On Thu, Feb 25, 2016 at 12:07 AM, Andy Lutomirski <l...@amacapital.net> wrote: > [resend -- thank you Gmail for sucking] > > On Wed, Feb 24, 2016 at 10:00 PM, H. Peter Anvin <h...@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 <l...@kernel.org> >>> AuthorDate: Tue, 23 Feb 2016 13:19:29 -0800 >>> Committer: Ingo Molnar <mi...@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. >
It is, indeed, broken. My test case doesn't notice because opportunistic sysexit papers over the issue. Grump. > 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) -- Andy Lutomirski AMA Capital Management, LLC
