mm: perform the mapping_map_writable() check after call_mmap()

In order for a F_SEAL_WRITE sealed memfd mapping to have an opportunity to
clear VM_MAYWRITE, we must be able to invoke the appropriate
vm_ops->mmap() handler to do so.  We would otherwise fail the
mapping_map_writable() check before we had the opportunity to avoid it.

This patch moves this check after the call_mmap() invocation.  Only memfd
actively denies write access causing a potential failure here (in
memfd_add_seals()), so there should be no impact on non-memfd cases.

This patch makes the userland-visible change that MAP_SHARED, PROT_READ
mappings of an F_SEAL_WRITE sealed memfd mapping will now succeed.

There is a delicate situation with cleanup paths assuming that a writable
mapping must have occurred in circumstances where it may now not have.  In
order to ensure we do not accidentally mark a writable file unwritable by
mistake, we explicitly track whether we have a writable mapping and unmap
only if we do.

[lstoakes@gmail.com: do not set writable_file_mapping in inappropriate case]
  Link: https://lkml.kernel.org/r/c9eb4cc6-7db4-4c2b-838d-43a0b319a4f0@lucifer.local
Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238
Link: https://lkml.kernel.org/r/55e413d20678a1bb4c7cce889062bbb07b0df892.1697116581.git.lstoakes@gmail.com


Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>