[PATCH 7.2 70/88] keys: Avoid the owner account dereference in named keyring lookup

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

7.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Cen Zhang <zzzccc427@gmail.com>

commit 51409aaa01f8602552c336009fd22b226b0cdf1d upstream.

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

Cc: stable@vger.kernel.org
Fixes: 2ea190d0a006 ("keys: skip keys from another user namespace")
Assisted-by: LLM
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/pm-key-management-objects-candidate-0002-v3-2baa8cad2bb554364e10@gmail.com
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 security/keys/keyring.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -1158,7 +1158,7 @@ struct key *find_keyring_by_name(const c
 	 * grants Search permission and that hasn't been revoked
 	 */
 	list_for_each_entry(keyring, &ns->keyring_name_list, name_link) {
-		if (!kuid_has_mapping(ns, keyring->user->uid))
+		if (!kuid_has_mapping(ns, keyring->uid))
 			continue;
 
 		if (test_bit(KEY_FLAG_REVOKED, &keyring->flags))