Re: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user recovery

[Andy Lutomirski]

On Tue, Jan 12, 2021 at 10:24 AM Luck, Tony <tony.luck@xxxxxxxxx> wrote:
>
> On Tue, Jan 12, 2021 at 09:21:21AM -0800, Andy Lutomirski wrote:
> > Well, we need to do *something* when the first __get_user() trips the
> > #MC.  It would be nice if we could actually fix up the page tables
> > inside the #MC handler, but, if we're in a pagefault_disable() context
> > we might have locks held.  Heck, we could have the pagetable lock
> > held, be inside NMI, etc.  Skipping the task_work_add() might actually
> > make sense if we get a second one.
> >
> > We won't actually infinite loop in pagefault_disable() context -- if
> > we would, then we would also infinite loop just from a regular page
> > fault, too.
>
> Fixing the page tables inside the #MC handler to unmap the poison
> page would indeed be a good solution. But, as you point out, not possible
> because of locks.
>
> Could we take a more drastic approach? We know that this case the kernel
> is accessing a user address for the current process. Could the machine
> check handler just re-write %cr3 to point to a kernel-only page table[1].
> I.e. unmap the entire current user process.

That seems scary, especially if we're in the middle of a context
switch when this happens.  We *could* make it work, but I'm not at all
convinced it's wise.

>
> Then any subsequent access to user space will page fault. Maybe have a
> flag in the task structure to help the #PF handler understand what just
> happened.
>
> The code we execute in the task_work handler can restore %cr3

This would need to be integrated with something much more local IMO.
Maybe it could be scoped to pagefault_disable()/pagefault_enable()?

--Andy