[PATCH 4.15 198/202] devpts: fix error handling in devpts_mntget()

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

4.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Biggers <ebiggers@xxxxxxxxxx>

commit c9cc8d01fb04117928830449388512a5047569c9 upstream.

If devpts_ptmx_path() returns an error code, then devpts_mntget()
dereferences an ERR_PTR():

    BUG: unable to handle kernel paging request at fffffffffffffff5
    IP: devpts_mntget+0x13f/0x280 fs/devpts/inode.c:173

Fix it by returning early in the error paths.

Reproducer:

    #define _GNU_SOURCE
    #include <fcntl.h>
    #include <sched.h>
    #include <sys/ioctl.h>
    #define TIOCGPTPEER _IO('T', 0x41)

    int main()
    {
        for (;;) {
            int fd = open("/dev/ptmx", 0);
            unshare(CLONE_NEWNS);
            ioctl(fd, TIOCGPTPEER, 0);
        }
    }

Fixes: 311fc65c9fb9 ("pty: Repair TIOCGPTPEER")
Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/devpts/inode.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -168,11 +168,11 @@ struct vfsmount *devpts_mntget(struct fi
 	dput(path.dentry);
 	if (err) {
 		mntput(path.mnt);
-		path.mnt = ERR_PTR(err);
+		return ERR_PTR(err);
 	}
 	if (DEVPTS_SB(path.mnt->mnt_sb) != fsi) {
 		mntput(path.mnt);
-		path.mnt = ERR_PTR(-ENODEV);
+		return ERR_PTR(-ENODEV);
 	}
 	return path.mnt;
 }