TLDRVuln - Cybersecurity Vulnerabilities Made Simple

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

ALSA: seq: avoid stale FIFO cells during resize

snd_seq_fifo_resize() still needs to publish the replacement pool

before it waits for FIFO users. A blocking snd_seq_read() holds

f->use_lock while it sleeps, so concurrent senders must be able to

queue to the new pool and wake that reader instead of failing against a

closing old pool.

However, snd_seq_fifo_event_in() duplicates an event before it takes

f->lock, and snd_seq_read() can dequeue a cell and later call

snd_seq_fifo_cell_putback() if copy_to_user() or

snd_seq_expand_var_event() fails. If resize swaps f->pool and detaches

oldhead in between, either path can relink an old-pool cell after the

snapshot. That stale cell sits outside the drained oldhead list, keeps

oldpool->counter elevated, and can leave snd_seq_pool_delete() waiting

for the retired pool to drain.

Keep the existing swap-before-wait ordering in snd_seq_fifo_resize(),

but reject stale cells before any FIFO relink. Revalidate event-in cells

under f->lock and retry them against the published replacement pool, and

free stale putback cells instead of linking them back into the FIFO.

The buggy scenario involves two paths, with each column showing the

order within that path:

resize path: relink path:

1. Allocate newpool. 1. Take f->use_lock.

2. Swap f->pool to newpool and 2. Duplicate or dequeue an old-pool

detach oldhead. cell before oldpool closes.

3. Mark oldpool closing and 3. Reach a later relink point after

wait for FIFO users. resize published newpool.

4. Free oldhead and delete 4. Relink the old-pool cell after

oldpool. resize detached oldhead.

5. Drop f->use_lock.

The reproducer reports a resize ioctl blocked in the expected pool

teardown path:

signal: resize iteration=98 target_pool=4 exceeded 250ms

(elapsed=251ms)

diagnostic: resize_tid=651 wchan=snd_seq_pool_done

diagnostic: resize_tid=651 stack=

snd_seq_pool_done+0x5b/0x140

snd_seq_pool_delete+0x7a/0x90

snd_seq_fifo_resize+0x193/0x1e0

snd_seq_ioctl_set_client_pool+0x214/0x260

snd_seq_ioctl+0x119/0x540

__x64_sys_ioctl+0xd1/0x120

do_syscall_64+0xbb/0x2f0

entry_SYSCALL_64_after_hwframe+0x77/0x7f

A second run with larger pools hit the same target path:

signal: resize iteration=32 target_pool=64 exceeded 250ms