Re: [PATCH v4.9] x86/uaccess: Don't leak the AC flag into __put_user() value evaluation

[Nathan Chancellor <natechancellor@gmail.com>]

On Sun, Mar 03, 2019 at 07:27:24PM +0300, Maxim Zhukov wrote:
> From: Andy Lutomirski <luto@kernel.org>
> 
> When calling __put_user(foo(), ptr), the __put_user() macro would call
> foo() in between __uaccess_begin() and __uaccess_end().  If that code
> were buggy, then those bugs would be run without SMAP protection.
> 
> Fortunately, there seem to be few instances of the problem in the
> kernel. Nevertheless, __put_user() should be fixed to avoid doing this.
> Therefore, evaluate __put_user()'s argument before setting AC.
> 
> This issue was noticed when an objtool hack by Peter Zijlstra complained
> about genregs_get() and I compared the assembly output to the C source.
> 
>  [ bp: Massage commit message and fixed up whitespace. ]
> 
> Fixes: 11f1a4b9755f ("x86: reorganize SMAP handling in user space accesses")
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Borislav Petkov <bp@suse.de>
> Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Denys Vlasenko <dvlasenk@redhat.com>
> Cc: stable@vger.kernel.org
> Link: http://lkml.kernel.org/r/20190225125231.845656645@infradead.org
> ---
>  arch/x86/include/asm/uaccess.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index a8d85a687cf4..39bbc225558c 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -292,7 +292,7 @@ do {									\
>  		__put_user_asm(x, ptr, retval, "l", "k", "ir", errret);	\
>  		break;							\
>  	case 8:								\
> -		__put_user_asm_u64((__typeof__(*ptr))(x), ptr, retval,	\
> +		__put_user_asm_u64(x, ptr, retval,	\
>  				   errret);				\
>  		break;							\
>  	default:							\
> @@ -427,8 +427,10 @@ do {									\
>  #define __put_user_nocheck(x, ptr, size)			\
>  ({								\
>  	int __pu_err;						\
> +	__typeof__(*(ptr)) __pu_val;				\
>  	__uaccess_begin();					\
> -	__put_user_size((x), (ptr), (size), __pu_err, -EFAULT);	\
> +	__pu_val = x;						\

This is incorrect, __pu_val's assignment should be before
__uaccess_begin like in the original patch (which is the point of the
patch in the first place).

Cheers,
Nathan

> +	__put_user_size(__pu_val, (ptr), (size), __pu_err, -EFAULT);	\
>  	__uaccess_end();					\
>  	__builtin_expect(__pu_err, 0);				\
>  })
> -- 
> 2.21.0
>