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

[Luck, Tony]

On Tue, Jan 12, 2021 at 10:57:07AM -0800, Andy Lutomirski wrote:
> 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.

Scary? It's terrifying!

But we know that the fault happend in a get_user() or copy_from_user() call
(i.e. an RIP with an extable recovery address).  Does context switch
access user memory?

-Tony