[PATCH] keys: Fix key_user use-after-free during ownership changes

[Chengfeng Ye <nicoyip.dev@gmail.com>]

keyctl_chown_key() replaces key->user while holding key->sem and drops
the old key_user reference after releasing the semaphore. The /proc/keys
iterators and find_keyring_by_name() instead dereference key->user while
holding unrelated locks.

This allows the following interleaving:

  CPU 0 (/proc/keys)            CPU 1 (KEYCTL_CHOWN)
  load old key->user
                                replace key->user
                                key_user_put(old)
                                  kfree(old)
  read old->uid

Serialize the namespace-mapping reads and the pointer replacement with
key_user_lock. This lock already protects final key_user removal, so the
old object cannot be freed while its uid is being read. Keep reading the
quota-owner UID rather than key->uid because those values can legitimately
differ for thread keyrings.

Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
 security/keys/internal.h | 10 ++++++++++
 security/keys/keyctl.c   |  2 ++
 security/keys/keyring.c  |  2 +-
 security/keys/proc.c     |  4 ++--
 4 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/security/keys/internal.h b/security/keys/internal.h
index b7b622bc36a1..741d547ba5c4 100644
--- a/security/keys/internal.h
+++ b/security/keys/internal.h
@@ -70,6 +70,16 @@ extern struct key_user	root_key_user;
 extern struct key_user *key_user_lookup(kuid_t uid);
 extern void key_user_put(struct key_user *user);
 
+static inline kuid_t key_user_uid(const struct key *key)
+{
+	kuid_t uid;
+
+	spin_lock(&key_user_lock);
+	uid = key->user->uid;
+	spin_unlock(&key_user_lock);
+	return uid;
+}
+
 /*
  * Key quota limits.
  * - root has its own separate limits to everyone else
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index d14ace88e529..c17924609317 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -1036,8 +1036,10 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
 			atomic_inc(&newowner->nikeys);
 		}
 
+		spin_lock(&key_user_lock);
 		zapowner = key->user;
 		key->user = newowner;
+		spin_unlock(&key_user_lock);
 		key->uid = uid;
 	}
 
diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index 15bf4af8f282..49f4be934525 100644
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -1158,7 +1158,7 @@ struct key *find_keyring_by_name(const char *name, bool uid_keyring)
 	 * 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, key_user_uid(keyring)))
 			continue;
 
 		if (test_bit(KEY_FLAG_REVOKED, &keyring->flags))
diff --git a/security/keys/proc.c b/security/keys/proc.c
index 4f4e2c1824f1..8d6d26652aab 100644
--- a/security/keys/proc.c
+++ b/security/keys/proc.c
@@ -68,7 +68,7 @@ static struct rb_node *key_serial_next(struct seq_file *p, struct rb_node *n)
 	n = rb_next(n);
 	while (n) {
 		struct key *key = rb_entry(n, struct key, serial_node);
-		if (kuid_has_mapping(user_ns, key->user->uid))
+		if (kuid_has_mapping(user_ns, key_user_uid(key)))
 			break;
 		n = rb_next(n);
 	}
@@ -100,7 +100,7 @@ static struct key *find_ge_key(struct seq_file *p, key_serial_t id)
 		return NULL;
 
 	for (;;) {
-		if (kuid_has_mapping(user_ns, minkey->user->uid))
+		if (kuid_has_mapping(user_ns, key_user_uid(minkey)))
 			return minkey;
 		n = rb_next(&minkey->serial_node);
 		if (!n)
-- 
2.43.0