linux-kernel - Re: [patch V2 02/14] x86/fpu: Prevent state corruption in __fpu__restore

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Date:   Mon, 7 Jun 2021 10:49:02 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.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>,
        syzbot+2067e764dbcd10721e2e@...kaller.appspotmail.com,
        stable@...r.kernel.org
Subject: Re: [patch V2 02/14] x86/fpu: Prevent state corruption in
 __fpu__restore_sig()

On Sun, Jun 06, 2021 at 01:47:44AM +0200, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> The non-compacted slowpath uses __copy_from_user() and copies the entire
> user buffer into the kernel buffer, verbatim.  This means that the kernel
> buffer may now contain entirely invalid state on which XRSTOR will #GP.
> validate_user_xstate_header() can detect some of that corruption, but that
> leaves the onus on callers to clear the buffer.
> 
> Prior to XSAVES support it was possible just to reinitialize the buffer,
> completely, but with supervisor states that is not longer possible as the
> buffer clearing code split got it backwards. Fixing that is possible, but
> not corrupting the state in the first place is more robust.
> 
> Avoid corruption of the kernel XSAVE buffer by using copy_user_to_xstate()
> which validates the XSAVE header contents before copying the actual states
> to the kernel. copy_user_to_xstate() was previously only called for
> compacted-format kernel buffers, but it works for both compacted and
> non-compacted forms.
> 
> Using it for the non-compacted form is slower because of multiple
> __copy_from_user() operations, but that cost is less important than robust
> code in an already slow path.
> 
> [ Changelog polished by Dave Hansen ]
> 
> Fixes: b860eb8dce59 ("x86/fpu/xstate: Define new functions for clearing fpregs and xstates")
> Reported-by: syzbot+2067e764dbcd10721e2e@...kaller.appspotmail.com
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: stable@...r.kernel.org
> ---
> V2: Removed the make validate_user_xstate_header() static hunks (Borislav)
> ---
>  arch/x86/kernel/fpu/signal.c |    9 +--------
>  1 file changed, 1 insertion(+), 8 deletions(-)

Very nice.

Reviewed-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette