[resend -- thank you Gmail for sucking]
On Wed, Feb 24, 2016 at 10:00 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.
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)
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