keyctl_chown_key() replaces key->user under key->sem and drops the reference to the previous owner after releasing the semaphore. Readers which do not hold that semaphore can still be using the previous owner when key_user_put() frees it. For example, namespace filtering in /proc/keys can race with chown: /proc/keys reader keyctl_chown_key() user = key->user key->user = newowner key_user_put(old) kfree(old) read user->uid An earlier instrumented run reported: BUG: KASAN: slab-use-after-free in proc_keys_start+0x353/0x440 Read of size 4 at addr ffff8881128abbc0 by task poc/88 Call Trace: proc_keys_start+0x353/0x440 seq_read_iter+0x25d/0x1190 proc_reg_read_iter+0x19e/0x260 Allocated by task 86: key_user_lookup+0x1b4/0x540 keyctl_chown_key+0x3cc/0xbf0 Freed by task 87: kfree+0x149/0x330 keyctl_chown_key+0x7b0/0xbf0 Take key_user_lock explicitly while /proc/keys and find_keyring_by_name() copy the owner's UID. find_ge_key() is the initial/seek lookup reached by the reported proc_keys_start() fault; key_serial_next() applies the same UID filter to subsequent entries. find_keyring_by_name() reads the same replaceable owner when looking up named keyrings. key_serial_lock and keyring_name_lock protect these traversals but do not exclude chown. Taking key->sem would sleep under those spinlocks. The embedded key->user->lock cannot protect the dereference needed to acquire it. key_user_put() already holds key_user_lock for the final reference decrement and tree removal. A reader selecting an owner before removal blocks its final release until the UID copy finishes. After removal, the lock and reference-count ordering make the preceding owner replacement visible, including when another task drops the final reference. Mark the pointer loads with READ_ONCE() and the chown store with WRITE_ONCE(); the existing lifetime locking provides the protection. Retain the quota-owner UID rather than substituting key->uid, since they may differ for thread keyrings. Other owner accesses already have lifetime protection. key_alloc() uses a referenced owner before publishing the key. Final key_put() cannot overlap chown, which holds a key reference. key_gc_unused_keys() handles unreferenced keys: serial-tree removal excludes /proc/keys readers, and keyring_destroy() removes named keyrings under keyring_name_lock before the owner reference is dropped. /proc/key-users already holds key_user_lock during traversal. Quota reservation and instantiation need the separate accounting serialization. Fixes: 454804ab0302 ("keys: make procfiles per-user-namespace") Fixes: 2ea190d0a006 ("keys: skip keys from another user namespace") Cc: stable@vger.kernel.org Assisted-by: GPT-6-Astra Signed-off-by: Chengfeng Ye --- Changes in v4: - Move the quota-reservation and instantiation scopes from v3 1/2 to the complete accounting fix in 2/2; retain the combined lock coverage. - Isolate the namespace-reader lifetime fix and make it independent of the accounting patch by relying on existing lookup/final-release locking. - Pair READ_ONCE() pointer snapshots with WRITE_ONCE() in chown; explain lifetime ordering and preserve the quota-owner UID filtering semantics. - Include analysis of of the other owner access sites in the changelog v3: https://lore.kernel.org/r/20260927162528.943886-2-nicoyip.dev@gmail.com/ v2: https://lore.kernel.org/r/20260904080940.575882-1-nicoyip.dev@gmail.com/ security/keys/keyctl.c | 2 +- security/keys/keyring.c | 7 ++++++- security/keys/proc.c | 15 +++++++++++++-- 3 files changed, 20 insertions(+), 4 deletions(-) diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c index d14ace88e529c..c89a4d6efed40 100644 --- a/security/keys/keyctl.c +++ b/security/keys/keyctl.c @@ -1039,3 +1039,3 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group) zapowner = key->user; - key->user = newowner; + WRITE_ONCE(key->user, newowner); key->uid = uid; diff --git a/security/keys/keyring.c b/security/keys/keyring.c index 15bf4af8f2821..4a13749bc54cd 100644 --- a/security/keys/keyring.c +++ b/security/keys/keyring.c @@ -1148,6 +1148,7 @@ struct key *find_keyring_by_name(const char *name, bool uid_keyring) { struct user_namespace *ns = current_user_ns(); struct key *keyring; + kuid_t uid; if (!name) return ERR_PTR(-EINVAL); @@ -1158,7 +1159,11 @@ 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)) + spin_lock(&key_user_lock); + uid = READ_ONCE(keyring->user)->uid; + spin_unlock(&key_user_lock); + + if (!kuid_has_mapping(ns, uid)) continue; if (test_bit(KEY_FLAG_REVOKED, &keyring->flags)) diff --git a/security/keys/proc.c b/security/keys/proc.c index 4f4e2c1824f18..aa42ea4becec7 100644 --- a/security/keys/proc.c +++ b/security/keys/proc.c @@ -68,7 +68,13 @@ 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)) + kuid_t uid; + + spin_lock(&key_user_lock); + uid = READ_ONCE(key->user)->uid; + spin_unlock(&key_user_lock); + + if (kuid_has_mapping(user_ns, uid)) break; n = rb_next(n); } @@ -80,6 +86,7 @@ static struct key *find_ge_key(struct seq_file *p, key_serial_t id) struct user_namespace *user_ns = seq_user_ns(p); struct rb_node *n = key_serial_tree.rb_node; struct key *minkey = NULL; + kuid_t uid; while (n) { struct key *key = rb_entry(n, struct key, serial_node); @@ -100,7 +107,11 @@ 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)) + spin_lock(&key_user_lock); + uid = READ_ONCE(minkey->user)->uid; + spin_unlock(&key_user_lock); + + if (kuid_has_mapping(user_ns, uid)) return minkey; n = rb_next(&minkey->serial_node); if (!n) -- 2.43.0