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

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Message-ID: <20241113202105.py5imjdy7pctccqi@jpoimboe>
Date: Wed, 13 Nov 2024 12:21:05 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>, Amit Shah <amit@...nel.org>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, x86@...nel.org,
	linux-doc@...r.kernel.org, amit.shah@....com,
	thomas.lendacky@....com, bp@...en8.de, tglx@...utronix.de,
	peterz@...radead.org, corbet@....net, mingo@...hat.com,
	dave.hansen@...ux.intel.com, hpa@...or.com, seanjc@...gle.com,
	pbonzini@...hat.com, daniel.sneddon@...ux.intel.com,
	kai.huang@...el.com, sandipan.das@....com,
	boris.ostrovsky@...cle.com, Babu.Moger@....com,
	david.kaplan@....com, dwmw@...zon.co.uk
Subject: Re: [RFC PATCH v2 1/3] x86: cpu/bugs: update SpectreRSB comments for
 AMD

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