[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. 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)
