CVE-2017-15265 (published 2017-10-16T18:29Z)

Race condition in the ALSA subsystem in the Linux kernel before 4.13.8 allows local users to cause a denial of service (use-after-free) or possibly have unspecified other impact via crafted /dev/snd/seq ioctl calls, related to sound/core/seq/seq_clientmgr.c and sound/core/seq/seq_ports.c.

Severity: MEDIUM
CVSS v2 base score: 6.9 (AV:L/AC:M/Au:N/C:C/I:C/A:C)
CVSS v3 base score: 7.0, HIGH (CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H)

Weakness information:
cwe_id: CWE-362
cwe_name: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
description: The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.
extended_description: 
            This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider.
            A race condition occurs within concurrent environments, and is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc.
            A race condition violates these properties, which are closely related:
               
                  Exclusivity - the code sequence is given exclusive access to the shared resource, i.e., no other code sequence can modify properties of the shared resource before the original sequence has completed execution.
                  Atomicity - the code sequence is behaviorally atomic, i.e., no other thread or process can concurrently execute the same sequence of instructions (or a subset) against the same resource.
               
            A race condition exists when an "interfering code sequence" can still access the shared resource, violating exclusivity. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single "x++" statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x).
            The interfering code sequence could be "trusted" or "untrusted." A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.
         
         
cwe_id: CWE-416
cwe_name: Use After Free
description: The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
extended_description: 
             
             
             
			 
             
             
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Takashi Iwai
author_date: 2017-10-09 11:09:20+02:00
msg: ALSA: seq: Fix use-after-free at creating a port

There is a potential race window opened at creating and deleting a
port via ioctl, as spotted by fuzzing.  snd_seq_create_port() creates
a port object and returns its pointer, but it doesn't take the
refcount, thus it can be deleted immediately by another thread.
Meanwhile, snd_seq_ioctl_create_port() still calls the function
snd_seq_system_client_ev_port_start() with the created port object
that is being deleted, and this triggers use-after-free like:

 BUG: KASAN: use-after-free in snd_seq_ioctl_create_port+0x504/0x630 [snd_seq] at addr ffff8801f2241cb1
 =============================================================================
 BUG kmalloc-512 (Tainted: G    B          ): kasan: bad access detected
 -----------------------------------------------------------------------------
 INFO: Allocated in snd_seq_create_port+0x94/0x9b0 [snd_seq] age=1 cpu=3 pid=4511
 	___slab_alloc+0x425/0x460
 	__slab_alloc+0x20/0x40
  	kmem_cache_alloc_trace+0x150/0x190
	snd_seq_create_port+0x94/0x9b0 [snd_seq]
	snd_seq_ioctl_create_port+0xd1/0x630 [snd_seq]
 	snd_seq_do_ioctl+0x11c/0x190 [snd_seq]
 	snd_seq_ioctl+0x40/0x80 [snd_seq]
 	do_vfs_ioctl+0x54b/0xda0
 	SyS_ioctl+0x79/0x90
 	entry_SYSCALL_64_fastpath+0x16/0x75
 INFO: Freed in port_delete+0x136/0x1a0 [snd_seq] age=1 cpu=2 pid=4717
 	__slab_free+0x204/0x310
 	kfree+0x15f/0x180
 	port_delete+0x136/0x1a0 [snd_seq]
 	snd_seq_delete_port+0x235/0x350 [snd_seq]
 	snd_seq_ioctl_delete_port+0xc8/0x180 [snd_seq]
 	snd_seq_do_ioctl+0x11c/0x190 [snd_seq]
 	snd_seq_ioctl+0x40/0x80 [snd_seq]
 	do_vfs_ioctl+0x54b/0xda0
 	SyS_ioctl+0x79/0x90
 	entry_SYSCALL_64_fastpath+0x16/0x75
 Call Trace:
  [<ffffffff81b03781>] dump_stack+0x63/0x82
  [<ffffffff81531b3b>] print_trailer+0xfb/0x160
  [<ffffffff81536db4>] object_err+0x34/0x40
  [<ffffffff815392d3>] kasan_report.part.2+0x223/0x520
  [<ffffffffa07aadf4>] ? snd_seq_ioctl_create_port+0x504/0x630 [snd_seq]
  [<ffffffff815395fe>] __asan_report_load1_noabort+0x2e/0x30
  [<ffffffffa07aadf4>] snd_seq_ioctl_create_port+0x504/0x630 [snd_seq]
  [<ffffffffa07aa8f0>] ? snd_seq_ioctl_delete_port+0x180/0x180 [snd_seq]
  [<ffffffff8136be50>] ? taskstats_exit+0xbc0/0xbc0
  [<ffffffffa07abc5c>] snd_seq_do_ioctl+0x11c/0x190 [snd_seq]
  [<ffffffffa07abd10>] snd_seq_ioctl+0x40/0x80 [snd_seq]
  [<ffffffff8136d433>] ? acct_account_cputime+0x63/0x80
  [<ffffffff815b515b>] do_vfs_ioctl+0x54b/0xda0
  .....

