[lkml]   [2020]   [Mar]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] x86/optprobe: Fix OPTPROBE vs UACCESS
On Thu, Mar 05, 2020 at 07:40:06PM +0900, Masami Hiramatsu wrote:
> Hi Peter,
> On Thu, 5 Mar 2020 10:21:30 +0100
> Peter Zijlstra <> wrote:
> >
> > While looking at an objtool UACCESS warning, it suddenly occurred to me
> > that it is entirely possible to have an OPTPROBE right in the middle of
> > an UACCESS region.
> >
> > In this case we must of course clear FLAGS.AC while running the KPROBE.
> > Luckily the trampoline already saves/restores [ER]FLAGS, so all we need
> > to do is inject a CLAC. Unfortunately we cannot use ALTERNATIVE() in the
> > trampoline text, so we have to frob that manually.
> Good catch! so this prevents optprobe handler to access user space
> avoiding SMAP feature.

Yes that, but also worse, since the patch referenced by Fixes, x86_64 no
longer saves/restores FLAGS on context switch, and if the OPTPROBE were
to (accidentally) call into schedule() (say through preempt_enable()),
the next task could also run without SMAP for a while.

> Acked-by: Masami Hiramatsu <>


 \ /
  Last update: 2020-03-05 13:01    [W:0.116 / U:3.652 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site