Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in get_user() inline asm"

From: Matthias Kaehlcke <mka@chromium.org>
To: Andrey Rybainin <ryabinin.a.a@gmail.com>
Cc: "Josh Poimboeuf" <jpoimboe@redhat.com>,
	"Chris J Arges" <chris.j.arges@canonical.com>,
	"Borislav Petkov" <bp@suse.de>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"H . Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Douglas Anderson" <dianders@chromium.org>,
	"Michael Davidson" <md@google.com>,
	"Greg Hackmann" <ghackmann@google.com>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	"Stephen Hines" <srhines@google.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Bernhard Rosenkränzer" <Bernhard.Rosenkranzer@linaro.org>
Subject: Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in get_user() inline asm"
Date: Thu, 13 Jul 2017 14:14:14 -0700	[thread overview]
Message-ID: <20170713211414.GH95735@google.com> (raw)
In-Reply-To: <75850bb7-a60e-057d-d88b-afa0c79e94a0@gmail.com>

El Thu, Jul 13, 2017 at 11:20:04PM +0300 Andrey Rybainin ha dit:

> 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?
>
>
> ---
>  arch/x86/include/asm/uaccess.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index a059aac9e937..121204387978 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -157,15 +157,18 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
>   * Clang/LLVM cares about the size of the register, but still wants
>   * the base register for something that ends up being a pair.
>   */
> +
> +register unsigned long __current_sp asm(_ASM_SP);
> +
>  #define get_user(x, ptr) \
>  ({ \
>   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) \
> +     : "=a" (__ret_gu), "=r" (__val_gu), \
> +       "+r" (__current_sp) \
>       : "0" (ptr), "i" (sizeof(*(ptr)))); \
>   (x) = (__force __typeof__(*(ptr))) __val_gu; \
>   __builtin_expect(__ret_gu, 0); \

Thanks for the suggestion, however it fails to build with both gcc and clang:

fs/ioctl.c:585:6: error: use of undeclared identifier '__current_sp'
        if (get_user(count, &argp->dest_count)) {
            ^
arch/x86/include/asm/uaccess.h:168:16: note: expanded from macro 'get_user'
                       "+r" (__current_sp)
      \

The references I found refer to __current_sp as an intrinsic function
for ARM32.