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

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Message-Id: <38AF04BE-7F39-450F-8C26-879C9934E3D6@amacapital.net>
Date:   Tue, 12 Jan 2021 14:04:55 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-edac <linux-edac@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user recovery


> On Jan 12, 2021, at 12:52 PM, Luck, Tony <tony.luck@...el.com> wrote:
> 
> 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@...el.com> 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?

No, but NMI can.

The case that would be very very hard to deal with is if we get an NMI just before IRET/SYSRET and get #MC inside that NMI.

What we should probably do is have a percpu list of pending memory failure cleanups and just accept that we’re going to sometimes get a second MCE (or third or fourth) before we can get to it.

Can we do the cleanup from an interrupt?  IPI-to-self might be a credible approach, if so.