Re: [PATCH] keys: Fix linking a duplicate key to a keyring's assoc_array

[Jarkko Sakkinen]

On Thu, Mar 23, 2023 at 02:04:12PM +0100, Petr Pavlu wrote:
> When making a DNS query inside the kernel using dns_query(), the request
> code can in rare cases end up creating a duplicate index key in the
> assoc_array of the destination keyring. It is eventually found by
> a BUG_ON() check in the assoc_array implementation and results in
> a crash.
> 
> Example report:
> [2158499.700025] kernel BUG at ../lib/assoc_array.c:652!
> [2158499.700039] invalid opcode: 0000 [#1] SMP PTI
> [2158499.700065] CPU: 3 PID: 31985 Comm: kworker/3:1 Kdump: loaded Not tainted 5.3.18-150300.59.90-default #1 SLE15-SP3
> [2158499.700096] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 11/12/2020
> [2158499.700351] Workqueue: cifsiod cifs_resolve_server [cifs]
> [2158499.700380] RIP: 0010:assoc_array_insert+0x85f/0xa40
> [2158499.700401] Code: ff 74 2b 48 8b 3b 49 8b 45 18 4c 89 e6 48 83 e7 fe e8 95 ec 74 00 3b 45 88 7d db 85 c0 79 d4 0f 0b 0f 0b 0f 0b e8 41 f2 be ff <0f> 0b 0f 0b 81 7d 88 ff ff ff 7f 4c 89 eb 4c 8b ad 58 ff ff ff 0f
> [2158499.700448] RSP: 0018:ffffc0bd6187faf0 EFLAGS: 00010282
> [2158499.700470] RAX: ffff9f1ea7da2fe8 RBX: ffff9f1ea7da2fc1 RCX: 0000000000000005
> [2158499.700492] RDX: 0000000000000000 RSI: 0000000000000005 RDI: 0000000000000000
> [2158499.700515] RBP: ffffc0bd6187fbb0 R08: ffff9f185faf1100 R09: 0000000000000000
> [2158499.700538] R10: ffff9f1ea7da2cc0 R11: 000000005ed8cec8 R12: ffffc0bd6187fc28
> [2158499.700561] R13: ffff9f15feb8d000 R14: ffff9f1ea7da2fc0 R15: ffff9f168dc0d740
> [2158499.700585] FS:  0000000000000000(0000) GS:ffff9f185fac0000(0000) knlGS:0000000000000000
> [2158499.700610] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [2158499.700630] CR2: 00007fdd94fca238 CR3: 0000000809d8c006 CR4: 00000000003706e0
> [2158499.700702] Call Trace:
> [2158499.700741]  ? key_alloc+0x447/0x4b0
> [2158499.700768]  ? __key_link_begin+0x43/0xa0
> [2158499.700790]  __key_link_begin+0x43/0xa0
> [2158499.700814]  request_key_and_link+0x2c7/0x730
> [2158499.700847]  ? dns_resolver_read+0x20/0x20 [dns_resolver]
> [2158499.700873]  ? key_default_cmp+0x20/0x20
> [2158499.700898]  request_key_tag+0x43/0xa0
> [2158499.700926]  dns_query+0x114/0x2ca [dns_resolver]
> [2158499.701127]  dns_resolve_server_name_to_ip+0x194/0x310 [cifs]
> [2158499.701164]  ? scnprintf+0x49/0x90
> [2158499.701190]  ? __switch_to_asm+0x40/0x70
> [2158499.701211]  ? __switch_to_asm+0x34/0x70
> [2158499.701405]  reconn_set_ipaddr_from_hostname+0x81/0x2a0 [cifs]
> [2158499.701603]  cifs_resolve_server+0x4b/0xd0 [cifs]
> [2158499.701632]  process_one_work+0x1f8/0x3e0
> [2158499.701658]  worker_thread+0x2d/0x3f0
> [2158499.701682]  ? process_one_work+0x3e0/0x3e0
> [2158499.701703]  kthread+0x10d/0x130
> [2158499.701723]  ? kthread_park+0xb0/0xb0
> [2158499.701746]  ret_from_fork+0x1f/0x40
> 
> The situation occurs as follows:
> * Some kernel facility invokes dns_query() to resolve a hostname, for
>   example, "abcdef". The function registers its global DNS resolver
>   cache as current->cred.thread_keyring and passes the query to
>   request_key_net() -> request_key_tag() -> request_key_and_link().
> * Function request_key_and_link() creates a keyring_search_context
>   object. Its match_data.cmp method gets set via a call to
>   type->match_preparse() (resolves to dns_resolver_match_preparse()) to
>   dns_resolver_cmp().
> * Function request_key_and_link() continues and invokes
>   search_process_keyrings_rcu() which returns that a given key was not
>   found. The control is then passed to request_key_and_link() ->
>   construct_alloc_key().
> * Concurrently to that, a second task similarly makes a DNS query for
>   "abcdef." and its result gets inserted into the DNS resolver cache.
> * Back on the first task, function construct_alloc_key() first runs
>   __key_link_begin() to determine an assoc_array_edit operation to
>   insert a new key. Index keys in the array are compared exactly as-is,
>   using keyring_compare_object(). The operation finds that "abcdef" is
