mm/shmem: honour F_SEAL_EXEC when an ACL is set

The owner of a memfd sealed with F_SEAL_EXEC can still change its
execute bits by setting a POSIX access ACL. With CONFIG_TMPFS_POSIX_ACL,
this lets an unprivileged owner make a non-executable memfd executable,
bypassing vm.memfd_noexec=2.

simple_set_acl() updates i_mode directly, bypassing the seal check in
shmem_setattr(). Add shmem_set_acl() to check the resulting mode. Share
the F_SEAL_EXEC check with shmem_setattr() using memfd_check_seals_mode()
in include/linux/memfd.h.

Fix generated by an LLM and sent by Chris Mason in private.

Fixes: 6fd7353829ca ("mm/memfd: add F_SEAL_EXEC")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
Acked-by: Pedro Falcato <pfalcato@suse.de>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
[ david: use common helper; simplify description ]
Link: https://patch.msgid.link/20261029-fixes_shmem-v4-4-392fc715d8f6@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

0 files changed, 0 insertions, 0 deletions