linux-kernel - Re: [PATCH] x86/sev-es: Do not use copy_from_kernel

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Message-ID: <CAMBK9=b9V6WxYZNrVPNV5vzBZ-mT_noBAxod=ijqrV3bUXAUyA@mail.gmail.com>
Date:   Wed, 6 Sep 2023 16:25:17 -0700
From:   Adam Dunlap <acdunlap@...gle.com>
To:     Dave Hansen <dave.hansen@...el.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>,
        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 Zijlstra (Intel)" <peterz@...radead.org>,
        Joerg Roedel <jroedel@...e.de>, Jacob Xu <jacobhxu@...gle.com>
Subject: Re: [PATCH] x86/sev-es: Do not use copy_from_kernel_nofault in early
 #VC handler

> Usually, we'll add some gunk in arch/x86/boot/compressed/misc.h to
> override the troublesome implementation.  In this case, it would make a
> lot of sense to somehow avoid touching boot_cpu_data.x86_virt_bits in
> the first place.

Thanks for the comment. I realize this patch is doing something a bit misleading
here. In this case, "early" does not refer to the compressed kernel, but
actually the regular kernel but in the stage with this early #VC handler
vc_boot_ghcb (instead of the usual vc_raw_handle_exception). This #VC handler
triggers for the first time on a cpuid instruction in secondary_startup_64, but
boot_cpu_data.x86_virt_bits is not initialized until setup_arch inside of
start_kernel, which is at the end of secondary_startup_64.

The compressed kernel already has its own implementation of vc_decode_insn which
simply calls memcpy instead of copy_from_kernel_nofault, so the point of this
patch is to do the same thing for the early stages of the post-decompression
kernel.

So unless I'm misunderstanding, adding a version of copy_from_kernel_nofault to
the compressed directory wouldn't solve the problem since the problem does not
occur during the compressed stage.