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

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Message-ID: <79c179acaa6ec4e1cf112ae2dfce8370694a5089.camel@xry111.site>
Date:   Mon, 14 Aug 2023 17:00:12 +0800
From:   Xi Ruoyao <xry111@...111.site>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org,
        "Borislav Petkov (AMD)" <bp@...en8.de>,
        Rainer Fiebig <jrf@...lbox.org>
Subject: Does srso safe RET mitigation require microcode update?

Hi,

There seems a difference between Documentation/admin-guide/hw-
vuln/srso.rst and the actual behavior.  The documentation says:

   First of all, it is required that the latest microcode be loaded for
   mitigations to be effective.

And:

    * 'Vulnerable: no microcode':

      The processor is vulnerable, no microcode extending IBPB
      functionality to address the vulnerability has been applied.

Per the text, if there is no firmware update, the system is just
vulnerable.  But on a real Zen 3 system, the spec_rstack_overflow file
contains "Mitigation: safe RET, no microcode".

So we are puzzled now: is this system vulnerable or mitigated?

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University