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

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Message-ID: <CALCETrVz_24iYcwzG621Rjp0zzVYoO55Q4Yojtidf6Gr-G==8w@mail.gmail.com>
Date:	Thu, 25 Feb 2016 00:07:18 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Brian Gerst <brgerst@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/urgent] x86/entry/32: Add an ASM_CLAC to entry_SYSENTER_32

[resend -- thank you Gmail for sucking]

On Wed, Feb 24, 2016 at 10:00 PM, H. Peter Anvin <hpa@...or.com> wrote:
> 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?
>

I think that's more trouble than it's worth, and it'll undo a bunch of
the context tracking cleanups that deferring it made possible,
especially since this only matters in a configuration (32-bit SMAP)
that no one uses. [1]

*However*, I just realized that I have no idea why the 32-bit sysenter
path is safe against NT being set.  I fixed it on compat, and now I'm
confused as to the status on 32-bit.  If we need to fix up NT, I think
we can fold AC into that.

Also, I'll try to benchmark this soon.

[1] http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=de9e478b9d49f3a0214310d921450cf5bb4a21e6
(it didn't even boot through most of 4.5-rc)