linux-kernel - Re: [PATCH v3] x86: fpu: Keep xfd_state always in sync with MSR_IA32

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Date:   Fri, 19 May 2023 17:03:23 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Adamos Ttofari <attofari@...zon.de>, chang.seok.bae@...el.com
Cc:     attofari@...zon.de, abusse@...zon.de,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Kyle Huey <me@...ehuey.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] x86: fpu: Keep xfd_state always in sync with
 MSR_IA32_XFD

On Fri, May 19 2023 at 11:23, Adamos Ttofari wrote:
> Commit 672365477ae8 ("x86/fpu: Update XFD state where required") and
> commit 8bf26758ca96 ("x86/fpu: Add XFD state to fpstate") introduced a
> per CPU variable xfd_state to keep the MSR_IA32_XFD value cached. In
> order to avoid unnecessary writes to the MSR.
>
> On CPU hotplug MSR_IA32_XFD is reset to the init_fpstate.xfd, which
> wipes out any stale state. But the per CPU cached xfd value is not
> reset, which brings them out of sync.
>
> As a consequence a subsequent xfd_update_state() might fail to update
> the MSR which in turn can result in XRSTOR raising a #NM in kernel
> space, which crashes the kernel.
>
> To address the issue mentioned, initialize xfd_state together with
> MSR_IA32_XFD.
>
> Fixes: 672365477ae8 ("x86/fpu: Update XFD state where required")
>
> Signed-off-by: Adamos Ttofari <attofari@...zon.de>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>