linux-kernel - Re: [patch V3 65/66] x86/fpu/signal: Handle #PF in the direct restore path

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Date:   Wed, 23 Jun 2021 10:45:04 +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 65/66] x86/fpu/signal: Handle #PF in the direct
 restore path

On Fri, Jun 18, 2021 at 04:19:28PM +0200, Thomas Gleixner wrote:
> If *RSTOR raises an exception, then the slow path is taken. That's wrong
> because if the reason was not #PF then going through the slow path is waste
> of time because that will end up with the same conclusion that the data is
> invalid.
> 
> Now that the wrapper around *RSTOR return an negative error code, which is
> the negated trap number, it's possible to differentiate.
> 
> If the *RSTOR raised #PF then handle it directly in the fast path and if it
> was some other exception, e.g. #GP, then give up and do not try the fast
> path.
> 
> This removes the legacy frame FRSTOR code from the slow path because FRSTOR
> is not a ia32_fxstate frame and is therefore handled in the fast path.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  arch/x86/kernel/fpu/signal.c |   65 ++++++++++++++++++++-----------------------
>  1 file changed, 31 insertions(+), 34 deletions(-)
> 
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -270,11 +270,17 @@ static int restore_hwregs_from_user(void
>  	}
>  }
>  
> -static int restore_fpregs_from_user(void __user *buf, u64 xrestore, bool fx_only)
> +/*
> + * Attempt to restore the FPU registers directly from user memory.
> + * Pagefaults are handled and any errors returned are fatal.
> + */
> +static int restore_fpregs_from_user(void __user *buf, u64 xrestore,
> +				    bool fx_only, unsigned int size)
>  {
>  	struct fpu *fpu = &current->thread.fpu;
>  	int ret ;
>  
> +retry:
>  	fpregs_lock();
>  	pagefault_disable();
>  	ret = restore_hwregs_from_user(buf, xrestore, fx_only);
> @@ -291,15 +297,16 @@ static int restore_fpregs_from_user(void
>  		 * invalidate the FPU register state otherwise the task
>  		 * might preempt current and return to user space with
>  		 * corrupted FPU registers.
> -		 *
> -		 * In case current owns the FPU registers then no further
> -		 * action is required. The fixup in the slow path will
> -		 * handle it correctly.
>  		 */
>  		if (test_thread_flag(TIF_NEED_FPU_LOAD))
>  			__cpu_invalidate_fpregs_state();
>  		fpregs_unlock();
> -		return ret;
> +
> +		if (ret == -EFAULT)
> +			ret = fault_in_pages_readable(buf, size);
> +		if (!ret)
> +			goto retry;
> +		return ret == -EFAULT ? ret : -EINVAL;

Uuuh, this is gonna make people stop and think, with all those different
ret assignments and cases. :)

In any case, __fpu_restore_sig() is starting to become somewhat saner
again, nice!

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

-- 
Regards/Gruss,
    Boris.

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