From: Nadav Amit <nadav.amit@gmail.com>
To: Yu Zhao <yuzhao@google.com>
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
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
Date: Sun, 17 Jan 2021 18:49:53 -0800 [thread overview]
Message-ID: <0BB9D243-F527-409D-8A9E-612DFD4EE993@gmail.com> (raw)
In-Reply-To: <YASPEYs0QVI88xfM@google.com>
> On Jan 17, 2021, at 11:25 AM, Yu Zhao <yuzhao@google.com> wrote:
>
> 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:
>>>
>>> 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:
>>>>>
>>>>> 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.
>>>>>>>>>
>>>>>>>>> Unfortunately, due to limited space in page-struct this would only
>>>>>>>>> be possible for 64-bit (and my implementation is only for x86-64).
>>>>>>>>
>>>>>>>> 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.
>>>>>>>
>>>>>>> Discourage, discourage. Better now than later.
>>>>>>>
>>>>>>> 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.
>>>>>>
>>>>>> 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.
>>>>>
>>>>> 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?
>>>>
>>>> [ +Mel Gorman who implemented ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH ]
>>>>
>>>> IIUC, there are at least two bugs in x86 implementation.
>>>>
>>>> First, there is a missing memory barrier in tlbbatch_add_mm() between
>>>> inc_mm_tlb_gen() and the read of mm_cpumask().
>>>
>>> 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.
>>
>> Yes, you are correct.
>>
>>>> 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.
>>>
>>> This isn't a problem either because flush_required is per thread.
>>
>> 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.
>
> 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
“you guys” complain that my reproducers do not work for you (is it PTI that
I disable? idle=poll? 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”).
Note that Andrea asked me not to hijack this thread and have a different one
on this issue.
next prev parent reply other threads:[~2021-01-18 2:50 UTC|newest]
Thread overview: 120+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-19 4:30 [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect Nadav Amit
2020-12-19 19:15 ` Andrea Arcangeli
[not found] ` <EDC00345-B46E-4396-8379-98E943723809@gmail.com>
2020-12-19 22:06 ` Nadav Amit
2020-12-20 2:20 ` Andrea Arcangeli
2020-12-21 4:36 ` Nadav Amit
2020-12-21 5:12 ` Yu Zhao
2020-12-21 5:25 ` Nadav Amit
2020-12-21 5:39 ` Nadav Amit
2020-12-21 7:29 ` Yu Zhao
2020-12-22 20:34 ` Andy Lutomirski
2020-12-22 20:58 ` Nadav Amit
2020-12-22 21:34 ` Andrea Arcangeli
2020-12-20 2:01 ` Andy Lutomirski
2020-12-20 2:49 ` Andrea Arcangeli
2020-12-20 5:08 ` Andy Lutomirski
2020-12-21 18:03 ` Andrea Arcangeli
2020-12-21 18:22 ` Andy Lutomirski
2020-12-20 6:05 ` Yu Zhao
2020-12-20 8:06 ` Nadav Amit
2020-12-20 9:54 ` Yu Zhao
2020-12-21 3:33 ` Nadav Amit
2020-12-21 4:44 ` Yu Zhao
2020-12-21 17:27 ` Peter Xu
2020-12-21 18:31 ` Nadav Amit
2020-12-21 19:16 ` Yu Zhao
2020-12-21 19:55 ` Linus Torvalds
