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Message-ID: <20230814101134.GBZNn91uTmNImxRDXr@fat_crate.local>
Date: Mon, 14 Aug 2023 12:11:34 +0200
From: Borislav Petkov <bp@...en8.de>
To: Rainer Fiebig <jrf@...lbox.org>
Cc: Xi Ruoyao <xry111@...111.site>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: Re: Does srso safe RET mitigation require microcode update?
On Mon, Aug 14, 2023 at 11:47:54AM +0200, Rainer Fiebig wrote:
> "* 'Mitigation: safe RET':
>
> Software-only mitigation. It complements the extended IBPB microcode
> patch functionality by addressing User->Kernel and Guest->Host
> transitions protection."
>
> Now, what does that mean: partial mitigation or also no mitigation
> without microcode?
It *complements*! Meaning, for a full safe RET mitigation, you need the
microcode on Zen3/4.
Would that explain it better?
"First of all, it is required that the latest microcode be loaded for
mitigations to be effective. That means that for the User->User
aspect of the mitigation to be effective, one needs the updated
microcode for Zen3 and Zen4 generations. In addition, one needs to
*enable* the User->User mitigation which is part of the spectre_v2_user
controls:
- conditional IBPB:
where each process can select whether it needs an IBPB issued around
it PR_SPEC_DISABLE/_ENABLE etc, see Spectre Side Channels. That is the
default setting
- strict:
i.e., always on - by supplying spectre_v2_user=on on the kernel
command line."
IOW, without microcode, you do not mitigate the User->User aspect of the
mitigation but you mitigate the other vectors with safe RET.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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