keys: Avoid the owner account dereference in named keyring lookup

Overview

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

keys: Avoid the owner account dereference in named keyring lookup

Named keyring lookup must keep the storage containing the owner UID alive through the namespace mapping check. find_keyring_by_name() reads keyring->user->uid under keyring_name_lock, but that lock protects the keyring's name entry and allocation, not its separate key_user.

A named session-keyring join can overlap a privileged KEYCTL_CHOWN on another CPU. If the keyring holds the last reference to its old dynamic key_user and the ownership transfer succeeds, the following ordering is possible:

Named join                         Chown
find_keyring_by_name()              keyctl_chown_key()
  read_lock(keyring_name_lock)       down_write(key->sem)
  load old keyring->user
                                     replace key->user and key->uid
                                     up_write(key->sem)
                                     key_put(key)
                                     key_user_put(old user): free
  read old user->uid
  read_unlock(keyring_name_lock)

Chown neither takes keyring_name_lock nor key_session_mutex, so it can free the old account between the pointer load and the UID read. The lookup then reads freed memory even though the keyring itself is alive.

Use keyring->uid for the mapping check. key_alloc() initializes this inline owner UID and keyctl_chown_key() updates it on chown. The existing name lock keeps its containing keyring allocated throughout the check, so lookup no longer depends on the account's lifetime. This also matches the owner UID used by the permission check.

KASAN report as below:

BUG: KASAN: slab-use-after-free in find_keyring_by_name+0x577/0x5d0
Read of size 4 at addr ffff8881128ae2ec by task keycase/500

Affected

• Linux >= 2ea190d0a006ce5218baa6e798512652446a605a < 77249d4b1bb5ad43552b7023c4a0978937aa7a56
• Linux >= 2ea190d0a006ce5218baa6e798512652446a605a < 0b865fa477d94ac96c26848aab1fd176397de3f6
• Linux >= 2ea190d0a006ce5218baa6e798512652446a605a < 6763329f673d261641c79212abf773af77b2c922
• Linux >= 2ea190d0a006ce5218baa6e798512652446a605a < aa03c8fb4334f14d218020f7431debf2b7766865
• Linux >= 2ea190d0a006ce5218baa6e798512652446a605a < c9664f780f861dd9f11f120c7a03ed0993671ebb
• Linux >= 2ea190d0a006ce5218baa6e798512652446a605a < b0830a26e2c3008a6e21863126b4a0e68b2761cd
• Linux >= 2ea190d0a006ce5218baa6e798512652446a605a < 5c6a9512887a6cc694d594d9c0b57c0841d215b5
• Linux >= 2ea190d0a006ce5218baa6e798512652446a605a < 40b12b93ae4ab9e22c3d571e9b09fd08afad53f6
• Linux >= 2ea190d0a006ce5218baa6e798512652446a605a < 51409aaa01f8602552c336009fd22b226b0cdf1d
• Linux 2.6.30

Remediation

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