Hey, We discovered a potential bug in the devpts implementation via TIOCGPTPEER ioctl()s today. We've tackled a similar problem already in:
commit 311fc65c9fb9c966bca8e6f3ff8132ce57344ab9 Author: Eric W. Biederman <ebied...@xmission.com> Date: Thu Aug 24 15:13:29 2017 -0500 pty: Repair TIOCGPTPEER Most libcs will *still* look at /dev/ptmx when opening the master fd of pty device. Usually, /dev/ptmx will nowadays be either a symlink to /dev/pts/ptmx or it will be a second device node with permissions 666 whereas /dev/pts/ptmx will usually have permissions 000. Afaik, we've also always supported making /dev/ptmx a bind-mount to /dev/pts/ptmx or at least I haven't observed any issues with this so far and it's something fairly common in containers. So in short, it should be legal to do: mount --bind /dev/pts/ptmx /dev/ptmx chmod 666 /dev/ptmx However, for any libc implementation or program that uses TIOCGPTPEER the /proc/<pid>/fd/{0,1,2} symlinks are broken (currently affects at least glibc 2.27) with bind-mounts of /dev/pts/ptmx to /dev/ptmx. A quick reproducer is: unshare --mount mount --bind /dev/pts/ptmx /dev/ptmx chmod 666 /dev/ptmx script ls -al /proc/self/fd/0 Let's assume the slave device index I received was 5 then I would expect to see: ls -al /proc/self/fd/0 lrwx------ 1 chb chb 64 Mar 7 16:41 /proc/self/fd/0 -> /dev/pts/5 But what I actually see is: ls -al /proc/self/fd/0 lrwx------ 1 chb chb 64 Mar 7 16:41 /proc/self/fd/0 -> / I think the explanation for this is fairly straightforward. When userspace does: master = open("/dev/ptmx", O_RDWR | O_NOCTTY); slave = ioctl(master, TIOCGPTPEER, O_RDWR | O_NOCTTY); and /dev/ptmx is a bind-mount of /dev/pts/ptmx looking up the root mount of the dentry for the slave it appears to the kernel as if the dentry is escaping it's bind-mount: ├─/dev udev devtmpfs rw,nosuid,relatime,size=4001260k,nr_inodes=1000315,mode=755 │ ├─/dev/pts devpts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 │ └─/dev/ptmx devpts[/ptmx] devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 since the root mount of the dentry is /dev/pts but the root mount of /dev/ptmx is /dev if I'm correct so similar to what Linus pointed out in a previous discussion (see [1]) before. So we still record the "wrong" vfsmount when /dev/ptmx is a bind-mount and then hit the problem when we call devpts_mntget() in drivers/tty/pty.c. So I thought about this and - in case my analysis is correct - the solution didn't seem obvious to me as a bind-mount has no concept of what it's "parent" is (Which in this case should be the devpts mount at /dev/pts.). Christian [1]: https://lkml.org/lkml/2017/8/23/826