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

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

[Kyle Zeng]

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@openai.com>
---
 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
--- 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,
 
 			memcpy(&tmpev, &cell->event, aligned_size);
 			tmpev.data.ext.len &= ~SNDRV_SEQ_EXT_MASK;
+			tmpev.data.ext.ptr = NULL;
 			if (copy_to_user(buf, &tmpev, aligned_size)) {
 				err = -EFAULT;
 				break;
-- 
2.50.0

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

[Takashi Iwai]

On Sun, 07 Jun 2026 02:41:29 +0200,
Kyle Zeng wrote:
> 
> 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@openai.com>

Applied to for-next branch now.  Thanks.


Takashi