[PATCH] ALSA: seq: dummy: fix UMP event stack overread

[PATCH] ALSA: seq: dummy: fix UMP event stack overread

[Kyle Zeng]

The dummy sequencer port forwards events by copying an incoming
struct snd_seq_event into a stack temporary, rewriting source and
destination, and dispatching the temporary to subscribers. That legacy
event storage is smaller than struct snd_seq_ump_event.

When a UMP event reaches the dummy client, the copy leaves the UMP flag
set but only provides legacy-sized stack storage. The subscriber
delivery path then uses snd_seq_event_packet_size() and copies a
UMP-sized packet from that stack object, reading past the end of the
temporary.

Use the existing union __snd_seq_event storage and copy the packet size
reported for the incoming event before rewriting the common routing
fields. This preserves the full UMP packet for UMP events while keeping
legacy event handling unchanged.

Fixes: 32cb23a0f911 ("ALSA: seq: dummy: Allow UMP conversion")
Signed-off-by: Kyle Zeng <kylebot@openai.com>
---
 sound/core/seq/seq_dummy.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/sound/core/seq/seq_dummy.c b/sound/core/seq/seq_dummy.c
index af45f328ae99..8abe80985dad 100644
--- a/sound/core/seq/seq_dummy.c
+++ b/sound/core/seq/seq_dummy.c
@@ -9,6 +9,7 @@
 #include <linux/module.h>
 #include <sound/core.h>
 #include "seq_clientmgr.h"
+#include "seq_memory.h"
 #include <sound/initval.h>
 #include <sound/asoundef.h>
 
@@ -81,19 +82,21 @@ dummy_input(struct snd_seq_event *ev, int direct, void *private_data,
 	    int atomic, int hop)
 {
 	struct snd_seq_dummy_port *p;
-	struct snd_seq_event tmpev;
+	union __snd_seq_event tmpev;
+	size_t size;
 
 	p = private_data;
 	if (ev->source.client == SNDRV_SEQ_CLIENT_SYSTEM ||
 	    ev->type == SNDRV_SEQ_EVENT_KERNEL_ERROR)
 		return 0; /* ignore system messages */
-	tmpev = *ev;
+	size = snd_seq_event_packet_size(ev);
+	memcpy(&tmpev, ev, size);
 	if (p->duplex)
-		tmpev.source.port = p->connect;
+		tmpev.legacy.source.port = p->connect;
 	else
-		tmpev.source.port = p->port;
-	tmpev.dest.client = SNDRV_SEQ_ADDRESS_SUBSCRIBERS;
-	return snd_seq_kernel_client_dispatch(p->client, &tmpev, atomic, hop);
+		tmpev.legacy.source.port = p->port;
+	tmpev.legacy.dest.client = SNDRV_SEQ_ADDRESS_SUBSCRIBERS;
+	return snd_seq_kernel_client_dispatch(p->client, &tmpev.legacy, atomic, hop);
 }
 
 /*
-- 
2.51.0

Re: [PATCH] ALSA: seq: dummy: fix UMP event stack overread

[Takashi Iwai]

On Fri, 05 Jun 2026 10:02:04 +0200,
Kyle Zeng wrote:
> 
> The dummy sequencer port forwards events by copying an incoming
> struct snd_seq_event into a stack temporary, rewriting source and
> destination, and dispatching the temporary to subscribers. That legacy
> event storage is smaller than struct snd_seq_ump_event.
> 
> When a UMP event reaches the dummy client, the copy leaves the UMP flag
> set but only provides legacy-sized stack storage. The subscriber
> delivery path then uses snd_seq_event_packet_size() and copies a
> UMP-sized packet from that stack object, reading past the end of the
> temporary.
> 
> Use the existing union __snd_seq_event storage and copy the packet size
> reported for the incoming event before rewriting the common routing
> fields. This preserves the full UMP packet for UMP events while keeping
> legacy event handling unchanged.
> 
> Fixes: 32cb23a0f911 ("ALSA: seq: dummy: Allow UMP conversion")
> Signed-off-by: Kyle Zeng <kylebot@openai.com>

Applied now.  Thanks.


Takashi