From: Nadav Amit <nadav.amit@gmail.com>
To: Yu Zhao <yuzhao@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
linux-mm <linux-mm@kvack.org>, Peter Xu <peterx@redhat.com>,
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@vger.kernel.org, minchan@kernel.org,
Andy Lutomirski <luto@kernel.org>, Will Deacon <will@kernel.org>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
Date: Sun, 20 Dec 2020 19:33:09 -0800 [thread overview]
Message-ID: <3680387D-65F1-4078-A19D-F77DE8544B96@gmail.com> (raw)
In-Reply-To: <X98fZOiLNmnDQKhN@google.com>
> On Dec 20, 2020, at 1:54 AM, Yu Zhao <yuzhao@google.com> wrote:
>
> On Sun, Dec 20, 2020 at 12:06:38AM -0800, Nadav Amit wrote:
>>> On Dec 19, 2020, at 10:05 PM, Yu Zhao <yuzhao@google.com> wrote:
>>>
>>> On Sat, Dec 19, 2020 at 01:34:29PM -0800, Nadav Amit wrote:
>>>> [ cc’ing some more people who have experience with similar problems ]
>>>>
>>>>> On Dec 19, 2020, at 11:15 AM, Andrea Arcangeli <aarcange@redhat.com> wrote:
>>>>>
>>>>> Hello,
>>>>>
>>>>> On Fri, Dec 18, 2020 at 08:30:06PM -0800, Nadav Amit wrote:
>>>>>> Analyzing this problem indicates that there is a real bug since
>>>>>> mmap_lock is only taken for read in mwriteprotect_range(). This might
>>>>>
>>>>> Never having to take the mmap_sem for writing, and in turn never
>>>>> blocking, in order to modify the pagetables is quite an important
>>>>> feature in uffd that justifies uffd instead of mprotect. It's not the
>>>>> most important reason to use uffd, but it'd be nice if that guarantee
>>>>> would remain also for the UFFDIO_WRITEPROTECT API, not only for the
>>>>> other pgtable manipulations.
>>>>>
>>>>>> Consider the following scenario with 3 CPUs (cpu2 is not shown):
>>>>>>
>>>>>> cpu0 cpu1
>>>>>> ---- ----
>>>>>> userfaultfd_writeprotect()
>>>>>> [ write-protecting ]
>>>>>> mwriteprotect_range()
>>>>>> mmap_read_lock()
>>>>>> change_protection()
>>>>>> change_protection_range()
>>>>>> ...
>>>>>> change_pte_range()
>>>>>> [ defer TLB flushes]
>>>>>> userfaultfd_writeprotect()
>>>>>> mmap_read_lock()
>>>>>> change_protection()
>>>>>> [ write-unprotect ]
>>>>>> ...
>>>>>> [ unprotect PTE logically ]
>>>>>> ...
>>>>>> [ page-fault]
>>>>>> ...
>>>>>> wp_page_copy()
>>>>>> [ set new writable page in PTE]
>>>
>>> I don't see any problem in this example -- wp_page_copy() calls
>>> ptep_clear_flush_notify(), which should take care of the stale entry
>>> left by cpu0.
>>>
>>> That being said, I suspect the memory corruption you observed is
>>> related this example, with cpu1 running something else that flushes
>>> conditionally depending on pte_write().
>>>
>>> Do you know which type of pages were corrupted? file, anon, etc.
>>
>> First, Yu, you are correct. My analysis is incorrect, but let me have
>> another try (below). To answer your (and Andrea’s) question - this happens
>> with upstream without any changes, excluding a small fix to the selftest,
>> since it failed (got stuck) due to missing wake events. [1]
>>
>> We are talking about anon memory.
>>
>> So to correct myself, I think that what I really encountered was actually
>> during MM_CP_UFFD_WP_RESOLVE (i.e., when the protection is removed). The
>> problem was that in this case the “write”-bit was removed during unprotect.
>
> Thanks. You are right about when the problem happens: UFD write-
> UNprotecting. But it's not UFD write-UNprotecting that removes the
> writable bit -- the bit can only be removed during COW or UFD
> write-protecting. So your original example was almost correct, except
> the last line describing cpu1.
The scenario is a bit confusing, so stay with me. The idea behind uffd
unprotect is indeed only to mark the PTE logically as uffd-unprotected, and
not to *set* the writable bit, allowing the #PF handler to do COW or
whatever correctly upon #PF.
However, the problem that we have is that if a page is already writable,
write-unprotect *clears* the writable bit, making it write-protected (at
least for anonymous pages). This is not good from performance point-of-view,
but also a correctness issue, as I pointed out.
In some more detail: mwriteprotect_range() uses vm_get_page_prot() to
compute the new protection. For anonymous private memory, at least on x86,
this means the write-bit in the protection is clear. So later,
change_pte_range() *clears* the write-bit during *unprotection*.
That’s the reason the second part of my patch - the change to preserve_write
- fixes the problem.
> The problem is how do_wp_page() handles non-COW pages. (For COW pages,
> do_wp_page() works correctly by either reusing an existing page or
> make a new copy out of it.) In UFD case, the existing page may not
> have been properly write-protected. As you pointed out, the tlb flush
> may not be done yet. Making a copy can potentially race with the
> writer on cpu2.
Just to clarify the difference - You regard a scenario of UFFD
write-protect, while I am pretty sure the problem I encountered is during
write-unprotect.
I am not sure we are on the same page (but we may be). The problem I have is
with cow_user_page() that is called by do_wp_page() before any TLB flush
took place (either by change_protection_range() or by do_wp_page() which
does flush, but after the copy).
Let me know if you regard a different scenario.
> Should we fix the problem by ensuring integrity of the copy? IMO, no,
> because do_wp_page() shouldn't copy at all in this case. It seems it
> was recently broken by
>
> be068f29034f mm: fix misplaced unlock_page in do_wp_page()
> 09854ba94c6a mm: do_wp_page() simplification
>
> I haven't study them carefully. But if you could just revert them and
> run the test again, we'd know where exactly to look at next.
These patches regard the wp_page_reuse() case, which makes me think we
are not on the same page. I do not see a problem with wp_page_reuse()
since it does not make a copy of the page. If you can explain what I
am missing, it would be great.
next prev parent reply other threads:[~2020-12-21 4:33 UTC|newest]
Thread overview: 121+ 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
2020-12-19 21:34 ` Nadav Amit
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 [this message]
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
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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