ALSA: seq: avoid stale FIFO cells during resize

Overview

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 (elapsed=251ms) diagnostic: resize_tid=663 wchan=snd_seq_pool_done diagnostic: resize_tid=663 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

Affected

• Linux >= 2d7d54002e396c180db0c800c1046f0a3c471597 < ae429220c7f9747579bc4682c574585230e971cc
• Linux >= 2d7d54002e396c180db0c800c1046f0a3c471597 < dcf784abee6df2f5c47e36a1d9e400065993bc04
• Linux >= 2d7d54002e396c180db0c800c1046f0a3c471597 < 98a965cb1e767b54b6bbd831b6cea26f521a8f51
• Linux >= 2d7d54002e396c180db0c800c1046f0a3c471597 < 1c4c35fb68d520241f7d9f1c356b5e2370fd8364
• Linux >= 2d7d54002e396c180db0c800c1046f0a3c471597 < e546128291f8d688dcb931827e2efd2aa6c0734d
• Linux df7a2d0af9f7ab73cf22a7b62bbc0bad93a02110
• Linux 28e0ebdd57cd3287ac821068364b74c70af3861c
• Linux d4b8e8a32d74052fe292618ae19f794944ddc673
• Linux e91325f27c4d98d1088e9e86b3ab475f752eeb80
• Linux 9ebfed203c0d93449a074c5c2fd5bb3e58518f0d
• Linux 3492352e1f34673708fa7409b4b1d97545aa2f1c
• Linux a90d7447e4a154ad26e3b9e09a0878680be49339
• Linux 74a2c1ff88a4e0960623d17f7db56ac5a60bb0cf
• Linux c36ef6467420f8982e6d4d6f93d0634a85a5ee45
• Linux >= 3.2.91 < 3.3
• Linux >= 3.10.107 < 3.11
• Linux >= 3.12.73 < 3.13
• Linux >= 3.16.46 < 3.17
• Linux >= 3.18.50 < 3.19
• Linux >= 4.1.40 < 4.2
• Linux >= 4.4.60 < 4.5
• Linux >= 4.9.21 < 4.10
• Linux >= 4.10.9 < 4.11
• Linux 4.11

Remediation

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