We may fix this in a few different ways, and in this patch, it's fixed
simply by taking the refcount properly at snd_seq_create_port() and
letting the caller unref the object after use.  Also, there is another
potential use-after-free by sprintf() call in snd_seq_create_port(),
and this is moved inside the lock.

This fix covers CVE-2017-15265.

Reported-and-tested-by: Michael23 Yu <ycqzsy@gmail.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: seq_clientmgr.c
change_type: ModificationType.MODIFY
diff: @@ -1259,6 +1259,7 @@ static int snd_seq_ioctl_create_port(struct snd_seq_client *client, void *arg)
 	struct snd_seq_port_info *info = arg;
 	struct snd_seq_client_port *port;
 	struct snd_seq_port_callback *callback;
+	int port_idx;
 
 	/* it is not allowed to create the port for an another client */
 	if (info->addr.client != client->number)
@@ -1269,7 +1270,9 @@ static int snd_seq_ioctl_create_port(struct snd_seq_client *client, void *arg)
 		return -ENOMEM;
 
 	if (client->type == USER_CLIENT && info->kernel) {
-		snd_seq_delete_port(client, port->addr.port);
+		port_idx = port->addr.port;
+		snd_seq_port_unlock(port);
+		snd_seq_delete_port(client, port_idx);
 		return -EINVAL;
 	}
 	if (client->type == KERNEL_CLIENT) {
@@ -1290,6 +1293,7 @@ static int snd_seq_ioctl_create_port(struct snd_seq_client *client, void *arg)
 
 	snd_seq_set_port_info(port, info);
 	snd_seq_system_client_ev_port_start(port->addr.client, port->addr.port);
+	snd_seq_port_unlock(port);
 
 	return 0;
 }

programming_language: C
filename: seq_ports.c
change_type: ModificationType.MODIFY
diff: @@ -122,7 +122,9 @@ static void port_subs_info_init(struct snd_seq_port_subs_info *grp)
 }
 
 
-/* create a port, port number is returned (-1 on failure) */
+/* create a port, port number is returned (-1 on failure);
+ * the caller needs to unref the port via snd_seq_port_unlock() appropriately
+ */
 struct snd_seq_client_port *snd_seq_create_port(struct snd_seq_client *client,
 						int port)
 {
@@ -151,6 +153,7 @@ struct snd_seq_client_port *snd_seq_create_port(struct snd_seq_client *client,
 	snd_use_lock_init(&new_port->use_lock);
 	port_subs_info_init(&new_port->c_src);
 	port_subs_info_init(&new_port->c_dest);
+	snd_use_lock_use(&new_port->use_lock);
 
 	num = port >= 0 ? port : 0;
 	mutex_lock(&client->ports_mutex);
@@ -165,9 +168,9 @@ struct snd_seq_client_port *snd_seq_create_port(struct snd_seq_client *client,
 	list_add_tail(&new_port->list, &p->list);
 	client->num_ports++;
 	new_port->addr.port = num;	/* store the port number in the port */
+	sprintf(new_port->name, "port-%d", num);
 	write_unlock_irqrestore(&client->ports_lock, flags);
 	mutex_unlock(&client->ports_mutex);
-	sprintf(new_port->name, "port-%d", num);
 
 	return new_port;
 }

programming_language: C