[PATCH] ALSA: seq: Don't leak the extension cell pointer in the bounce payload

[HyeongJun An <sammiee5311@gmail.com>]

The bounce_error_event() embeds the failed event in the bounce payload
by pointing data.ext.ptr at it.  When that event is a queued
variable-length event, its own data.ext.ptr holds the address of its
first extension cell, put there by snd_seq_event_dup().  The payload
goes out verbatim through snd_seq_expand_var_event(), so the address
reaches userspace.

That is the same address commit 705dd6dcbc0e ("ALSA: seq: Clear
variable event pointer on read") removed from the event header.  The
read path still clears it there, just above the call that expands the
payload.

Embed a sanitised copy instead, treated exactly as snd_seq_read()
treats the header.  A stack copy is enough because delivery is
synchronous and snd_seq_event_dup() copies before returning.

An unprivileged client reaches this by setting SNDRV_SEQ_FILTER_BOUNCE,
queueing a variable-length event to a port that does not exist and
reading the bounce back.  Eight bytes on 64-bit, from its own pool.

Fixes: efc86691e4d8 ("ALSA: seq: Fix kernel heap address leak in bounce_error_event()")
Assisted-by: Claude:claude-opus-5
Signed-off-by: HyeongJun An <sammiee5311@gmail.com>
---
Unrelated, and not part of the change: the payload does not match struct
snd_seq_event_bounce in include/uapi/sound/asequencer.h - the err field is
not sent.  Nothing regressed, since the kernel never produced the event
before efc86691e4d8.  I can fix that separately if you want it.

 sound/core/seq/seq_clientmgr.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
index 28782e1776fa..9ec6c1c0cabc 100644
--- a/sound/core/seq/seq_clientmgr.c
+++ b/sound/core/seq/seq_clientmgr.c
@@ -528,7 +528,7 @@ static int bounce_error_event(struct snd_seq_client *client,
 			      struct snd_seq_event *event,
 			      int err, int atomic, int hop)
 {
-	struct snd_seq_event bounce_ev;
+	struct snd_seq_event bounce_ev, quoted;
 	int result;
 
 	if (client == NULL ||
@@ -548,15 +548,19 @@ static int bounce_error_event(struct snd_seq_client *client,
 		 * For user clients, send SNDRV_SEQ_EVENT_BOUNCE with the
 		 * original event embedded as variable-length data.  This
 		 * avoids exposing data.quote.event (a kernel pointer) to
-		 * userspace.  The variable-length path in snd_seq_event_dup()
-		 * copies the event data from data.ext.ptr into chained cells,
-		 * and snd_seq_expand_var_event() copies only the data content
-		 * -- never the pointer -- to userspace.
+		 * userspace.  Sanitise the embedded copy too - a queued
+		 * variable-length event carries the address of its own
+		 * extension cell, and the payload goes out verbatim.
 		 */
+		quoted = *event;
+		if (snd_seq_ev_is_variable(&quoted)) {
+			quoted.data.ext.len &= ~SNDRV_SEQ_EXT_MASK;
+			quoted.data.ext.ptr = NULL;
+		}
 		bounce_ev.type = SNDRV_SEQ_EVENT_BOUNCE;
 		bounce_ev.flags = SNDRV_SEQ_EVENT_LENGTH_VARIABLE;
 		bounce_ev.data.ext.len = sizeof(struct snd_seq_event);
-		bounce_ev.data.ext.ptr = (char *)event;
+		bounce_ev.data.ext.ptr = (char *)&quoted;
 	} else {
 		/*
 		 * For kernel clients, quote the event pointer directly.
-- 
2.43.0