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

[Brian Gerst]

On Thu, Feb 25, 2016 at 8:47 AM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
> On Thu, Feb 25, 2016 at 3:03 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>
>> On Feb 24, 2016 10:01 PM, "H. Peter Anvin" <hpa@xxxxxxxxx> 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@xxxxxxxxxx>
>>> > AuthorDate: Tue, 23 Feb 2016 13:19:29 -0800
>>> > Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
>>> > 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).

Looking deeper, the sysexit path doesn't restore eflags, so we'd need
to manually restore AC and IOPL.

--
Brian Gerst