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
