[alsa-devel] [PATCH 4/5] ALSA: seq: Protect in-kernel ioctl calls with mutex

[Takashi Iwai]

ALSA OSS sequencer calls the ioctl function indirectly via
snd_seq_kernel_client_ctl().  While we already applied the protection
against races between the normal ioctls and writes via the client's
ioctl_mutex, this code path was left untouched.  And this seems to be
the cause of still remaining some rare UAF as spontaneously triggered
by syzkaller.

For the sake of robustness, wrap the ioctl_mutex also for the call via
snd_seq_kernel_client_ctl(), too.

Reported-by: syzbot+e4c8abb920efa77bace9 at syzkaller.appspotmail.com
Signed-off-by: Takashi Iwai <tiwai at suse.de>
---
 sound/core/seq/seq_clientmgr.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
index 933bde3843d9..976404691261 100644
--- a/sound/core/seq/seq_clientmgr.c
+++ b/sound/core/seq/seq_clientmgr.c
@@ -2340,14 +2340,19 @@ int snd_seq_kernel_client_ctl(int clientid, unsigned int cmd, void *arg)
 {
 	const struct ioctl_handler *handler;
 	struct snd_seq_client *client;
+	int err;
 
 	client = clientptr(clientid);
 	if (client == NULL)
 		return -ENXIO;
 
 	for (handler = ioctl_handlers; handler->cmd > 0; ++handler) {
-		if (handler->cmd == cmd)
-			return handler->func(client, arg);
+		if (handler->cmd == cmd) {
+			mutex_lock(&client->ioctl_mutex);
+			err = handler->func(client, arg);
+			mutex_unlock(&client->ioctl_mutex);
+			return err;
+		}
 	}
 
 	pr_debug("ALSA: seq unknown ioctl() 0x%x (type='%c', number=0x%02x)\n",
-- 
2.16.4