linux-kernel - Re: [PATCH v2 2/2] x86/bugs: Don't fill RSB on context switch with eIBRS

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Message-ID: <20241205233245.4xaicvusl5tfp2oi@jpoimboe>
Date: Thu, 5 Dec 2024 15:32:45 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org, amit@...nel.org, kvm@...r.kernel.org,
	amit.shah@....com, thomas.lendacky@....com, bp@...en8.de,
	tglx@...utronix.de, peterz@...radead.org,
	pawan.kumar.gupta@...ux.intel.com, 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, andrew.cooper3@...rix.com
Subject: Re: [PATCH v2 2/2] x86/bugs: Don't fill RSB on context switch with
 eIBRS

On Thu, Nov 21, 2024 at 12:07:19PM -0800, Josh Poimboeuf wrote:
> User->user Spectre v2 attacks (including RSB) across context switches
> are already mitigated by IBPB in cond_mitigation(), if enabled globally
> or if either the prev or the next task has opted in to protection.  RSB
> filling without IBPB serves no purpose for protecting user space, as
> indirect branches are still vulnerable.

Question for Intel/AMD folks: where is it documented that IBPB clears
the RSB?  I thought I'd seen this somewhere but I can't seem to find it.

-- 
Josh