...mai.com>
Subject: Re: [PATCH 06/24] x86,kvm: Fix indirect calls in emulator

On Tue, Jan 23, 2018 at 04:25:45PM +0100, Peter Zijlstra wrote:
> Replace the indirect calls with CALL_NOSPEC.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/x86/kvm/emulate.c |    9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -25,6 +25,7 @@
>  #include <asm/kvm_emulate.h>
>  #include <linux/stringify.h>
>  #include <asm/debugreg.h>
> +#include <asm/nospec-branch.h>
>  
>  #include "x86.h"
>  #include "tss.h"
> @@ -1021,8 +1022,8 @@ static __always_inline u8 test_cc(unsign
>  	void (*fop)(void) = (void *)em_setcc + 4 * (condition & 0xf);
>  
>  	flags = (flags & EFLAGS_MASK) | X86_EFLAGS_IF;
> -	asm("push %[flags]; popf; call *%[fastop]"
> -	    : "=a"(rc) : [fastop]"r"(fop), [flags]"r"(flags));
> +	asm("push %[flags]; popf; " CALL_NOSPEC
> +	    : "=a"(rc) : [thunk_target]"r"(fop), [flags]"r"(flags));

Oh, "thunk_target" is magical.

-- 
Regards/Gruss,
    Boris.

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