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

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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 705dd6dcbc ("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: efc86691e4 ("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>
Link: https://patch.msgid.link/20260811131835.3837024-1-sammiee5311@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
HyeongJun An 2026-08-11 22:18:35 +09:00 committed by Takashi Iwai
parent 108704eeca
commit 59e1592d3c
1 changed files with 10 additions and 6 deletions
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16
sound/core/seq/seq_clientmgr.c
View File

@ -541,7 +541,7 @@ 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 ||
@ -561,15 +561,19 @@ 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.