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

[Luck, Tony]

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.

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

-Tony

[1] Does such a thing already exist? If not, I'd need some help/pointers
to create it.