Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed mappings by default

[David Hildenbrand]

On 04.05.23 23:27, Lorenzo Stoakes wrote:
> Writing to file-backed mappings which require folio dirty tracking using
> GUP is a fundamentally broken operation, as kernel write access to GUP
> mappings do not adhere to the semantics expected by a file system.
>
> A GUP caller uses the direct mapping to access the folio, which does not
> cause write notify to trigger, nor does it enforce that the caller marks
> the folio dirty.
>
> The problem arises when, after an initial write to the folio, writeback
> results in the folio being cleaned and then the caller, via the GUP
> interface, writes to the folio again.
>
> As a result of the use of this secondary, direct, mapping to the folio no
> write notify will occur, and if the caller does mark the folio dirty, this
> will be done so unexpectedly.
>
> For example, consider the following scenario:-
>
> 1. A folio is written to via GUP which write-faults the memory, notifying
>     the file system and dirtying the folio.
> 2. Later, writeback is triggered, resulting in the folio being cleaned and
>     the PTE being marked read-only.
> 3. The GUP caller writes to the folio, as it is mapped read/write via the
>     direct mapping.
> 4. The GUP caller, now done with the page, unpins it and sets it dirty
>     (though it does not have to).
>
> This change updates both the PUP FOLL_LONGTERM slow and fast APIs. As
> pin_user_pages_fast_only() does not exist, we can rely on a slightly
> imperfect whitelisting in the PUP-fast case and fall back to the slow case
> should this fail.

Thanks a lot, this looks pretty good to me!

I started writing some selftests (assuming none would be in the works) using
iouring and and the gup_tests interface. So far, no real surprises for the general
GUP interaction [1].


There are two things I noticed when registering an iouring fixed buffer (that differ
now from generic gup_test usage):


(1) Registering a fixed buffer targeting an unsupported MAP_SHARED FS file now fails with
    EFAULT (from pin_user_pages()) instead of EOPNOTSUPP (from io_pin_pages()).

The man page for io_uring_register documents:

       EOPNOTSUPP
              User buffers point to file-backed memory.

... we'd have to do some kind of errno translation in io_pin_pages(). But the
translation is not simple (sometimes we want to forward EOPNOTSUPP). That also
applies once we remove that special-casing in io_uring code.

... maybe we can simply update the manpage (stating that older kernels returned
EOPNOTSUPP) and start returning EFAULT?


(2) Registering a fixed buffer targeting a MAP_PRIVATE FS file fails with EOPNOTSUPP
    (from io_pin_pages()). As discussed, there is nothing wrong with pinning all-anon
    pages (resulting from breaking COW).

That could be easily be handled (allow any !VM_MAYSHARE), and would automatically be
handled once removing the iouring special-casing.


[1]

