linux-kernel - Re: [patch V3 15/66] x86/fpu: Fail ptrace() requests that try to set invalid MXCSR values

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Date:   Mon, 21 Jun 2021 12:17:43 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     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>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [patch V3 15/66] x86/fpu: Fail ptrace() requests that try to set invalid MXCSR values

On Fri, Jun 18 2021 at 16:18, Thomas Gleixner wrote:

> From: Andy Lutomirski <luto@...nel.org>
>
> There is no benefit from accepting and silently changing an invalid MXCSR
> value supplied via ptrace().  Instead, return -EINVAL on invalid input.
>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
> V2: New patch. Picked up from Andy.
> ---
>  arch/x86/kernel/fpu/regset.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> ---
> --- a/arch/x86/kernel/fpu/regset.c
> +++ b/arch/x86/kernel/fpu/regset.c
> @@ -64,8 +64,9 @@ int xfpregs_set(struct task_struct *targ
>  	if (ret)
>  		return ret;
>  
> -	/* Mask invalid MXCSR bits (for historical reasons). */
> -	newstate.mxcsr &= mxcsr_feature_mask;
> +	/* Do not allow an invalid MXCSR value. */
> +	if (newstate.mxcsr & ~mxcsr_feature_mask)
> +		ret = -EINVAL;

This obviously needs to be:

               return -EINVAL;

/me goes to find a brown paperbag