ALSA: seq: avoid stale FIFO cells during resize

Summary

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

Timeline

1. Aug 15, 2026, 5:57 AM UTCCVE published
2. Oct 21, 2026, 12:45 PM UTCRecord last updated

Affected versions

• Linux Linux

  From 2d7d54002e396c180db0c800c1046f0a3c471597 · fixed in ae429220c7f9747579bc4682c574585230e971cc; From 2d7d54002e396c180db0c800c1046f0a3c471597 · fixed in dcf784abee6df2f5c47e36a1d9e400065993bc04; From 2d7d54002e396c180db0c800c1046f0a3c471597 · fixed in 98a965cb1e767b54b6bbd831b6cea26f521a8f51; From 2d7d54002e396c180db0c800c1046f0a3c471597 · fixed in 1c4c35fb68d520241f7d9f1c356b5e2370fd8364; From 2d7d54002e396c180db0c800c1046f0a3c471597 · fixed in e546128291f8d688dcb931827e2efd2aa6c0734d; From 3.2.91 · fixed in 3.3; From 3.10.107 · fixed in 3.11; From 3.12.73 · fixed in 3.13; From 3.16.46 · fixed in 3.17; From 3.18.50 · fixed in 3.19; From 4.1.40 · fixed in 4.2; From 4.4.60 · fixed in 4.5; From 4.9.21 · fixed in 4.10; From 4.10.9 · fixed in 4.11

• Linux Linux

  4.11

• KernelLinux

  From 4.11.0 · fixed in 6.6.160; From 6.7.0 · fixed in 6.12.114; From 6.13.0 · fixed in 6.18.40; From 6.19.0 · fixed in 7.1.5

References 5