>   not yet present in the destination keyring.
> * Function construct_alloc_key() continues and checks if a given key is
>   already present on some keyring by again calling
>   search_process_keyrings_rcu(). This search is done using
>   dns_resolver_cmp() and "abcdef" gets matched with now present key
>   "abcdef.".
> * The found key is linked on the destination keyring by calling
>   __key_link() and using the previously calculated assoc_array_edit
>   operation. This inserts the "abcdef." key in the array but creates
>   a duplicity because the same index key is already present.
> 
> Fix the problem by postponing __key_link_begin() in
> construct_alloc_key() until an actual key which should be linked into
> the destination keyring is determined.
> 
> Signed-off-by: Petr Pavlu <petr.pavlu@xxxxxxxx>
> ---
>  security/keys/request_key.c | 35 ++++++++++++++++++++++++-----------
>  1 file changed, 24 insertions(+), 11 deletions(-)
> 
> diff --git a/security/keys/request_key.c b/security/keys/request_key.c
> index 2da4404276f0..04eb7e4cedad 100644
> --- a/security/keys/request_key.c
> +++ b/security/keys/request_key.c
> @@ -398,17 +398,21 @@ static int construct_alloc_key(struct keyring_search_context *ctx,
>  	set_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags);
>  
>  	if (dest_keyring) {
> -		ret = __key_link_lock(dest_keyring, &ctx->index_key);
> +		ret = __key_link_lock(dest_keyring, &key->index_key);
>  		if (ret < 0)
>  			goto link_lock_failed;
> -		ret = __key_link_begin(dest_keyring, &ctx->index_key, &edit);
> -		if (ret < 0)
> -			goto link_prealloc_failed;
>  	}
>  
> -	/* attach the key to the destination keyring under lock, but we do need
> +	/*
> +	 * Attach the key to the destination keyring under lock, but we do need
>  	 * to do another check just in case someone beat us to it whilst we
> -	 * waited for locks */
> +	 * waited for locks.
> +	 *
> +	 * The caller might specify a comparison function which looks for keys
> +	 * that do not exactly match but are still equivalent from the caller's
> +	 * perspective. The __key_link_begin() operation must be done only after
> +	 * an actual key is determined.
> +	 */
>  	mutex_lock(&key_construction_mutex);
>  
>  	rcu_read_lock();
> @@ -417,12 +421,16 @@ static int construct_alloc_key(struct keyring_search_context *ctx,
>  	if (!IS_ERR(key_ref))
>  		goto key_already_present;
>  
> -	if (dest_keyring)
> +	if (dest_keyring) {
> +		ret = __key_link_begin(dest_keyring, &key->index_key, &edit);
> +		if (ret < 0)
> +			goto link_alloc_failed;
>  		__key_link(dest_keyring, key, &edit);
> +	}
>  
>  	mutex_unlock(&key_construction_mutex);
>  	if (dest_keyring)
> -		__key_link_end(dest_keyring, &ctx->index_key, edit);
> +		__key_link_end(dest_keyring, &key->index_key, edit);
>  	mutex_unlock(&user->cons_lock);
>  	*_key = key;
>  	kleave(" = 0 [%d]", key_serial(key));
> @@ -435,10 +443,13 @@ static int construct_alloc_key(struct keyring_search_context *ctx,
>  	mutex_unlock(&key_construction_mutex);
>  	key = key_ref_to_ptr(key_ref);
>  	if (dest_keyring) {
> +		ret = __key_link_begin(dest_keyring, &key->index_key, &edit);
> +		if (ret < 0)
> +			goto link_alloc_failed_unlocked;
>  		ret = __key_link_check_live_key(dest_keyring, key);
>  		if (ret == 0)
>  			__key_link(dest_keyring, key, &edit);
> -		__key_link_end(dest_keyring, &ctx->index_key, edit);
> +		__key_link_end(dest_keyring, &key->index_key, edit);
>  		if (ret < 0)
>  			goto link_check_failed;
>  	}
> @@ -453,8 +464,10 @@ static int construct_alloc_key(struct keyring_search_context *ctx,
>  	kleave(" = %d [linkcheck]", ret);
>  	return ret;
>  
> -link_prealloc_failed:
> -	__key_link_end(dest_keyring, &ctx->index_key, edit);
> +link_alloc_failed:
> +	mutex_unlock(&key_construction_mutex);
> +link_alloc_failed_unlocked:
> +	__key_link_end(dest_keyring, &key->index_key, edit);
>  link_lock_failed:
>  	mutex_unlock(&user->cons_lock);
>  	key_put(key);
> -- 
> 2.35.3
> 

A good catch, thanks.

Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>

BR, Jarkko