[PATCH 5.10] ALSA: seq: Clear variable event pointer on read

[Roman Demidov]

From: Kyle Zeng <kylebot@xxxxxxxxxx>

commit 705dd6dcbc0ea87351c660c1a6443f85f1001c76 upstream.

snd_seq_read() copies a queued variable-length event header to userspace
before expanding the payload. Queued variable-length events use
SNDRV_SEQ_EXT_CHAINED internally, and data.ext.ptr points at the first
extension cell.

The read side strips SNDRV_SEQ_EXT_* bits from data.ext.len before the
copy, but it leaves data.ext.ptr untouched. A userspace sequencer client
can therefore write a direct variable event to itself and read back the
extension-cell kernel address from the returned header.

Clear the temporary header pointer before copy_to_user(). The original
queued event remains unchanged and is still passed to
snd_seq_expand_var_event(), so payload expansion keeps using the
internal chain.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Kyle Zeng <kylebot@xxxxxxxxxx>
Link: https://patch.msgid.link/20260607004129.61345-1-kylebot@xxxxxxxxxx
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
Signed-off-by: Roman Demidov <roman.demidov.nn@xxxxxxxxx>
---
Backport fix for CVE-2026-74339

 sound/core/seq/seq_clientmgr.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
index 0363670a56e7..27c8af14a39e 100644
--- a/sound/core/seq/seq_clientmgr.c
+++ b/sound/core/seq/seq_clientmgr.c
@@ -441,6 +441,7 @@ static ssize_t snd_seq_read(struct file *file, char __user *buf, size_t count,
 			struct snd_seq_event tmpev;
 			tmpev = cell->event;
 			tmpev.data.ext.len &= ~SNDRV_SEQ_EXT_MASK;
+			tmpev.data.ext.ptr = NULL;
 			if (copy_to_user(buf, &tmpev, sizeof(struct snd_seq_event))) {
 				err = -EFAULT;
 				break;
-- 
2.53.0