From: Yu Zhao <yuzhao@google.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
Andy Lutomirski <luto@kernel.org>, Peter Xu <peterx@redhat.com>,
Nadav Amit <nadav.amit@gmail.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>,
Will Deacon <will@kernel.org>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
Date: Wed, 23 Dec 2020 03:06:30 -0700 [thread overview]
Message-ID: <X+MWppLjiR7hLgg9@google.com> (raw)
In-Reply-To: <CAHk-=wjNv1GQn+8stK419HAqK0ofkJ1vOR9YSWSNjbW3T5as9A@mail.gmail.com>
On Wed, Dec 23, 2020 at 01:44:42AM -0800, Linus Torvalds wrote:
> On Tue, Dec 22, 2020 at 4:01 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > The more I look at the mprotect code, the less I like it. We seem to
> > be much better about the TLB flushes in other places (looking at
> > mremap, for example). The mprotect code seems to be very laissez-faire
> > about the TLB flushing.
>
> No, this doesn't help.
>
> > Does adding a TLB flush to before that
> >
> > pte_unmap_unlock(pte - 1, ptl);
> >
> > fix things for you?
>
> It really doesn't fix it. Exactly because - as pointed out earlier -
> the actual page *copy* happens outside the pte lock.
I appreciate all the pointers. It seems to me it does.
> So what can happen is:
>
> - CPU 1 holds the page table lock, while doing the write protect. It
> has cleared the writable bit, but hasn't flushed the TLB's yet
>
> - CPU 2 did *not* have the TLB entry, sees the new read-only state,
> takes a COW page fault, and reads the PTE from memory (into
> vmf->orig_pte)
In handle_pte_fault(), we lock page table and check pte_write(), so
we either see a RW pte before CPU 1 runs or a RO one with no stale tlb
entries after CPU 1 runs, assume CPU 1 flushes tlb while holding the
same page table lock (not mmap_lock).
> - CPU 2 correctly decides it needs to be a COW, and copies the page contents
>
> - CPU 3 *does* have a stale TLB (because TLB invalidation hasn't
> happened yet), and writes to that page in users apce
>
> - CPU 1 now does the TLB invalidate, and releases the page table lock
>
> - CPU 2 gets the page table lock, sees that its PTE matches
> vmf->orig_pte, and switches it to be that writable copy of the page.
>
> where the copy happened before CPU 3 had stopped writing to the page.
>
> So the pte lock doesn't actually matter, unless we actually do the
> page copy inside of it (on CPU2), in addition to doing the TLB flush
> inside of it (on CPU1).
>
> mprotect() is actually safe for two independent reasons: (a) it does
> the mmap_sem for writing (so mprotect can't race with the COW logic at
> all), and (b) it changes the vma permissions so turning something
> read-only actually disables COW anyway, since it won't be a COW, it
> will be a SIGSEGV.
>
> So mprotect() is irrelevant, other than the fact that it shares some
> code with that "turn it read-only in the page tables".
>
> fork() is a much closer operation, in that it actually triggers that
> COW behavior, but fork() takes the mmap_sem for writing, so it avoids
> this too.
>
> So it's really just userfaultfd and that kind of ilk that is relevant
> here, I think. But that "you need to flush the TLB before releasing
> the page table lock" was not true (well, it's true in other
> circumstances - just not *here*), and is not part of the solution.
>
> Or rather, if it's part of the solution here, it would have to be
> matched with that "page copy needs to be done under the page table
> lock too".
>
> Linus
>
next prev parent reply other threads:[~2020-12-23 10:06 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
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 [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=X+MWppLjiR7hLgg9@google.com \
--to=yuzhao@google.com \
--cc=aarcange@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=luto@kernel.org \
--cc=mike.kravetz@oracle.com \
--cc=minchan@kernel.org \
--cc=nadav.amit@gmail.com \
--cc=peterx@redhat.com \
--cc=peterz@infradead.org \
--cc=rppt@linux.vnet.ibm.com \
--cc=stable@vger.kernel.org \
--cc=torvalds@linux-foundation.org \
--cc=will@kernel.org \
--cc=xemul@openvz.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).