[PATCH v2 0/3] permit write-sealed memfd read-only shared mappings

[Lorenzo Stoakes <lstoakes@gmail.com>]

The man page for fcntl() describing memfd file seals states the following
about F_SEAL_WRITE:-

    Furthermore, trying to create new shared, writable memory-mappings via
    mmap(2) will also fail with EPERM.

With emphasis on _writable_. In turns out in fact that currently the kernel
simply disallows _all_ new shared memory mappings for a memfd with
F_SEAL_WRITE applied, rendering this documentation inaccurate.

This matters because users are therefore unable to obtain a shared mapping
to a memfd after write sealing altogether, which limits their
usefulness. This was reported in the discussion thread [1] originating from
a bug report [2].

This is a product of both using the struct address_space->i_mmap_writable
atomic counter to determine whether writing may be permitted, and the
kernel adjusting this counter when _any_ VM_SHARED mapping is performed.

It seems sensible that we should only update this mapping if VM_MAYWRITE is
specified, i.e. whether it is possible that this mapping could at any point
be written to.

If we do so then all we need to do to permit write seals to function as
documented is to clear VM_MAYWRITE when mapping read-only. It turns out
this functionality already exists for F_SEAL_FUTURE_WRITE - we can
therefore simply adapt this logic to do the same for F_SEAL_WRITE.

The final change required is to invoke call_mmap() before
mapping_map_writable() - doing so ensures that the memfd-relevant
shmem_mmap() or hugetlbfs_file_mmap() custom mmap handlers will be called
before the writable test, enabling us to clear VM_MAYWRITE first.

Thanks to Andy Lutomirski for the suggestion!

[1]:https://lore.kernel.org/all/20230324133646.16101dfa666f253c4715d965@linux-foundation.org/
[2]:https://bugzilla.kernel.org/show_bug.cgi?id=217238

v2:
- Removed RFC tag.
- Correct incorrect goto pointed out by Jan.
- Reworded cover letter as suggested by Jan.

v1:
https://lore.kernel.org/all/cover.1680560277.git.lstoakes@gmail.com/

Lorenzo Stoakes (3):
  mm: drop the assumption that VM_SHARED always implies writable
  mm: update seal_check_[future_]write() to include F_SEAL_WRITE as well
  mm: perform the mapping_map_writable() check after call_mmap()

 fs/hugetlbfs/inode.c |  2 +-
 include/linux/fs.h   |  4 ++--
 include/linux/mm.h   | 24 ++++++++++++++++++------
 kernel/fork.c        |  2 +-
 mm/filemap.c         |  2 +-
 mm/madvise.c         |  2 +-
 mm/mmap.c            | 22 +++++++++++-----------
 mm/shmem.c           |  2 +-
 8 files changed, 36 insertions(+), 24 deletions(-)

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2.40.1