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

[Lorenzo Stoakes <lstoakes@gmail.com>]

This patch series is in two parts:-

1. Currently there are a number of places in the kernel where we assume
   VM_SHARED implies that a mapping is writable. Let's be slightly less
   strict and relax this restriction in the case that VM_MAYWRITE is not
   set.

   This should have no noticeable impact as the lack of VM_MAYWRITE implies
   that the mapping can not be made writable via mprotect() or any other
   means.

2. Align the behaviour of F_SEAL_WRITE and F_SEAL_FUTURE_WRITE on mmap().
   The latter already clears the VM_MAYWRITE flag for a sealed read-only
   mapping, we simply extend this to F_SEAL_WRITE too.

   For this to have effect, we must also invoke call_mmap() before
   mapping_map_writable().

As this is quite a fundamental change on the assumptions around VM_SHARED
and since this causes a visible change to userland (in permitting read-only
shared mappings on F_SEAL_WRITE mappings), I am putting forward as an RFC
to see if there is anything terribly wrong with it.

I suspect even if the patch series as a whole is unpalatable, there are
probably things we can salvage from it in any case.

Thanks to Andy Lutomirski who inspired the series!

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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2.40.0