[PATCH] hugetlbfs: enforce F_SEAL_EXEC in setattr

[Eva Kurchatova]

Commit 6fd735518263 ('mm/memfd: add F_SEAL_EXEC') added the F_SEAL_EXEC
seal to memfd, with the corresponding enforcement implemented in
shmem_setattr(): once F_SEAL_EXEC is set, fchmod() of any of the execute
bits (S_IXUSR|S_IXGRP|S_IXOTH) is rejected with -EPERM.

The hugetlbfs equivalent in hugetlbfs_setattr() was missed, even though
hugetlbfs memfd files carry the same 'info->seals' bitmap and the same
F_SEAL_EXEC bit can be added through memfd_create(... MFD_HUGETLB ...)
plus fcntl(F_ADD_SEALS, F_SEAL_EXEC). As a result, the seal is silently
ignored on hugetlbfs memfds and fchmod() may turn execute bits back on
after the seal has been applied.

Mirror the shmem check in hugetlbfs_setattr() so that ATTR_MODE changes
that flip any execute bit on a sealed inode return -EPERM. This also
fixes the memfd_test 'hugetlbfs' SEAL-EXEC subtest, which previously
aborted on:

    fchmod(/memfd:kern_memfd_seal_exec (deleted), 00777) didn't fail as expected

Fixes: 6fd735518263 ("mm/memfd: add F_SEAL_EXEC")
Signed-off-by: Eva Kurchatova <eva.kurchatova@xxxxxxxxxxxxx>
---
 fs/hugetlbfs/inode.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 216e1a0dd0b2..812bf8e21b91 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -841,6 +841,11 @@ static int hugetlbfs_setattr(struct mnt_idmap *idmap,
 	if (error)
 		return error;
 
+	if ((info->seals & F_SEAL_EXEC) && (ia_valid & ATTR_MODE)) {
+		if ((inode->i_mode ^ attr->ia_mode) & 0111)
+			return -EPERM;
+	}
+
 	trace_hugetlbfs_setattr(inode, dentry, attr);
 
 	if (ia_valid & ATTR_SIZE) {
-- 
2.54.0