CVE-2026-64479: ALSA: seq: Fix uninitialised heap leak in snd_seq_event_dup()

In the Linux kernel, the following vulnerability has been resolved:

ALSA: seq: Fix uninitialised heap leak in sndseqeventdup()

sndseqeventdup() copies an incoming event into a pool cell and, in the UMP-enabled build, clears the trailing cell->ump.raw.extra word that the memcpy() did not cover. The guard deciding whether to clear it compares the copied size against sizeof(cell->event):

memcpy(&cell->ump, event, size); if (size < sizeof(cell->event)) cell->ump.raw.extra = 0;

For a legacy (non-UMP) event, size == sizeof(struct sndseqevent) == sizeof(cell->event), so the condition is false and the extra word keeps stale data. The cell pool is allocated with kvmalloc() (not zeroed) and cells are reused via a free list, so that word holds uninitialised heap or leftover event data.

When such a cell is delivered to a UMP client (client->midiversion > 0) that set SNDRVSEQFILTERNOCONVERT -- so the legacy event reaches it unconverted -- sndseqread() reads it out as the larger struct sndsequmpevent and copies the stale word to user space, a 4-byte kernel heap infoleak to an unprivileged /dev/snd/seq client.

Compare against sizeof(cell->ump) instead, so the trailing word is zeroed for every event shorter than the UMP cell.