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

[Takashi Iwai <tiwai@suse.de>]

On Tue, 11 Aug 2026 15:18:35 +0200,
HyeongJun An wrote:
> 
> 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>

Applied now to for-next branch now.

> ---
> 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.

Any further fix would be appreciated.


thanks,

Takashi