linux-kernel - Re: [patch V3 11/66] x86/fpu: Sanitize xstateregs

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Date:   Sun, 20 Jun 2021 23:30:23 +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 V3 11/66] x86/fpu: Sanitize xstateregs_set()

On Fri, Jun 18, 2021 at 04:18:34PM +0200, Thomas Gleixner wrote:
> xstateregs_set() operates on a stopped task and tries to copy the provided
> buffer into the task's fpu.state.xsave buffer.
> 
> Any error while copying or invalid state detected after copying results in
> wiping the target task's 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.
> 
> Fix this with the following modifications:
> 
>  1) If data has to be copied from userspace, allocate a buffer and copy from
>     user first.
> 
>  2) Use copy_kernel_to_xstate() unconditionally so that header checking
>     works correctly.
> 
>  3) Return on error without corrupting the target state.
> 
> This prevents corrupting states and lets the caller deal with the problem
> it caused in the first place.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  arch/x86/include/asm/fpu/xstate.h |    4 ---
>  arch/x86/kernel/fpu/regset.c      |   44 +++++++++++++++-----------------------
>  arch/x86/kernel/fpu/xstate.c      |   14 ++++++------
>  3 files changed, 26 insertions(+), 36 deletions(-)

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

Nice!

-- 
Regards/Gruss,
    Boris.

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