linux-kernel - Re: [PATCH] x86/boot/sev: Move SEV decompressor variables into the .data section

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Message-Id: <4dfe2339-7899-4469-8957-646d8195b4a4@app.fastmail.com>
Date: Wed, 04 Feb 2026 16:54:23 +0100
From: "Ard Biesheuvel" <ardb@...nel.org>
To: "Tom Lendacky" <thomas.lendacky@....com>, linux-kernel@...r.kernel.org,
 x86@...nel.org
Cc: "Thomas Gleixner" <tglx@...utronix.de>, "Ingo Molnar" <mingo@...hat.com>,
 "Borislav Petkov" <bp@...en8.de>,
 "Dave Hansen" <dave.hansen@...ux.intel.com>, stable@...r.kernel.org,
 "Kevin Hui" <kevinhui@...a.com>
Subject: Re: [PATCH] x86/boot/sev: Move SEV decompressor variables into the .data
 section



On Wed, 4 Feb 2026, at 16:01, Tom Lendacky wrote:
> As part of the work to remove the dependency on calling into the
> decompressor code (startup_64()) for a UEFI boot, a call to rmpadjust()
> was removed from sev_enable() in favor of checking the value of the
> snp_vmpl variable. When booting through a non-UEFI path and calling
> startup_64(), the call to sev_enable() is performed before the BSS section
> is zeroed. With the removal of the rmpadjust() call and the corresponding
> check of the return code, the snp_vmpl variable is checked. Since the
> kernel is running at VMPL0, the snp_vmpl variable will not have been set
> and should be the default value of 0. However, since the call occurs
> before the BSS is zeroed, the snp_vmpl variable may not actually be zero,
> which will cause the guest boot to fail.
>
> Since the decompressor relocates itself, the BSS would need to be cleared
> both before and after the relocation, but this would, in effect, cause all
> of the changes to BSS variables before relocation to be lost after
> relocation.
>
> Instead, move the snp_vmpl variable into the .data section so that it is
> initialized and the value made safe during relocation. As a pre-caution
> against future changes, move other SEV-related decompressor variables into
> the .data section, too.
>
> Fixes: 68a501d7fd82 ("x86/boot: Drop redundant RMPADJUST in SEV SVSM 
> presence check")
> Cc: stable@...r.kernel.org
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Tested-by: Kevin Hui <kevinhui@...a.com>
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>

Reviewed-by: Ard Biesheuvel <ardb@...nel.org>