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Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <YASPEYs0QVI88xfM@google.com>
Date:   Sun, 17 Jan 2021 18:49:53 -0800
Cc:     Will Deacon <will@kernel.org>,
        Laurent Dufour <ldufour@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vinayak Menon <vinmenon@codeaurora.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Xu <peterx@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        linux-mm <linux-mm@kvack.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Pavel Emelyanov <xemul@openvz.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        stable <stable@vger.kernel.org>,
        Minchan Kim <minchan@kernel.org>, surenb@google.com,
        Mel Gorman <mgorman@suse.de>, cai@lca.pw
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 <C7D5A74C-25BF-458A-AAD9-61E484B9F225@gmail.com>
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 <2C7AE23B-ACA3-4D55-A907-AF781C5608F0@gmail.com>
 <20210112214337.GA10434@willie-the-truck> <YAO/9YVceghRYo4T@google.com>
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 <YASPEYs0QVI88xfM@google.com>
To:     Yu Zhao <yuzhao@google.com>
Precedence: bulk

> On Jan 17, 2021, at 11:25 AM, Yu Zhao <yuzhao@google.com> wrote:
>=20
> On Sun, Jan 17, 2021 at 02:13:43AM -0800, Nadav Amit wrote:
>>> On Jan 17, 2021, at 1:16 AM, Yu Zhao <yuzhao@google.com> wrote:
>>>=20
>>> On Sat, Jan 16, 2021 at 11:32:22PM -0800, Nadav Amit wrote:
>>>>> On Jan 16, 2021, at 8:41 PM, Yu Zhao <yuzhao@google.com> wrote:
>>>>>=20
>>>>> On Tue, Jan 12, 2021 at 09:43:38PM +0000, Will Deacon wrote:
>>>>>> On Tue, Jan 12, 2021 at 12:38:34PM -0800, Nadav Amit wrote:
>>>>>>>> On Jan 12, 2021, at 11:56 AM, Yu Zhao <yuzhao@google.com> =
wrote:
>>>>>>>> On Tue, Jan 12, 2021 at 11:15:43AM -0800, Nadav Amit wrote:
>>>>>>>>> I will send an RFC soon for per-table deferred TLB flushes =
tracking.
>>>>>>>>> The basic idea is to save a generation in the page-struct that =
tracks
>>>>>>>>> when deferred PTE change took place, and track whenever a TLB =
flush
>>>>>>>>> completed. In addition, other users - such as mprotect - would =
use
>>>>>>>>> the tlb_gather interface.
>>>>>>>>>=20
>>>>>>>>> Unfortunately, due to limited space in page-struct this would =
only
>>>>>>>>> be possible for 64-bit (and my implementation is only for =
x86-64).
>>>>>>>>=20
>>>>>>>> I don't want to discourage you but I don't think this would end =
up
>>>>>>>> well. PPC doesn't necessarily follow one-page-struct-per-table =
rule,
>>>>>>>> and I've run into problems with this before while trying to do
>>>>>>>> something similar.
>>>>>>>=20
>>>>>>> Discourage, discourage. Better now than later.
>>>>>>>=20
>>>>>>> It will be relatively easy to extend the scheme to be per-VMA =
instead of
>>>>>>> per-table for architectures that prefer it this way. It does =
require
>>>>>>> TLB-generation tracking though, which Andy only implemented for =
x86, so I
>>>>>>> will focus on x86-64 right now.
>>>>>>=20
>>>>>> Can you remind me of what we're missing on arm64 in this area, =
please? I'm
>>>>>> happy to help get this up and running once you have something I =
can build
>>>>>> on.
>>>>>=20
>>>>> I noticed arm/arm64 don't support =
ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH.
>>>>> Would it be something worth pursuing? Arm has been using =
mm_cpumask,
>>>>> so it might not be too difficult I guess?
>>>>=20
>>>> [ +Mel Gorman who implemented ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH ]
>>>>=20
>>>> IIUC, there are at least two bugs in x86 implementation.
>>>>=20
>>>> First, there is a missing memory barrier in tlbbatch_add_mm() =
between
>>>> inc_mm_tlb_gen() and the read of mm_cpumask().
>>>=20
>>> In arch_tlbbatch_add_mm()? inc_mm_tlb_gen() has builtin barrier as =
its
>>> comment says -- atomic update ops that return values are also full
>>> memory barriers.
>>=20
>> Yes, you are correct.
>>=20
>>>> Second, try_to_unmap_flush() clears flush_required after flushing. =
Another
>>>> thread can call set_tlb_ubc_flush_pending() after the flush and =
before
>>>> flush_required is cleared, and the indication that a TLB flush is =
pending
>>>> can be lost.
>>>=20
>>> This isn't a problem either because flush_required is per thread.
>>=20
>> Sorry, I meant mm->tlb_flush_batched . It is not per-thread.
>> flush_tlb_batched_pending() clears it after flush and indications =
that
>> set_tlb_ubc_flush_pending() sets in between can be lost.
>=20
> Hmm, the PTL argument above flush_tlb_batched_pending() doesn't seem
> to hold when USE_SPLIT_PTE_PTLOCKS is set. Do you have a reproducer?
> KCSAN might be able to help in this case.

I do not have a reproducer. It is just based on my understanding of this
code.

I will give a short try for building a reproducer, although for some =
reason
=E2=80=9Cyou guys=E2=80=9D complain that my reproducers do not work for =
you (is it PTI that
I disable? idle=3Dpoll? running in a VM?). It is also not likely to be =
too
easy to build a reproducer that actually triggers a memory corruption.

Anyhow, apparently KCSAN has already shouted about this code, causing =
Qian
Cai to add "data_race()" to avoid KCSAN from shouting (9c1177b62a8c
"mm/rmap: annotate a data race at tlb_flush_batched=E2=80=9D).

Note that Andrea asked me not to hijack this thread and have a different =
one
on this issue.