Re: [RFC PATCH v2 1/3] x86: cpu/bugs: update SpectreRSB comments for AMD

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Tue, Nov 12, 2024 at 01:43:48PM -0800, Pawan Gupta wrote:
> On Mon, Nov 11, 2024 at 05:46:44PM -0800, Josh Poimboeuf wrote:
> > +	 * 1) RSB underflow ("Intel Retbleed")
> >  	 *
> >  	 *    Some Intel parts have "bottomless RSB".  When the RSB is empty,
> >  	 *    speculated return targets may come from the branch predictor,
> >  	 *    which could have a user-poisoned BTB or BHB entry.
> >  	 *
> > -	 *    AMD has it even worse: *all* returns are speculated from the BTB,
> > -	 *    regardless of the state of the RSB.
> > +	 *    When IBRS or eIBRS is enabled, the "user -> kernel" attack is
> > +	 *    mitigated by the IBRS branch prediction isolation properties, so
> > +	 *    the RSB buffer filling wouldn't be necessary to protect against
> > +	 *    this type of attack.
> >  	 *
> > -	 *    When IBRS or eIBRS is enabled, the "user -> kernel" attack
> > -	 *    scenario is mitigated by the IBRS branch prediction isolation
> > -	 *    properties, so the RSB buffer filling wouldn't be necessary to
> > -	 *    protect against this type of attack.
> > +	 *    The "user -> user" attack is mitigated by RSB filling on context
> > +	 *    switch.
> 
> user->user SpectreRSB is also mitigated by IBPB, so RSB filling is
> unnecessary when IBPB is issued. Also, when an appication does not opted-in
> for IBPB at context switch, spectre-v2 for that app is not mitigated,
> filling RSB is only a half measure in that case.
> 
> Is RSB filling really serving any purpose for userspace?

Indeed...

If we don't need to flush RSB for user->user, we'd only need to worry
about protecting the kernel.  Something like so?

  - eIBRS+!PBRSB:	no flush
  - eIBRS+PBRSB:	lite flush
  - everything else:	full flush

i.e., same logic as spectre_v2_determine_rsb_fill_type_at_vmexit(), but
also for context switches.

-- 
Josh