2020-12-21 20:21 ` Yu Zhao
2020-12-21 20:25 ` Linus Torvalds
2020-12-21 20:23 ` Nadav Amit
2020-12-21 20:26 ` Linus Torvalds
2020-12-21 21:24 ` Yu Zhao
2020-12-21 21:49 ` Nadav Amit
2020-12-21 22:30 ` Peter Xu
2020-12-21 22:55 ` Nadav Amit
2020-12-21 23:30 ` Linus Torvalds
2020-12-21 23:46 ` Nadav Amit
2020-12-22 19:44 ` Andrea Arcangeli
2020-12-22 20:19 ` Nadav Amit
2020-12-22 21:17 ` Andrea Arcangeli
2020-12-21 23:12 ` Yu Zhao
2020-12-21 23:33 ` Linus Torvalds
2020-12-22 0:00 ` Yu Zhao
2020-12-22 0:11 ` Linus Torvalds
2020-12-22 0:24 ` Yu Zhao
2020-12-21 23:22 ` Linus Torvalds
2020-12-22 3:19 ` Andy Lutomirski
2020-12-22 4:16 ` Linus Torvalds
2020-12-22 20:19 ` Andy Lutomirski
2021-01-05 15:37 ` Peter Zijlstra
2021-01-05 18:03 ` Andrea Arcangeli
2021-01-12 16:20 ` Peter Zijlstra
2021-01-12 11:43 ` Vinayak Menon
2021-01-12 15:47 ` Laurent Dufour
2021-01-12 16:57 ` Peter Zijlstra
2021-01-12 19:02 ` Laurent Dufour
2021-01-12 19:15 ` Nadav Amit
2021-01-12 19:56 ` Yu Zhao
2021-01-12 20:38 ` Nadav Amit
2021-01-12 20:49 ` Yu Zhao
2021-01-12 21:43 ` Will Deacon
2021-01-12 22:29 ` Nadav Amit
2021-01-12 22:46 ` Will Deacon
2021-01-13 0:31 ` Andy Lutomirski
2021-01-17 4:41 ` Yu Zhao
2021-01-17 7:32 ` Nadav Amit
2021-01-17 9:16 ` Yu Zhao
2021-01-17 10:13 ` Nadav Amit
2021-01-17 19:25 ` Yu Zhao
2021-01-18 2:49 ` Nadav Amit [this message]
2020-12-22 9:38 ` Nadav Amit
2020-12-22 19:31 ` Andrea Arcangeli
2020-12-22 20:15 ` Matthew Wilcox
2020-12-22 20:26 ` Andrea Arcangeli
2020-12-22 21:14 ` Yu Zhao
2020-12-22 22:02 ` Andrea Arcangeli
2020-12-22 23:39 ` Yu Zhao
2020-12-22 23:50 ` Linus Torvalds
2020-12-23 0:01 ` Linus Torvalds
2020-12-23 0:23 ` Yu Zhao
2020-12-23 2:17 ` Andrea Arcangeli
2020-12-23 9:44 ` Linus Torvalds
2020-12-23 10:06 ` Yu Zhao
2020-12-23 16:24 ` Peter Xu
2020-12-23 18:51 ` Andrea Arcangeli
2020-12-23 18:55 ` Andrea Arcangeli
2020-12-23 19:12 ` Yu Zhao
2020-12-23 19:32 ` Peter Xu
2020-12-23 0:20 ` Linus Torvalds
2020-12-23 2:56 ` Andrea Arcangeli
2020-12-23 3:36 ` Yu Zhao
2020-12-23 15:52 ` Peter Xu
2020-12-23 21:07 ` Andrea Arcangeli
2020-12-23 21:39 ` Andrea Arcangeli
2020-12-23 22:29 ` Yu Zhao
2020-12-23 23:04 ` Andrea Arcangeli
2020-12-24 1:21 ` Andy Lutomirski
2020-12-24 2:00 ` Andrea Arcangeli
2020-12-24 3:09 ` Nadav Amit
2020-12-24 3:30 ` Nadav Amit
2020-12-24 3:34 ` Yu Zhao
2020-12-24 4:01 ` Andrea Arcangeli
2020-12-24 5:18 ` Nadav Amit
2020-12-24 18:49 ` Andrea Arcangeli
2020-12-24 19:16 ` Andrea Arcangeli
2020-12-24 4:37 ` Nadav Amit
2020-12-24 3:31 ` Andrea Arcangeli
2020-12-23 23:39 ` Linus Torvalds
2020-12-24 1:01 ` Andrea Arcangeli
2020-12-22 21:14 ` Nadav Amit
2020-12-22 12:40 ` Nadav Amit
2020-12-22 18:30 ` Yu Zhao
2020-12-22 19:20 ` Nadav Amit
2020-12-23 16:23 ` Will Deacon
2020-12-23 19:04 ` Nadav Amit
2020-12-23 22:05 ` Andrea Arcangeli
2020-12-23 22:45 ` Nadav Amit
2020-12-23 23:55 ` Andrea Arcangeli
2020-12-21 21:55 ` Peter Xu
2020-12-21 23:13 ` Linus Torvalds
2020-12-21 19:53 ` Peter Xu
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