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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user recovery
Date:   Tue, 12 Jan 2021 14:04:55 -0800
Message-Id: <38AF04BE-7F39-450F-8C26-879C9934E3D6@amacapital.net>
References: <20210112205207.GA18195@agluck-desk2.amr.corp.intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>, 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>
In-Reply-To: <20210112205207.GA18195@agluck-desk2.amr.corp.intel.com>
To:     "Luck, Tony" <tony.luck@intel.com>
Precedence: bulk


> On Jan 12, 2021, at 12:52 PM, Luck, Tony <tony.luck@intel.com> wrote:
>=20
> =EF=BB=BFOn 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@intel.com> wrote:=

>>>=20
>>> 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.
>>>>=20
>>>> 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.
>>>=20
>>> 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 possibl=
e
>>> because of locks.
>>>=20
>>> 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.
>>=20
>> 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.
>=20
> Scary? It's terrifying!
>=20
> But we know that the fault happend in a get_user() or copy_from_user() cal=
l
> (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 b=
efore IRET/SYSRET and get #MC inside that NMI.

What we should probably do is have a percpu list of pending memory failure c=
leanups and just accept that we=E2=80=99re going to sometimes get a second M=
CE (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 ap=
proach, if so.