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

Overview

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

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

snd_seq_event_dup() 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 snd_seq_event) == 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->midi_version > 0) that set SNDRV_SEQ_FILTER_NO_CONVERT -- so the legacy event reaches it unconverted -- snd_seq_read() reads it out as the larger struct snd_seq_ump_event 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.

Affected

• Linux >= 6389f2c135311c4ce7c08c3b29145c8f95aacf1f < d7649aa11089a93ea2285c210397aa67e5800766
• Linux >= d7e2ce72833bb23a82b4201fbed7214cc04a4a8c < a224c84e5d3d35708c082c84ad12d81d90762195
• Linux >= 46397622a3fa8372b8fda0f04b33d16923b03b1b < ea672a9f6cc38f06fe69dd2c257ef8a3d4db179a
• Linux >= 46397622a3fa8372b8fda0f04b33d16923b03b1b < fb1aa5082847b98f44f9c6272aee9d0dca9244f0
• Linux >= 46397622a3fa8372b8fda0f04b33d16923b03b1b < 651ba82fe2a144bc7356d940bfd235c3810b0549
• Linux >= 46397622a3fa8372b8fda0f04b33d16923b03b1b < 6ded42615fa1f4949925afd0a8a9e1ab3bf96202
• Linux >= 46397622a3fa8372b8fda0f04b33d16923b03b1b < 435990e25bf1f4af3e6df12a6fbfd1f7ba4a97d4
• Linux 6.5

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.