# ./pin_longterm
# [INFO] detected hugetlb size: 2048 KiB
# [INFO] detected hugetlb size: 1048576 KiB
TAP version 13
1..50
# [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd
ok 1 Pinning succeeded as expected
# [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with tmpfile
ok 2 Pinning succeeded as expected
# [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with local tmpfile
ok 3 Pinning failed as expected
# [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (2048 kB)
ok 4 # SKIP need more free huge pages
# [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (1048576 kB)
ok 5 Pinning succeeded as expected
# [RUN] R/W longterm GUP-fast pin in MAP_SHARED file mapping ... with memfd
ok 6 Pinning succeeded as expected
# [RUN] R/W longterm GUP-fast pin in MAP_SHARED file mapping ... with tmpfile
ok 7 Pinning succeeded as expected
# [RUN] R/W longterm GUP-fast pin in MAP_SHARED file mapping ... with local tmpfile
ok 8 Pinning failed as expected
# [RUN] R/W longterm GUP-fast pin in MAP_SHARED file mapping ... with memfd hugetlb (2048 kB)
ok 9 # SKIP need more free huge pages
# [RUN] R/W longterm GUP-fast pin in MAP_SHARED file mapping ... with memfd hugetlb (1048576 kB)
ok 10 Pinning succeeded as expected
# [RUN] R/O longterm GUP pin in MAP_SHARED file mapping ... with memfd
ok 11 Pinning succeeded as expected
# [RUN] R/O longterm GUP pin in MAP_SHARED file mapping ... with tmpfile
ok 12 Pinning succeeded as expected
# [RUN] R/O longterm GUP pin in MAP_SHARED file mapping ... with local tmpfile
ok 13 Pinning succeeded as expected
# [RUN] R/O longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (2048 kB)
ok 14 # SKIP need more free huge pages
# [RUN] R/O longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (1048576 kB)
ok 15 Pinning succeeded as expected
# [RUN] R/O longterm GUP-fast pin in MAP_SHARED file mapping ... with memfd
ok 16 Pinning succeeded as expected
# [RUN] R/O longterm GUP-fast pin in MAP_SHARED file mapping ... with tmpfile
ok 17 Pinning succeeded as expected
# [RUN] R/O longterm GUP-fast pin in MAP_SHARED file mapping ... with local tmpfile
ok 18 Pinning succeeded as expected
# [RUN] R/O longterm GUP-fast pin in MAP_SHARED file mapping ... with memfd hugetlb (2048 kB)
ok 19 # SKIP need more free huge pages
# [RUN] R/O longterm GUP-fast pin in MAP_SHARED file mapping ... with memfd hugetlb (1048576 kB)
ok 20 Pinning succeeded as expected
# [RUN] R/W longterm GUP pin in MAP_PRIVATE file mapping ... with memfd
ok 21 Pinning succeeded as expected
# [RUN] R/W longterm GUP pin in MAP_PRIVATE file mapping ... with tmpfile
ok 22 Pinning succeeded as expected
# [RUN] R/W longterm GUP pin in MAP_PRIVATE file mapping ... with local tmpfile
ok 23 Pinning succeeded as expected
# [RUN] R/W longterm GUP pin in MAP_PRIVATE file mapping ... with memfd hugetlb (2048 kB)
ok 24 # SKIP need more free huge pages
# [RUN] R/W longterm GUP pin in MAP_PRIVATE file mapping ... with memfd hugetlb (1048576 kB)
ok 25 Pinning succeeded as expected
# [RUN] R/W longterm GUP-fast pin in MAP_PRIVATE file mapping ... with memfd
ok 26 Pinning succeeded as expected
# [RUN] R/W longterm GUP-fast pin in MAP_PRIVATE file mapping ... with tmpfile
ok 27 Pinning succeeded as expected
# [RUN] R/W longterm GUP-fast pin in MAP_PRIVATE file mapping ... with local tmpfile
ok 28 Pinning succeeded as expected
# [RUN] R/W longterm GUP-fast pin in MAP_PRIVATE file mapping ... with memfd hugetlb (2048 kB)
ok 29 # SKIP need more free huge pages
# [RUN] R/W longterm GUP-fast pin in MAP_PRIVATE file mapping ... with memfd hugetlb (1048576 kB)
ok 30 Pinning succeeded as expected
# [RUN] R/O longterm GUP pin in MAP_PRIVATE file mapping ... with memfd
ok 31 Pinning succeeded as expected
# [RUN] R/O longterm GUP pin in MAP_PRIVATE file mapping ... with tmpfile
ok 32 Pinning succeeded as expected
# [RUN] R/O longterm GUP pin in MAP_PRIVATE file mapping ... with local tmpfile
ok 33 Pinning succeeded as expected
# [RUN] R/O longterm GUP pin in MAP_PRIVATE file mapping ... with memfd hugetlb (2048 kB)
ok 34 # SKIP need more free huge pages
# [RUN] R/O longterm GUP pin in MAP_PRIVATE file mapping ... with memfd hugetlb (1048576 kB)
ok 35 Pinning succeeded as expected
# [RUN] R/O longterm GUP-fast pin in MAP_PRIVATE file mapping ... with memfd
ok 36 Pinning succeeded as expected
# [RUN] R/O longterm GUP-fast pin in MAP_PRIVATE file mapping ... with tmpfile
ok 37 Pinning succeeded as expected
# [RUN] R/O longterm GUP-fast pin in MAP_PRIVATE file mapping ... with local tmpfile
ok 38 Pinning succeeded as expected
# [RUN] R/O longterm GUP-fast pin in MAP_PRIVATE file mapping ... with memfd hugetlb (2048 kB)
ok 39 # SKIP need more free huge pages
# [RUN] R/O longterm GUP-fast pin in MAP_PRIVATE file mapping ... with memfd hugetlb (1048576 kB)
ok 40 Pinning succeeded as expected
# [RUN] iouring fixed buffer with MAP_SHARED file mapping ... with memfd
ok 41 Pinning succeeded as expected
# [RUN] iouring fixed buffer with MAP_SHARED file mapping ... with tmpfile
ok 42 Pinning succeeded as expected
# [RUN] iouring fixed buffer with MAP_SHARED file mapping ... with local tmpfile
ok 43 Pinning failed as expected
# [RUN] iouring fixed buffer with MAP_SHARED file mapping ... with memfd hugetlb (2048 kB)
ok 44 # SKIP need more free huge pages
# [RUN] iouring fixed buffer with MAP_SHARED file mapping ... with memfd hugetlb (1048576 kB)
ok 45 Pinning succeeded as expected
# [RUN] iouring fixed buffer with MAP_PRIVATE file mapping ... with memfd
ok 46 Pinning succeeded as expected
# [RUN] iouring fixed buffer with MAP_PRIVATE file mapping ... with tmpfile
ok 47 Pinning succeeded as expected
# [RUN] iouring fixed buffer with MAP_PRIVATE file mapping ... with local tmpfile
not ok 48 Pinning failed as expected
# [RUN] iouring fixed buffer with MAP_PRIVATE file mapping ... with memfd hugetlb (2048 kB)
ok 49 # SKIP need more free huge pages
# [RUN] iouring fixed buffer with MAP_PRIVATE file mapping ... with memfd hugetlb (1048576 kB)
ok 50 Pinning succeeded as expected
Bail out! 1 out of 50 tests failed
# Totals: pass:39 fail:1 xfail:0 xpass:0 skip:10 error:0


--
Thanks,

David / dhildenb