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

[Chengfeng Ye]

On Mon, Aug 24, 2026 at 1:04 AM Chengfeng Ye <nicoyip.dev at gmail.com> wrote:
>
> 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.
>
> Fixes: 454804ab0302 ("keys: make procfiles per-user-namespace")
> Signed-off-by: Chengfeng Ye <nicoyip.dev at 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
>

Sorry for mistakenly sending this patch twice.

Best,
Chengfeng