linux-kernel - Re: [tip:x86/urgent] x86/entry/32: Add an ASM_CLAC to entry_SYSENTER

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Date:	Wed, 24 Feb 2016 22:00:55 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	peterz@...radead.org, brgerst@...il.com, mingo@...nel.org,
	tglx@...utronix.de, torvalds@...ux-foundation.org, luto@...nel.org,
	linux-kernel@...r.kernel.org, luto@...capital.net, bp@...en8.de,
	dvlasenk@...hat.com, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86/entry/32: Add an ASM_CLAC to
 entry_SYSENTER_32

On 02/24/16 21:53, tip-bot for Andy Lutomirski wrote:
> Commit-ID:  04d1d281dcfe683a53cddfab8371fc8bb302b069
> Gitweb:     http://git.kernel.org/tip/04d1d281dcfe683a53cddfab8371fc8bb302b069
> Author:     Andy Lutomirski <luto@...nel.org>
> AuthorDate: Tue, 23 Feb 2016 13:19:29 -0800
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Wed, 24 Feb 2016 08:43:04 +0100
> 
> x86/entry/32: Add an ASM_CLAC to entry_SYSENTER_32
> 
> Both before and after 5f310f739b4c ("x86/entry/32: Re-implement
> SYSENTER using the new C path"), we relied on a uaccess very early
> in the SYSENTER path to clear AC.  After that change, though, we can
> potentially make it all the way into C code with AC set, which
> enlarges the attack surface for SMAP bypass by doing SYSENTER with
> AC set.
> 
> Strengthen the SMAP protection by addding the missing ASM_CLAC right
> at the beginning.
> 

Hmmm... this potentially adds a *lot* of unnecessary cycles to this
path.  Could we reinstate the early uaccess?

	-hpa