Date:   Tue, 21 Feb 2023 11:59:14 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Andrew Cooper <andrew.cooper3@citrix.com>
Cc:     KP Singh <kpsingh@kernel.org>, Dave Hansen <dave.hansen@intel.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        linux-kernel@vger.kernel.org, pjt@google.com, evn@google.com,
        tglx@linutronix.de, mingo@redhat.com, dave.hansen@linux.intel.com,
        x86@kernel.org, hpa@zytor.com, peterz@infradead.org,
        pawan.kumar.gupta@linux.intel.com, kim.phillips@amd.com,
        alexandre.chartre@oracle.com, daniel.sneddon@linux.intel.com,
        =?utf-8?B?Sm9zw6k=?= Oliveira <joseloliveira11@gmail.com>,
        Rodrigo Branco <rodrigo@kernelhacking.com>,
        Alexandra Sandulescu <aesa@google.com>,
        Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH RESEND] x86/speculation: Fix user-mode spectre-v2
 protection with KERNEL_IBRS
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On Mon, Feb 20, 2023 at 11:30:46PM +0000, Andrew Cooper wrote:
> 1) Intel's legacy IBRS
> 2) AMD's regular IBRS

Yeah, we don't do that in the kernel.

> 3) AMD's AutoIBRS
> 
> which all have different relevant behaviours for userspace.  Just so
> it's written out coherently in at least one place...
> 
> When SEV-SNP is enabled in firmware, whether or not it's being used by
> software, AutoIBRS keeps indirect predictions inhibited in all of
> ASID0.  That's all host userspace to the non-hypervisor devs reading
> this thread.

Yap.

> For any AMD configuration setting STIBP, there must be an IBPB after
> having set STIBP.   Setting STIBP alone does not evict previously
> created shared predictions.  This one can go subtly wrong for anyone who
> assumes that Intel STIBP and AMD STIBP have the same behaviour.

We will IBPB eventually... on the next context switch.

> Furthermore, extra care needs taking on vmexit because transitioning
> from the guest STIBP setting to the host STIBP setting can leave shared
> predictions intact.

From what I can tell from looking at the SVM code, we don't do anything
special there when restoring SPEC_CTRL.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette