Re: [tip:x86/urgent] x86/entry/32: Add an ASM_CLAC to entry_SYSENTER_32

[Brian Gerst <brgerst@gmail.com>]

On Thu, Feb 25, 2016 at 3:03 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>
> On Feb 24, 2016 10:01 PM, "H. Peter Anvin" <hpa@zytor.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@kernel.org>
>> > AuthorDate: Tue, 23 Feb 2016 13:19:29 -0800
>> > Committer:  Ingo Molnar <mingo@kernel.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.

32-bit still saves eflags in switch_to(), so NT can't leak to other
tasks.  But for consistency it should get the same treatment as 64-bit
(clear NT in sysenter entry and drop saving eflags in switch_to).

--
Brian Gerst