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

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Message-ID: <20241112214241.fzqq6sqszqd454ei@desk>
Date: Tue, 12 Nov 2024 13:43:48 -0800
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
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 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?