linux-kernel - Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in get

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Date:   Thu, 13 Jul 2017 14:12:45 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Andrey Rybainin <ryabinin.a.a@...il.com>,
        Chris J Arges <chris.j.arges@...onical.com>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>,
        Michael Davidson <md@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, Bernhard.Rosenkranzer@...aro.org
Subject: Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in
 get_user() inline asm"

El Thu, Jul 13, 2017 at 03:34:16PM -0500 Josh Poimboeuf ha dit:

> On Thu, Jul 13, 2017 at 11:20:04PM +0300, Andrey Rybainin wrote:
> > On 07/13/2017 09:47 PM, Matthias Kaehlcke wrote:
> > 
> > > Thanks for your analysis!
> > > 
> > >> What happens if you try the below patch instead of the revert?  Any
> > >> chance the offending instruction goes away?
> > >>
> > >> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> > >> index 11433f9..beac907 100644
> > >> --- a/arch/x86/include/asm/uaccess.h
> > >> +++ b/arch/x86/include/asm/uaccess.h
> > >> @@ -171,7 +171,7 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
> > >>  	might_fault();							\
> > >>  	asm volatile("call __get_user_%P4"				\
> > >>  		     : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp)	\
> > >> -		     : "0" (ptr), "i" (sizeof(*(ptr))));		\
> > >> +		     : "0" (ptr), "i" (sizeof(*(ptr))), "r" (__sp));	\
> > >>  	(x) = (__force __typeof__(*(ptr))) __val_gu;			\
> > >>  	__builtin_expect(__ret_gu, 0);					\
> > >>  })
> > > 
> > > The generated code is basically the same, only that now the value from
> > > the stack is stored in a register and written twice to RSP:
> > > 
> > 
> > AFAIR clang works much better with global named registers.
> > Could you check if the patch bellow helps?
> 
> And yet another one to try (clobbering sp) :-)
> 
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index 11433f9..21f0c39 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -166,12 +166,12 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
>  ({									\
>  	int __ret_gu;							\
>  	register __inttype(*(ptr)) __val_gu asm("%"_ASM_DX);		\
> -	register void *__sp asm(_ASM_SP);				\
>  	__chk_user_ptr(ptr);						\
>  	might_fault();							\
> -	asm volatile("call __get_user_%P4"				\
> -		     : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp)	\
> -		     : "0" (ptr), "i" (sizeof(*(ptr))));		\
> +	asm volatile("call __get_user_%P3"				\
> +		     : "=a" (__ret_gu), "=r" (__val_gu)			\
> +		     : "0" (ptr), "i" (sizeof(*(ptr)))			\
> +		     : "sp");						\
>  	(x) = (__force __typeof__(*(ptr))) __val_gu;			\
>  	__builtin_expect(__ret_gu, 0);					\
>  })

This compiles with both gcc and clang, clang does not corrupt the
stack pointer. I wouldn't be able to tell though if it forces a stack
frame if it doesn't already exist, as the original patch intends.