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Date:   Fri, 11 Jun 2021 11:45:52 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [patch 06/41] x86/fpu: Sanitize xstateregs_set()

On 6/11/21 9:15 AM, Thomas Gleixner wrote:
> xstateregs_set() operates on a stopped task and tries to copy the provided
> buffer into the tasks fpu.state.xsave buffer.
> 
> Any error while copying or invalid state detected after copying results in
> wiping the target tasks FPU state completely including supervisor states.
> 
> That's just wrong. The caller supplied invalid data or has a problem with
> unmapped memory, so there is absolutely no justification to corrupt the
> target state.
> 
> @@ -1146,14 +1146,16 @@ int copy_kernel_to_xstate(struct xregs_s
>  	 */
>  	xsave->header.xfeatures |= hdr.xfeatures;
>  
> +	/* mxcsr reserved bits must be masked to zero for security reasons. */
> +	xsave->i387.mxcsr &= mxcsr_feature_mask;

This comment is vague.  At least it should say:

A subsequent XRSTOR(S) will fail if MXCSR has bits set that are not
accepted by the current CPU.  Mask out unsupported bits.

But a much nicer fix IMO would be to just return an error.

--Andy

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