dcache: don't put a mountpoint on a dentry that's being removed

Overview

In the Linux kernel, the following vulnerability has been resolved:

dcache: don't put a mountpoint on a dentry that's being removed

rmdir(), unlink() and rename() call dont_mount() on the victim and then detach_mounts() with the victim's inode locked. do_lock_mount() takes the inode lock of the mountpoint and checks cant_mount() so a mount can't show up after detach_mounts().

But attach_recursive_mnt() makes a second mountpoint for the root of the source mount so that the mounts already located at the destination can be put on top of it. No inode is locked for that one and d_set_mounted() only refuses a dentry that is unlinked. Between detach_mounts() and d_delete() the victim is still hashed:

rmrace: b passed dont_mount() and detach_mounts(), sleeping T2: move_mount(S, /tmp/plcant/x, BENEATH) = 0 errno 0 () T1: rmdir(/tmp/plcant/d/b) = 0 errno 0 109 107 0:61 /d/b//deleted /tmp/plcant/x rw,relatime - tmpfs tmpfs rw 108 109 0:63 / /tmp/plcant/x rw,relatime - tmpfs T rw

A bind mount of the directory that's being removed is moved beneath an existing mount while the rmdir() is located between the two calls. The mount ends up on the removed directory and nothing will ever detach it.

Check cant_mount() in d_set_mounted() as well. dont_mount() raises the flag under d_lock before detach_mounts() runs so either the mountpoint is set first and detach_mounts() finds the mount or the flag is seen and the mount is refused.

Affected

• Linux >= 1064f874abc0d05eeed8993815f584d847b72486 < 5e6bfd9f249f2c74bb63c0f06ad959da3bee0971
• Linux >= 1064f874abc0d05eeed8993815f584d847b72486 < 793cb67b0f52d00c67bd0e76bc57455b5c70ebb8
• Linux >= 1064f874abc0d05eeed8993815f584d847b72486 < 390ddcea2a8aca411b931a8e2863a80ccca8d2b3
• Linux >= 1064f874abc0d05eeed8993815f584d847b72486 < 30a1625022411a387cce9239408ece9955bc9c58
• Linux >= 1064f874abc0d05eeed8993815f584d847b72486 < 75f4197f1e11d2eb75c2d6f916a7d4764db621e5
• Linux 5e6ef5ef6eaf690faad3a7c6c665ebf45d7b6ccf
• Linux 839d42687dfce0ed0ea2c6bd8d707cc0e276fbe7
• Linux 808e83e5add13152bd4a88346487de68395c136b
• Linux 6de9d08a9718ee9c12ba6343c50141c2d633e3dc
• Linux >= 4.1.40 < 4.2
• Linux >= 4.4.54 < 4.5
• Linux >= 4.9.15 < 4.10
• Linux >= 4.10.3 < 4.11
• Linux 4.11

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.