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Date:   Tue, 12 Jan 2021 10:23:57 -0800
From:   "Luck, Tony" <tony.luck@intel.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Borislav Petkov <bp@alien8.de>, X86 ML <x86@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Darren Hart <dvhart@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-edac <linux-edac@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user
 recovery
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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.