linux-kernel - Re: [PATCH 1/1][BUG][TAKE2] KEYRINGS: find_keyring_by

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Date:	Sat, 24 Apr 2010 00:23:19 +0900
From:	Toshiyuki Okajima <toshi.okajima@...fujitsu.com>
To:	David Howells <dhowells@...hat.com>
Cc:	keyrings@...ux-nfs.org, security@...nel.org,
	linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1][BUG][TAKE2] KEYRINGS: find_keyring_by_name() can
 gain the freed keyring

On Fri, 23 Apr 2010 12:33:57 +0100
David Howells <dhowells@...hat.com> wrote:

> Better still, atomic_inc_not_zero().  How about the attached patch?
Your fix looks good to me. But, if usage count of the keyring is 0,
I think it better to return -ENOKEY immediately. 

Like this.
> +			/* we've got a match but we might end up racing with
> +			 * key_cleanup() if the keyring is currently 'dead'
> +			 * (ie. it has a zero usage count) */
> +			if (!atomic_inc_not_zero(&keyring->usage))

> +				continue;
			=>	break;


And my previous figure description(in first patch) was a bit wrong. 
Please replace it with my new one:

[Figure Description](Example)
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
|(cleaner)				(user)
| free_user(user)			sys_keyctl()
|  |                                      |
|  key_put(user->session_keyring)    keyctl_get_keyring_ID()
|  ||	//=> keyring->usage = 0        |
|  |schedule_work(&key_cleanup_task)  lookup_user_key()
|  ||                                      |
|  kmem_cache_free(,user)             |
|  .                                  |[KEY_SPEC_USER_KEYRING]
|  .                                  install_user_keyrings()
|  .                                       ||
| key_cleanup() [<= worker_thread()]  ||
|  |                                       ||
|  [spin_lock(&key_serial_lock)]      |[mutex_lock(&key_user_keyr..mutex)]
|  |                                       ||
|  atomic_read() == 0                 ||
|  |{ rb_ease(&key->serial_node,) }   ||
|  |                                       ||
|  [spin_unlock(&key_serial_lock)]    |find_keyring_by_name()
|  |                                       |||
|  keyring_destroy(keyring)           ||[read_lock(&keyring_name_lock)]
|  ||                                      |||
|  |[write_lock(&keyring_name_lock)]  ||atomic_inc(&keyring->usage)
|  |.                                 ||| *** GET freeing keyring ***
|  |.                                 ||[read_unlock(&keyring_name_lock)]
|  ||                                      ||
|  |list_del()                        |[mutex_unlock(&key_user_k..mutex)]
|  ||                                      |
|  |[write_unlock(&keyring_name_lock)] ** INVALID keyring is returned **
|  |                                        .
|  kmem_cache_free(,keyring)           .
|                                           . 
|                                      atomic_dec(&keyring->usage)
v                                         *** DESTROYED ***
TIME 
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 

Best Regards, 
Toshiyuki Okajima
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