From: Peter Xu <peterx@redhat.com>
To: Nadav Amit <nadav.amit@gmail.com>
Cc: Yu Zhao <yuzhao@google.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@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: Mon, 21 Dec 2020 14:53:57 -0500 [thread overview]
Message-ID: <20201221195357.GI6640@xz-x1> (raw)
In-Reply-To: <76B4F49B-ED61-47EA-9BE4-7F17A26B610D@gmail.com>
On Mon, Dec 21, 2020 at 10:31:57AM -0800, Nadav Amit wrote:
> > On Dec 21, 2020, at 9:27 AM, Peter Xu <peterx@redhat.com> wrote:
> >
> > Hi, Nadav,
> >
> > On Sun, Dec 20, 2020 at 12:06:38AM -0800, Nadav Amit wrote:
> >
> > [...]
> >
> >> 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.
> >> Sorry for the strange formatting to fit within 80 columns:
> >
> > I assume I can ignore the race mentioned in the commit message but only refer
> > to this one below. However I'm still confused. Please see below.
> >
> >> [ Start: PTE is writable ]
> >>
> >> cpu0 cpu1 cpu2
> >> ---- ---- ----
> >> [ Writable PTE
> >> cached in TLB ]
> >
> > Here cpu2 got writable pte in tlb. But why?
> >
> > If below is an unprotect, it means it must have been protected once by
> > userfaultfd, right? If so, the previous change_protection_range() which did
> > the wr-protect should have done a tlb flush already before it returns (since
> > pages>0 - we protected one pte at least). Then I can't see why cpu2 tlb has
> > stall data.
>
> Thanks, Peter. Just as you can munprotect() a region which was not protected
> before, you can ufff-unprotect a region that was not protected before. It
> might be that the user tried to unprotect a large region, which was
> partially protected and partially unprotected.
>
> The selftest obviously blindly unprotect some regions to check for bugs.
>
> So to your question - it was not write-protected (think about initial copy
> without write-protecting).
If that's the only case, how about we don't touch the ptes at all? Instead of
playing with preserve_write, I'm thinking something like this right before
ptep_modify_prot_start(), even for uffd_wp==true:
if (uffd_wp && pte_uffd_wp(old_pte)) {
WARN_ON_ONCE(pte_write(old_pte));
continue;
}
if (uffd_wp_resolve && !pte_uffd_wp(old_pte))
continue;
Then we can also avoid the heavy operations on changing ptes back and forth.
>
> > If I assume cpu2 doesn't have that cached tlb, then "write to old page" won't
> > happen either, because cpu1/cpu2 will all go through the cow path and pgtable
> > lock should serialize them.
> >
> >> userfaultfd_writeprotect()
> >> [ write-*unprotect* ]
> >> mwriteprotect_range()
> >> mmap_read_lock()
> >> change_protection()
> >>
> >> change_protection_range()
> >> ...
> >> change_pte_range()
> >> [ *clear* “write”-bit ]
> >> [ defer TLB flushes]
> >> [ page-fault ]
> >> …
> >> wp_page_copy()
> >> cow_user_page()
> >> [ copy page ]
> >> [ write to old
> >> page ]
> >> …
> >> set_pte_at_notify()
> >>
> >> [ End: cpu2 write not copied form old to new page. ]
> >
> > Could you share how to reproduce the problem? I would be glad to give it a
> > shot as well.
>
> You can run the selftests/userfaultfd with my small patch [1]. I ran it with
> the following parameters: “ ./userfaultfd anon 100 100 “. I think that it is
> more easily reproducible with “mitigations=off idle=poll” as kernel
> parameters.
>
> [1] https://lore.kernel.org/patchwork/patch/1346386/
Thanks.
>
> >
> >> [1] https://lore.kernel.org/patchwork/patch/1346386
> >
> > PS: Sorry to not have read the other series of yours. It seems to need some
> > chunk of time so I postponed it a bit due to other things; but I'll read at
> > least the fixes very soon.
>
> Thanks again, I will post RFCv2 with some numbers soon.
I read the patch 1/3 of the series. Would it be better to post them separately
just in case Andrew would like to pick them earlier?
Since you seem to be heavily working on uffd-wp - I do still have a few uffd-wp
fixes locally even for anonymous. I think they're related to some corner cases
like either thp or migration entry convertions, but anyway I'll see whether I
should post them even earlier (I planned to add smap/pagemap support for
uffd-wp so maybe I can even write some test case to verify some of them). Just
a FYI...
Thanks,
--
Peter Xu
prev parent reply other threads:[~2020-12-21 19:55 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
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 [this message]
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