[PATCH v3 1/2] keys: Protect key_user lifetime during ownership changes
Jarkko Sakkinen
jarkko at kernel.org
Mon Oct 5 00:58:57 UTC 2026
While preparing PR some changes in this patch started to bother me so
let's do a sanity check.
On Mon, Sep 28, 2026 at 12:25:27AM +0800, Chengfeng Ye wrote:
> 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
>
> Serialize pointer replacement and the affected readers with key_user_lock,
> which already protects final key_user removal. Take the lock explicitly
> while /proc/keys and find_keyring_by_name() copy the owner's UID. Retain
> the quota-owner UID rather than substituting key->uid, since they may
> differ for thread keyrings.
>
> Also hold the lock across owner accesses in key_payload_reserve() and
> across the instantiated-key count increments. Instantiation need not hold
> the target key's semaphore, so these paths need the same lifetime
> protection. Leave key-state publication and the chown accounting transfer
> outside the new critical sections for the separate accounting fix.
>
> Fixes: 5801649d8b83 ("[PATCH] keys: let keyctl_chown() change a key's owner")
> Cc: stable at vger.kernel.org
> Signed-off-by: Chengfeng Ye <nicoyip.dev at gmail.com>
> ---
> Changes in v3:
> - Split from v2 as patch 1/2; see the cover letter for the full split.
> - Rebase onto current mainline and retain explicit reader-side locking.
>
> v2: https://lore.kernel.org/r/20260904080940.575882-1-nicoyip.dev@gmail.com/
>
> security/keys/key.c | 13 +++++++++++--
> security/keys/keyctl.c | 2 ++
> security/keys/keyring.c | 7 ++++++-
> security/keys/proc.c | 15 +++++++++++++--
> 4 files changed, 32 insertions(+), 5 deletions(-)
>
> diff --git a/security/keys/key.c b/security/keys/key.c
> index b34a64d81d47..d0d583194b05 100644
> --- a/security/keys/key.c
> +++ b/security/keys/key.c
> @@ -21,6 +21,7 @@ struct rb_root key_serial_tree; /* tree of keys indexed by serial */
> DEFINE_SPINLOCK(key_serial_lock);
>
> struct rb_root key_user_tree; /* tree of quota records indexed by UID */
> +/* Protects key_user_tree and key ownership changes. */
This type of extras in bug fixes do not make any possible sense and set
an extra backporting cost.
> DEFINE_SPINLOCK(key_user_lock);
>
> unsigned int key_quota_root_maxkeys = 1000000; /* root's key count quota */
> @@ -380,9 +381,12 @@ int key_payload_reserve(struct key *key, size_t datalen)
>
> /* contemplate the quota adjustment */
> if (delta != 0 && test_bit(KEY_FLAG_IN_QUOTA, &key->flags)) {
> - unsigned maxbytes = uid_eq(key->user->uid, GLOBAL_ROOT_UID) ?
> - key_quota_root_maxbytes : key_quota_maxbytes;
> unsigned long flags;
> + unsigned int maxbytes;
> +
> + spin_lock(&key_user_lock);
> + maxbytes = uid_eq(key->user->uid, GLOBAL_ROOT_UID) ?
> + key_quota_root_maxbytes : key_quota_maxbytes;
>
> spin_lock_irqsave(&key->user->lock, flags);
>
> @@ -396,6 +400,7 @@ int key_payload_reserve(struct key *key, size_t datalen)
> key->quotalen += delta;
> }
> spin_unlock_irqrestore(&key->user->lock, flags);
> + spin_unlock(&key_user_lock);
I don't understand the necessity of key_user_lock here.
> }
>
> /* change the recorded data length if that didn't generate an error */
> @@ -447,7 +452,9 @@ static int __key_instantiate_and_link(struct key *key,
>
> if (ret == 0) {
> /* mark the key as being instantiated */
> + spin_lock(&key_user_lock);
> atomic_inc(&key->user->nikeys);
> + spin_unlock(&key_user_lock);
> mark_key_instantiated(key, 0);
> notify_key(key, NOTIFY_KEY_INSTANTIATED, 0);
>
> @@ -604,7 +611,9 @@ int key_reject_and_link(struct key *key,
> /* can't instantiate twice */
> if (key->state == KEY_IS_UNINSTANTIATED) {
> /* mark the key as being negatively instantiated */
> + spin_lock(&key_user_lock);
> atomic_inc(&key->user->nikeys);
> + spin_unlock(&key_user_lock);
atomic_inc() is wrapped under spinlock in the two changes above. Why
is that necessary?
> mark_key_instantiated(key, -error);
> notify_key(key, NOTIFY_KEY_INSTANTIATED, -error);
> key_set_expiry(key, ktime_get_real_seconds() + timeout);
> 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);
This is where I get it as you swap the key->user.
> key->uid = uid;
> }
>
> diff --git a/security/keys/keyring.c b/security/keys/keyring.c
> index 15bf4af8f282..5943e8b48c0f 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 = keyring->user->uid;
> + spin_unlock(&key_user_lock);
In this and similar site I do not get it. Why key->user->-lock could not
be instead?
> +
> + 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 4f4e2c1824f1..e507c500c068 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 = key->user->uid;
> + spin_unlock(&key_user_lock);
> +
> + if (kuid_has_mapping(user_ns, uid))
> break;
> n = rb_next(n);
Isn't procfs change at least out of scope? It
> }
> @@ -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 = 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
>
By putting global lock every possible site the bug does get fixed but
nothing really makes the case that this is ultimately least intrusive
way to fix this.
I could fix almost any possible race condition if there was a preference
to use global lock for everything. I'm neither sure whether this is a
superset change that covers the bug fix or not.
Br, Jarkko
Br, Jarkko
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