linux-kernel - Re: [patch V2 09/52] x86/fpu: Reject invalid MXCSR values in copy_kernel_to

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Date:   Wed, 16 Jun 2021 17:02:45 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [patch V2 09/52] x86/fpu: Reject invalid MXCSR values in
 copy_kernel_to_xstate()

On Mon, Jun 14, 2021 at 05:44:17PM +0200, Thomas Gleixner wrote:
> Instead of masking out reserved bits, check them and reject the provided
> state as invalid if not zero.
> 
> Suggested-by: Andy Lutomirski <luto@...nel.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
> V2: New patch
> ---
>  arch/x86/kernel/fpu/xstate.c |   11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -1166,6 +1166,14 @@ int copy_kernel_to_xstate(struct xregs_s
>  	if (validate_user_xstate_header(&hdr))
>  		return -EINVAL;
>  
> +	if (xfeatures_mxcsr_quirk(hdr.xfeatures)) {

Since we're cleaning up this FPU stinking pile - that function needs to
have a verb in the name, something like:

	if (xfeatures_mxcsr_quirk_needed(...))

but that's unrelated to here and as a note to whoever gets to get to it
first.

> +		const u32 *mxcsr = kbuf + offsetof(struct fxregs_state, mxcsr);
> +
> +		/* Reserved bits in MXCSR must be zero. */
> +		if (*mxcsr & ~mxcsr_feature_mask)
> +			return -EINVAL;
> +	}

Btw, that function has another

	if (xfeatures_mxcsr_quirk(hdr.xfeatures)) {

branch already below the loop.

Should we merge both? Diff ontop of yours:

---
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 5d032c48f39d..30022d3fcd4a 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1172,6 +1172,11 @@ int copy_kernel_to_xstate(struct xregs_state *xsave, const void *kbuf)
 		/* Reserved bits in MXCSR must be zero. */
 		if (*mxcsr & ~mxcsr_feature_mask)
 			return -EINVAL;
+
+		offset = offsetof(struct fxregs_state, mxcsr);
+		size = MXCSR_AND_FLAGS_SIZE;
+
+		memcpy(&xsave->i387.mxcsr, kbuf + offset, size);
 	}
 
 	for (i = 0; i < XFEATURE_MAX; i++) {
@@ -1187,12 +1192,6 @@ int copy_kernel_to_xstate(struct xregs_state *xsave, const void *kbuf)
 		}
 	}
 
-	if (xfeatures_mxcsr_quirk(hdr.xfeatures)) {
-		offset = offsetof(struct fxregs_state, mxcsr);
-		size = MXCSR_AND_FLAGS_SIZE;
-		memcpy(&xsave->i387.mxcsr, kbuf + offset, size);
-	}
-
 	/*
 	 * The state that came in from userspace was user-state only.
 	 * Mask all the user states out of 'xfeatures':

-- 
Regards/Gruss,
    Boris.

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