[PATCH] ALSA: seq: Serialize compat port-info ioctls

[Bruno Produit <bruno.produit@trailofbits.com>]

From: Kyle Zeng <kylebot@openai.com>

The native sequencer ioctl path serializes handler calls with
client->ioctl_mutex, but the translated port-info compat path invokes the
same handlers through snd_seq_kernel_client_ctl() without taking that
mutex.

This lets concurrent compat CREATE_PORT requests pass the port-count
check before any request reaches the serialized insertion. The computed
integer port index can then exceed the address field range and wrap to an
existing index. Subsequent subscriber teardown can resolve the duplicate
address to the wrong port and access a freed subscriber.

Take ioctl_mutex while dispatching converted port-info requests, matching
the native ioctl path. All translated port-info commands share this
helper, so their accesses to the client port state are serialized as
well.

Fixes: b3defb791b26 ("ALSA: seq: Make ioctls race-free")
Cc: stable@vger.kernel.org
Assisted-by: Codex:gpt-5.6-sol gpt-6-astra
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Signed-off-by: Bruno Produit <bruno.produit@trailofbits.com>
---

Trail of Bits has a reproducer for this bug that triggers a
KASAN use-after-free and can it share if needed

 sound/core/seq/seq_compat.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/sound/core/seq/seq_compat.c b/sound/core/seq/seq_compat.c
index 22679dca9..80110501d 100644
--- a/sound/core/seq/seq_compat.c
+++ b/sound/core/seq/seq_compat.c
@@ -44,7 +44,9 @@ static int snd_seq_call_port_info_ioctl(struct snd_seq_client *client, unsigned
 		return -EFAULT;
 	data->kernel = NULL;
 
-	err = snd_seq_kernel_client_ctl(client->number, cmd, data);
+	scoped_guard(mutex, &client->ioctl_mutex) {
+		err = snd_seq_kernel_client_ctl(client->number, cmd, data);
+	}
 	if (err < 0)
 		return err;