On Thu, Feb 25, 2016 at 3:03 AM, Andy Lutomirski <l...@amacapital.net> wrote: > > On Feb 24, 2016 10:01 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.
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
