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

In order for an F_SEAL_WRITE sealed memfd mapping to have an opportunity
to clear VM_MAYWRITE in seal_check_write() we must be able to invoke
either the shmem_mmap() or hugetlbfs_file_mmap() f_ops->mmap() handler to
do so.

We would otherwise fail the mapping_map_writable() check before we had the
opportunity to clear VM_MAYWRITE.

However, the existing logic in mmap_region() performs this check BEFORE
calling call_mmap() (which invokes file->f_ops->mmap()).  We must enforce
this check AFTER the function call.

In order to avoid any risk of breaking call_mmap() handlers which assume
this will have been done first, we continue to mark the file writable
first, simply deferring enforcement of it failing until afterwards.

This enables mmap(..., PROT_READ, MAP_SHARED, fd, 0) mappings for memfd's
sealed via F_SEAL_WRITE to succeed, whereas previously they were not
permitted.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238
Link: https://lkml.kernel.org/r/d2748bc4077b53c60bcb06fccaf976cb2afee345.1696709413.git.lstoakes@gmail.com


Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
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: Jan Kara <jack@suse.cz>
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>