In the Linux kernel, the following vulnerability has been resolved:
ALSA: seq: Don’t leak the extension cell pointer in the bounce payload
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.
