linux-kernel - Re: [PATCH v6 1/2] x86/fpu: Allow PKRU to be (once again) written by ptrace.

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Date:   Thu, 1 Sep 2022 15:29:46 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kyle Huey <me@...ehuey.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Robert O'Callahan <robert@...llahan.org>,
        David Manouchehri <david.manouchehri@...eup.net>,
        Borislav Petkov <bp@...e.de>, stable@...r.kernel.org
Subject: Re: [PATCH v6 1/2] x86/fpu: Allow PKRU to be (once again) written by
 ptrace.

On Mon, Aug 29, 2022, Kyle Huey wrote:
> @@ -1246,6 +1246,21 @@ static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
>  		}
>  	}
>  
> +	/*
> +	 * Update the user protection key storage. Allow KVM to
> +	 * pass in a NULL pkru pointer if the mask bit is unset
> +	 * for its legacy ABI behavior.
> +	 */
> +	if (pkru)
> +		*pkru = 0;
> +
> +	if (hdr.xfeatures & XFEATURE_MASK_PKRU) {
> +		struct pkru_state *xpkru;
> +
> +		xpkru = __raw_xsave_addr(xsave, XFEATURE_PKRU);
> +		*pkru = xpkru->pkru;
> +	}

What about writing this as:

	if (hdr.xfeatures & XFEATURE_MASK_PKRU) {
		...

		*pkru = xpkru->pkru;
	} else if (pkru) {
		*pkru = 0;
	}

to make it slightly more obvious that @pkru must be non-NULL if the feature flag
is enabled?

Or we could be paranoid, though I'm not sure this is worthwhile.

	if ((hdr.xfeatures & XFEATURE_MASK_PKRU) &&
	    !WARN_ON_ONCE(!pkru)) {
		...

		*pkru = xpkru->pkru;
	} else if (pkru) {
		*pkru = 0;
	}


Otherwise, looks good from a KVM perspective.  Thanks!