linux-kernel - Re: [PATCH v2 0/2] x86/sev-es: Resolve early #VC handler UB

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Date:   Thu, 28 Sep 2023 22:51:57 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Adam Dunlap <acdunlap@...gle.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Kim Phillips <kim.phillips@....com>,
        Juergen Gross <jgross@...e.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Nikunj A Dadhania <nikunj@....com>,
        Dionna Glaze <dionnaglaze@...gle.com>,
        Peter Gonda <pgonda@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Khalid ElMously <khalid.elmously@...onical.com>,
        Jacob Xu <jacobhxu@...gle.com>
Subject: Re: [PATCH v2 0/2] x86/sev-es: Resolve early #VC handler UB


* Adam Dunlap <acdunlap@...gle.com> wrote:

> In v0 of this patch, I mistakenly claimed that
> boot_cpu_data.x86_virt_bits was not getting set to 0 in some
> configurations. However, it is in fact reliably being set to 0 because
> it is specified to be in the .data..read_mostly section, and .data is
> initialized at this point. The boot failures that we were seeing were
> actually due to the gcc 12.3 compiler exploiting the undefined
> behavior to return false from __is_canonical_address when
> boot_cpu_data.x86_virt_bits is 0.
> 
> This patch set applies the 2 changes posted by Dave Hansen: remove the
> UB by explicily checking for 0, and remove the period of time where
> x86_virt_bits may be temporarily set to the wrong value where the #VC
> handler could observe it.
> 
> 
> Adam Dunlap (2):
>   x86/sev-es: Allow copy_from_kernel_nofault in earlier boot
>   x86/sev-es: Only set x86_virt_bits to correct value

I've applied these fixes to tip:x86/mm, thanks!

Note that I switched around the order of patches: we first want
to enhance x86_virt_bits to be reliable, only then do we want to
change copy_from_kernel_nofault_allowed() to be able to rely on it,
to fix early #VC accesses, right?

I also improved the changelogs & titles a bit.

Thanks,

	Ingo