linux-kernel - Re: [PATCH v5 07/39] x86: Add user control-protection fault handler

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Message-ID: <828f1b3154227c06ac1787961016464a4c116cc2.camel@intel.com>
Date:   Fri, 3 Feb 2023 23:01:42 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v5 07/39] x86: Add user control-protection fault handler

On Fri, 2023-02-03 at 20:44 +0100, Borislav Petkov wrote:
> > I think we have to read it before we enable interrupts or use
> > fpregs_lock(). So reading it before saves disabling preemption
> > later.
> 
> So I'm a bit confused - there's that cond_local_irq_enable() which
> will
> enable interrupts if they were enabled before.
> 
> So if they were enabled before and you reenable them here, then that
> current could be the wrong one if we schedule in between, right?
> 
> IOW, shouldn't those two lines be swapped so that it says:
> 
>         tsk = current;
> 
>         cond_local_irq_enable(regs);
> 
> and you can be sure that tsk is always the right current which caused
> the #CP? Or am I way off again?

Since this path is only for exceptions coming from userspace, I think
it should be valid either way. It can't be during a task switch.
I can swap the lines if it looks odd, but unless I'm wrong about the
'current' validity I think it's negligibly better as is because it is
preemptible for as long as possible.