[PATCH v2] keys: Avoid the owner account dereference in named keyring lookup

[PATCH v2] keys: Avoid the owner account dereference in named keyring lookup

[Cen Zhang]

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

Fixes: 2ea190d0a006 ("keys: skip keys from another user namespace")
Assisted-by: LLM
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
---
Changes in v2:
- Trim the KASAN report to its two-line fault summary.

Link to v1: https://lore.kernel.org/r/pm-key-management-objects-candidate-0002-v2-dd6c20a1d92928b33ab7@gmail.com

diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index 15bf4af8f28218ec3f12c97630d1c76939af7eca..46f774be72967a9bd16b0ddfdf323eda36edeff4 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, keyring->uid))
 			continue;
 
 		if (test_bit(KEY_FLAG_REVOKED, &keyring->flags))

Re: [PATCH v2] keys: Avoid the owner account dereference in named keyring lookup

[Jarkko Sakkinen]

On Fri, Oct 09, 2026 at 01:11:59PM +0800, Cen Zhang wrote:
> 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
> 
> Fixes: 2ea190d0a006 ("keys: skip keys from another user namespace")
> Assisted-by: LLM
> Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
> ---
> Changes in v2:
> - Trim the KASAN report to its two-line fault summary.
> 
> Link to v1: https://lore.kernel.org/r/pm-key-management-objects-candidate-0002-v2-dd6c20a1d92928b33ab7@gmail.com
> 
> diff --git a/security/keys/keyring.c b/security/keys/keyring.c
> index 15bf4af8f28218ec3f12c97630d1c76939af7eca..46f774be72967a9bd16b0ddfdf323eda36edeff4 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, keyring->uid))
>  			continue;
>  
>  		if (test_bit(KEY_FLAG_REVOKED, &keyring->flags))

Thanks.


Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

Br, Jarkko