linux-kernel - Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of dynamic user state

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Date:   Tue, 29 Jun 2021 11:50:56 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     "Lutomirski, Andy" <luto@...nel.org>, Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of
 dynamic user state

On 6/29/21 11:35 AM, Bae, Chang Seok wrote:
>         if (likely(use_xsave())) {
> +               /*
> +                * MSR IA32_XFD write follows after this XSAVE(S). So if a
> +                * state component is in use, XFD should not be armed for
> +                * current. But, for potential changes in the future,
> +                * cross-check XINUSE and XFD values. If a XINUSE state
> +                * is XFD-armed, the following XSAVE(S) does not save the
> +                * state.
> +                *
> +                * Reference the shadow XFD value instead of reading the
> +                * MSR.
> +                */
> +               if (xfd_capable() && boot_cpu_has(X86_FEATURE_XGETBV1)) {
> +                       u64 current_xfd = (fpu->state_mask & xfd_capable()) ^ xfd_capable();
> +
> +                       WARN_ON_FPU(xgetbv(1) & current_xfd);
> +               }

The code looks fine.  But, as usual, I hate the comment.  Maybe:

		/*
		 * If XFD is armed for an xfeature, XSAVE* will not save
		 * its state.  Ensure XFD is clear for all features that
		 * are in use before XSAVE*.
		 */

BTW, the ->state_mask calculation is a little confusing to me.  I
understand that fpu->state_mask shouldn't have any bits set that are
unset in xgetbv(1).

This code seems to be asking the question: Are any dynamic features in
their init state *and* XFD-armed?

Is it actually important to make sure that they are dynamic features?
Is there *any* case where a feature (dynamic or not) can have XFD armed
and be out of its init state?