Messages in this thread Patch in this message | | | From | Christian Brauner <> | Subject | [PATCH] devpts: resolve devpts bind-mounts | Date | Thu, 8 Mar 2018 18:00:51 +0100 |
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Most libcs will still look at /dev/ptmx when opening the master fd of pty device. When /dev/ptmx is a bind-mount of /dev/pts/ptmx and the TIOCGPTPEER ioctl() is used to safely retrieve a file descriptor for the slave side of the pty based on the master fd the /proc/self/fd/{0,1,2} symlinks will point to "/". When the kernel tries to look up the root mount of the dentry for the slave file descriptor it will detect that the dentry is escaping it's bind-mount since the root mount of the dentry is /dev/pts where the devpts is mounted but the root mount of /dev/ptmx is /dev. Having bind-mounts of /dev/pts/ptmx to /dev/ptmx not working correctly is a regression. In addition, it is also a fairly common scenario in containers employing user namespaces. To handle this situation correctly we walk up the bind-mounts for the /dev/ptmx file.
Here's a little reproducer that presupposes a libc that uses TIOCGPTPEER in its openpty() implementation:
unshare --mount mount --bind /dev/pts/ptmx /dev/ptmx chmod 666 /dev/ptmx script ls -al /proc/self/fd/0
with output:
lrwx------ 1 chb chb 64 Mar 7 16:41 /proc/self/fd/0 -> /
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com> Suggested-by: Eric Biederman <ebiederm@xmission.com> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> --- fs/devpts/inode.c | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+)
diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c index e31d6ed3ec32..4059e3e69d57 100644 --- a/fs/devpts/inode.c +++ b/fs/devpts/inode.c @@ -163,6 +163,26 @@ struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi) path = filp->f_path; path_get(&path); + if ((DEVPTS_SB(path.mnt->mnt_sb) == fsi) && + (path.mnt->mnt_root == fsi->ptmx_dentry)) { + /* Walk upward while the start point is a bind mount of a single + * file. + */ + while (path.mnt->mnt_root == path.dentry) + if (follow_up(&path) == 0) + break; + + /* Is this path a valid devpts filesystem? */ + err = devpts_ptmx_path(&path); + if (err == 0) { + dput(path.dentry); + return path.mnt; + } + + path_put(&path); + path = filp->f_path; + path_get(&path); + } err = devpts_ptmx_path(&path); dput(path.dentry); -- 2.15.1
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