lkml.org 
[lkml]   [2018]   [Jan]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] x86/retpoline: Fill RSB on context switch for affected CPUs
From
Date
On 12/01/18 17:49, David Woodhouse wrote:
> When we context switch from a shallow call stack to a deeper one, as we
> 'ret' up the deeper side we may encounter RSB entries (predictions for
> where the 'ret' goes to) which were populated in userspace. This is
> problematic if we have neither SMEP nor KPTI (the latter of which marks
> userspace pages as NX for the kernel), as malicious code in userspace
> may then be executed speculatively. So overwrite the CPU's return
> prediction stack with calls which are predicted to return to an infinite
> loop, to "capture" speculation if this happens. This is required both
> for retpoline, and also in conjunction with IBRS for !SMEP && !KPTI.
>
> On Skylake+ the problem is slightly different, and an *underflow* of the
> RSB may cause errant branch predictions to occur. So there it's not so
> much overwrite, as *filling* the RSB to attempt to prevent it getting
> empty. This is only a partial solution for Skylake+ since there are many
> other conditions which may result in the RSB becoming empty. The full
> solution on Skylake+ is to use IBRS, which will prevent the problem even
> when the RSB becomes empty. With IBRS, the RSB-stuffing will not be
> required on context switch.

If you unconditionally fill the RSB on every entry to supervisor mode,
then there are never guest-controlled RSB values to be found.

With that property (and IBRS to protect Skylake+), you shouldn't need
RSB filling anywhere in the middle.

~Andrew

\
 
 \ /
  Last update: 2018-01-14 23:26    [W:0.302 / U:0.192 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site