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author | Christian Brauner <christian.brauner@ubuntu.com> | 2018-03-13 19:55:25 +0300 |
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committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2018-03-14 15:31:23 +0300 |
commit | a319b01d9095da6f6c54bd20c1f1300762506255 (patch) | |
tree | 4b9529b83936881d4ac2f41719c31a2a6a26f29d /fs | |
parent | 7d71109df186d630a41280670c8d71d0cf9b0da9 (diff) | |
download | linux-a319b01d9095da6f6c54bd20c1f1300762506255.tar.xz |
devpts: resolve devpts bind-mounts
Most libcs will still look at /dev/ptmx when opening the master fd of a 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 /. A very simply reproducer for this issue presupposing a libc
that uses TIOCGPTPEER in its openpty() implementation is:
unshare --mount
mount --bind /dev/pts/ptmx /dev/ptmx
chmod 666 /dev/ptmx
script
ls -al /proc/self/fd/0
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.
The reason for the current failure is that the kernel tries to verify the
useability of the devpts filesystem without resolving the /dev/ptmx
bind-mount first. This will lead it to detect that the dentry is escaping
its bind-mount. The reason is that while the devpts filesystem mounted at
/dev/pts has the devtmpfs mounted at /dev as its parent mount:
21 -- -- / /dev
-- 21 -- / /dev/pts
devtmpfs and devpts are on different devices
-- -- 0:6 / /dev
-- -- 0:20 / /dev/pts
This has the consequence that the pathname of the parent directory of the
devpts filesystem mount at /dev/pts is /. So if /dev/ptmx is a bind-mount
of /dev/pts/ptmx then the /dev/ptmx bind-mount and the devpts mount at
/dev/pts will end up being located on the same device which is recorded in
the superblock of their vfsmount. This means the parent directory of the
/dev/ptmx bind-mount will be /ptmx:
-- -- ---- /ptmx /dev/ptmx
Without the bind-mount resolution patch the kernel will now perform the
bind-mount escape check directly on /dev/ptmx. The function responsible for
this is devpts_ptmx_path() which calls pts_path() which in turn calls
path_parent_directory(). Based on the above explanation,
path_parent_directory() will yield / as the parent directory for the
/dev/ptmx bind-mount and not the expected /dev. Thus, the kernel detects
that /dev/ptmx is escaping its bind-mount and will set /proc/<pid>/fd/<nr>
to /.
This patch changes the logic to first resolve any bind-mounts. After the
bind-mounts have been resolved (i.e. we have traced it back to the
associated devpts mount) devpts_ptmx_path() can be called. In order to
guarantee correct path generation for the slave file descriptor the kernel
now requires that a pts directory is found in the parent directory of the
ptmx bind-mount. This implies that when doing bind-mounts the ptmx
bind-mount and the devpts mount should have a common parent directory. A
valid example is:
mount -t devpts devpts /dev/pts
mount --bind /dev/pts/ptmx /dev/ptmx
an invalid example is:
mount -t devpts devpts /dev/pts
mount --bind /dev/pts/ptmx /ptmx
This allows us to support:
- calling open on ptmx devices located inside non-standard devpts mounts:
mount -t devpts devpts /mnt
master = open("/mnt/ptmx", ...);
slave = ioctl(master, TIOCGPTPEER, ...);
- calling open on ptmx devices located outside the devpts mount with a
common ancestor directory:
mount -t devpts devpts /dev/pts
mount --bind /dev/pts/ptmx /dev/ptmx
master = open("/dev/ptmx", ...);
slave = ioctl(master, TIOCGPTPEER, ...);
while failing on ptmx devices located outside the devpts mount without a
common ancestor directory:
mount -t devpts devpts /dev/pts
mount --bind /dev/pts/ptmx /ptmx
master = open("/ptmx", ...);
slave = ioctl(master, TIOCGPTPEER, ...);
in which case save path generation cannot be guaranteed.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Suggested-by: Eric Biederman <ebiederm@xmission.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'fs')
-rw-r--r-- | fs/devpts/inode.c | 26 |
1 files changed, 16 insertions, 10 deletions
diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c index 71b901936113..542364bf923e 100644 --- a/fs/devpts/inode.c +++ b/fs/devpts/inode.c @@ -160,21 +160,27 @@ struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi) path = filp->f_path; path_get(&path); - /* Has the devpts filesystem already been found? */ - if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) + /* 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; + + /* devpts_ptmx_path() finds a devpts fs or returns an error. */ + if ((path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) || + (DEVPTS_SB(path.mnt->mnt_sb) != fsi)) err = devpts_ptmx_path(&path); dput(path.dentry); - if (err) { - mntput(path.mnt); - return ERR_PTR(err); - } + if (!err) { + if (DEVPTS_SB(path.mnt->mnt_sb) == fsi) + return path.mnt; - if (DEVPTS_SB(path.mnt->mnt_sb) != fsi) { - mntput(path.mnt); - return ERR_PTR(-ENODEV); + err = -ENODEV; } - return path.mnt; + mntput(path.mnt); + return ERR_PTR(err); } struct pts_fs_info *devpts_acquire(struct file *filp) |