linux-kernel - Re: Does srso safe RET mitigation require microcode update?

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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