[PATCH 10/18] fsmount: fix bypassing SB_MANDLOCK permission check

[Eric Biggers <ebiggers3@gmail.com>]

From: Eric Biggers <ebiggers@google.com>

fc->sb_flags can be modified up until fc->uapi_mutex is taken, so the
permission check for SB_MANDLOCK needs to happen under the mutex.

Also move the may_mount() check as early as possible.

Fixes: 0c65353ab9f5 ("vfs: Implement fsmount() to effect a pre-configured mount")
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/namespace.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 8ac9e8fb31c9f..7f0191bb5db46 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3237,6 +3237,9 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags, unsigned int, ms_flags
 	unsigned int mnt_flags = 0;
 	long ret;
 
+	if (!may_mount())
+		return -EPERM;
+
 	if ((flags & ~(FSMOUNT_CLOEXEC)) != 0)
 		return -EINVAL;
 
@@ -3275,11 +3278,6 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags, unsigned int, ms_flags
 
 	fc = f.file->private_data;
 
-	ret = -EPERM;
-	if (!may_mount() ||
-	    ((fc->sb_flags & SB_MANDLOCK) && !may_mandlock()))
-		goto err_fsfd;
-
 	/* There must be a valid superblock or we can't mount it */
 	ret = -EINVAL;
 	if (!fc->root)
@@ -3300,6 +3298,10 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags, unsigned int, ms_flags
 	if (fc->phase != FS_CONTEXT_AWAITING_MOUNT)
 		goto err_unlock;
 
+	ret = -EPERM;
+	if ((fc->sb_flags & SB_MANDLOCK) && !may_mandlock())
+		goto err_unlock;
+
 	newmount.mnt = vfs_create_mount(fc, mnt_flags);
 	if (IS_ERR(newmount.mnt)) {
 		ret = PTR_ERR(newmount.mnt);
-- 
2.18.0