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Subject: Re: [PATCH] x86/bugs: wrap X86_FEATURE_RSB_CTXSW with ifdef
 CONFIG_RETPOLINE
To:     Jon Kohler <jon@nutanix.com>
Cc:     David Woodhouse <dwmw@amazon.co.uk>, Jiri Kosina <jkosina@suse.cz>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Ravi Shankar <ravi.v.shankar@intel.com>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Tony Luck <tony.luck@intel.com>,
        Anthony Steinhauser <asteinhauser@google.com>,
        Mike Rapoport <rppt@kernel.org>,
        Anand K Mistry <amistry@google.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        linux-kernel@vger.kernel.org
References: <20210507155308.97649-1-jon@nutanix.com>
From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <cd3517fb-06c2-9188-1a03-3848db6aba3e@intel.com>
Date:   Fri, 7 May 2021 09:02:11 -0700
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In-Reply-To: <20210507155308.97649-1-jon@nutanix.com>
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Precedence: bulk

On 5/7/21 8:53 AM, Jon Kohler wrote:
> The only place X86_FEATURE_RSB_CTXSW is currently in use is in
> arch/x86/entry/entry_{32|64}.S, where its use is wrapped with
> ifdef CONFIG_RETPOLINE. If someone uses a system with
> X86_FEATURE_IBRS_ENHANCED and compiles without CONFIG_RETPOLINE
> but still has spectre v2 set to auto, the kernel log will
> print that eIBRS is enabled and that RSB stuffing is enabled;
> however, that stuffing would never occur.
> 
> To make this behavior more clear, wrap the enablement of
> X86_FEATURE_RSB_CTXSW and the resulting log message with ifdef
> CONFIG_RETPOLINE, such that it is compiled out along with the
> actions it controls.
> 
> This way seems more correct at first glance as this was the way
> the code was originally written in fdf82a7856b; however, when
> enhanced IBRS was added, there was a goto added under
> SPECTRE_V2_CMD_AUTO which bypasses going through retpoline_auto,
> where X86_FEATURE_RETPOLINE is set.
> 
> The other option would be to remove the CONFIG_RETPOLINE from
> the code in entry_{32|64}.S, such that it would always be
> compiled no matter what, such that these two areas match.

This kinda dances around the real issue: Does RSB stuffing have
mitigation value on enhanced IBRS systems?

If yes, then we should make the RSB stuffing code in entry*.S available
separately from CONFIG_RETPOLINE.

If no, is it because eIBRS systems are not vulnerable, or because RSB
stuffing has no mitigation value?

Either way, I'm not sure the approach in this patch is the one we want.