linux-kernel - Re: [PATCH v2 05/36] x86/entry/64/compat: Fix SYSENTER's NT flag before user memory access

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Message-ID: <20151007111038.GC5046@nazgul.tnic>
Date:	Wed, 7 Oct 2015 13:10:38 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 05/36] x86/entry/64/compat: Fix SYSENTER's NT flag
 before user memory access

On Mon, Oct 05, 2015 at 05:47:53PM -0700, Andy Lutomirski wrote:
> Clearing NT is part of the prologue, whereas loading up arg6 makes
> more sense to think about as part of syscall processing.  Reorder
> them.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/entry/entry_64_compat.S | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
> index a9360d40fb7f..aa76864a8a6b 100644
> --- a/arch/x86/entry/entry_64_compat.S
> +++ b/arch/x86/entry/entry_64_compat.S
> @@ -89,15 +89,6 @@ ENTRY(entry_SYSENTER_compat)
>  	sub	$(10*8), %rsp /* pt_regs->r8-11, bp, bx, r12-15 not saved */
>  
>  	/*
> -	 * no need to do an access_ok check here because rbp has been
> -	 * 32-bit zero extended
> -	 */
> -	ASM_STAC
> -1:	movl	(%rbp), %ebp
> -	_ASM_EXTABLE(1b, ia32_badarg)
> -	ASM_CLAC
> -
> -	/*
>  	 * Sysenter doesn't filter flags, so we need to clear NT
>  	 * ourselves.  To save a few cycles, we can check whether
>  	 * NT was set instead of doing an unconditional popfq.
> @@ -106,6 +97,15 @@ ENTRY(entry_SYSENTER_compat)
>  	jnz	sysenter_fix_flags
>  sysenter_flags_fixed:

Btw, do I see it correctly that we can save us this jumping to
sysenter_fix_flags and back to sysenter_flags_fixed? Label is jumped to
only once and the couple of insns there can be behind a JZ...

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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