CVE-2017-15265 (C, published 2017-10-16T18:29Z)
Description: value: 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 v3 base score: 7)
Weakness: CWE-362 \u2014 Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
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.
Repository: https://github.com/torvalds/linux
Files changed:
sound/core/seq/seq_clientmgr.c
sound/core/seq/seq_ports.c
Commit message: 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>


Vulnerable code:
		snd_seq_delete_port(client, port->addr.port);
/* create a port, port number is returned (-1 on failure) */
	sprintf(new_port->name, "port-%d", num);

Fixed code:
	int port_idx;
		port_idx = port->addr.port;
		snd_seq_port_unlock(port);
		snd_seq_delete_port(client, port_idx);
	snd_seq_port_unlock(port);
/* create a port, port number is returned (-1 on failure);
 * the caller needs to unref the port via snd_seq_port_unlock() appropriately
 */
	snd_use_lock_use(&new_port->use_lock);
	sprintf(new_port->name, "port-%d", num);