vfs: fsmount: add missing mntget()

sys_fsmount() needs to take a reference to the new mount when adding it
to the anonymous mount namespace.  Otherwise the filesystem can be
unmounted while it's still in use, as found by syzkaller.

Reported-by: Mark Rutland <[email protected]>
Reported-by: [email protected]
Reported-by: [email protected]
Fixes: 93766fbd26 ("vfs: syscall: Add fsmount() to create a mount for a superblock")
Signed-off-by: Eric Biggers <[email protected]>
Signed-off-by: Al Viro <[email protected]>

fs/namespace.c

@@ -3445,6 +3445,7 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags,
 	ns->root = mnt;
 	ns->mounts = 1;
 	list_add(&mnt->mnt_list, &ns->list);
+	mntget(newmount.mnt);
 
 	/* Attach to an apparent O_PATH fd with a note that we need to unmount
 	 * it, not just simply put it.