linux-kernel - Re: [patch V3 22/66] x86/fpu: Move fpu__write

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Message-ID: <87a6njezz1.ffs@nanos.tec.linutronix.de>
Date:   Mon, 21 Jun 2021 22:15:30 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Borislav Petkov <bp@...e.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 22/66] x86/fpu: Move fpu__write_begin() to regset

On Mon, Jun 21 2021 at 17:30, Borislav Petkov wrote:
>> @@ -89,7 +108,7 @@ int xfpregs_set(struct task_struct *targ
>>  	if (newstate.mxcsr & ~mxcsr_feature_mask)
>>  		ret = -EINVAL;
> 		^^^^^^^^^^^^^^
>
> Don't forget to fix that thing, btw.

Fixed localy at the right spot already.

>> -	fpu__prepare_write(fpu);
>> +	fpu_force_restore(fpu);
>>  
>>  	/* Copy the state  */
>>  	memcpy(&fpu->state.fxsave, &newstate, sizeof(newstate));
>> @@ -147,7 +166,7 @@ int xstateregs_set(struct task_struct *t
>>  		}
>>  	}
>>  
>> -	fpu__prepare_write(fpu);
>> +	fpu_force_restore(fpu);
>>  	ret = copy_kernel_to_xstate(&fpu->state.xsave, kbuf ?: tmpbuf);
>
> Ok, this here:
>
> fpu_force_restore() will unconditionally invalidate the fpregs but the
> following copy_kernel_to_xstate() call can return -EINVAL when the
> xstate header validation fails or mxcsr has reserved bits set so we will
> not have copied anything to the target thread.

Then the task has to restore FPU on the way to user space. Not a big
deal, it's traced - so what?

Thanks,

        tglx