devpts: fix error handling in devpts_mntget()
authorEric Biggers <ebiggers@google.com>
Wed, 31 Jan 2018 08:49:18 +0000 (00:49 -0800)
committerLinus Torvalds <torvalds@linux-foundation.org>
Wed, 31 Jan 2018 16:48:37 +0000 (08:48 -0800)
commitc9cc8d01fb04117928830449388512a5047569c9
treeb7e97b68733d1ee57025aed05373041d5912853f
parentc0cef30e4ff0dc025f4a1660b8f0ba43ed58426e
devpts: fix error handling in devpts_mntget()

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@googlegroups.com>
Cc: <stable@vger.kernel.org> # v4.13+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
fs/devpts/inode.c