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Subject[PATCH 3.10 020/139] ALSA: seq: Fix use-after-free at creating a port
From: Takashi Iwai <>

commit 71105998845fb012937332fe2e806d443c09e026 upstream.

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
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]
INFO: Freed in port_delete+0x136/0x1a0 [snd_seq] age=1 cpu=2 pid=4717
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]
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 <>
Suggested-by: Linus Torvalds <>
Cc: <>
Signed-off-by: Takashi Iwai <>
Signed-off-by: Willy Tarreau <>
sound/core/seq/seq_clientmgr.c | 6 +++++-
sound/core/seq/seq_ports.c | 7 +++++--
2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
index d449dde..7b5a790 100644
--- a/sound/core/seq/seq_clientmgr.c
+++ b/sound/core/seq/seq_clientmgr.c
@@ -1248,6 +1248,7 @@ static int snd_seq_ioctl_create_port(struct snd_seq_client *client,
struct snd_seq_client_port *port;
struct snd_seq_port_info info;
struct snd_seq_port_callback *callback;
+ int port_idx;

if (copy_from_user(&info, arg, sizeof(info)))
return -EFAULT;
@@ -1261,7 +1262,9 @@ static int snd_seq_ioctl_create_port(struct snd_seq_client *client,
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) {
@@ -1283,6 +1286,7 @@ static int snd_seq_ioctl_create_port(struct snd_seq_client *client,

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);

if (copy_to_user(arg, &info, sizeof(info)))
return -EFAULT;
diff --git a/sound/core/seq/seq_ports.c b/sound/core/seq/seq_ports.c
index ee0522a..a28d1ac 100644
--- a/sound/core/seq/seq_ports.c
+++ b/sound/core/seq/seq_ports.c
@@ -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)
@@ -153,6 +155,7 @@ struct snd_seq_client_port *snd_seq_create_port(struct snd_seq_client *client,
+ snd_use_lock_use(&new_port->use_lock);

num = port >= 0 ? port : 0;
@@ -167,9 +170,9 @@ struct snd_seq_client_port *snd_seq_create_port(struct snd_seq_client *client,
list_add_tail(&new_port->list, &p->list);
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);
- sprintf(new_port->name, "port-%d", num);

return new_port;
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