From: Andrea Arcangeli <aarcange@redhat.com>
To: Nadav Amit <nadav.amit@gmail.com>
Cc: Andy Lutomirski <luto@kernel.org>, 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 <stable@vger.kernel.org>, Minchan Kim <minchan@kernel.org>,
Yu Zhao <yuzhao@google.com>, Will Deacon <will@kernel.org>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
Date: Tue, 22 Dec 2020 16:34:14 -0500 [thread overview]
Message-ID: <X+JmVmDvBOYuw5Zl@redhat.com> (raw)
In-Reply-To: <719DF2CD-A0BC-4B67-9FBA-A9E0A98AA45E@gmail.com>
On Tue, Dec 22, 2020 at 12:58:18PM -0800, Nadav Amit wrote:
> I had somewhat similar ideas - saving in each page-struct the generation,
> which would allow to: (1) extend pte_same() to detect interim changes
> that were reverted (RO->RW->RO) and (2) per-PTE pending flushes.
What don't you feel safe about, what's the problem with RO->RO->RO, I
don't get it.
The pte_same is perfectly ok without sequence counter in my view, I
never seen anything that would not be ok with pte_same given all the
invariant are respected. It's actually a great optimization compared
to any unscalable sequence counter.
The counter would slowdown everything, having to increase a counter
every time you change a pte, no matter if it's a counter per pgtable
or per-vma or per-mm, sounds very bad.
I'd rather prefer to take mmap_lock_write across the whole userfaultfd
ioctl, than having to deal with a new sequence counter increase for
every pte modification on a heavily contended cacheline.
Also note the counter would have solved nothing for
userfaultfd_writeprotect, it's useless to detect stale TLB entries.
See how !pte_write check happens after the counter was already increased:
CPU0 CPU 1 CPU 2
------ -------- -------
userfaultfd_wrprotect(mode_wp = true)
PT lock
atomic set _PAGE_UFFD_WP and clear _PAGE_WRITE
false_shared_counter_counter++
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PT unlock
do_page_fault FAULT_FLAG_WRITE
userfaultfd_wrprotect(mode_wp = false)
PT lock
ATOMIC clear _PAGE_UFFD_WP <- problem
/* _PAGE_WRITE not set */
false_shared_counter_counter++
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PT unlock
XXXXXXXXXXXXXX BUG RACE window open here
PT lock
counter = false_shared_counter_counter
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FAULT_FLAG_WRITE is set by CPU
_PAGE_WRITE is still clear in pte
PT unlock
wp_page_copy
copy_user_page runs with stale TLB
pte_same(counter, orig_pte, pte) -> PASS
^^^^^^^ ^^^^
commit the copy to the pte with the lost writes
deferred tlb flush <- too late
XXXXXXXXXXXXXX BUG RACE window close here
================================================================================
next prev parent reply other threads:[~2020-12-22 21:34 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 [this message]
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
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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