btrfs: zstd: fix lost wakeup when waiting for a workspace

Summary

In the Linux kernel, the following vulnerability has been resolved: btrfs: zstd: fix lost wakeup when waiting for a workspace A writer can sleep forever in zstd_get_workspace() even though a workspace is free. When zstd_alloc_workspace() fails, the task is queued on zwsm->wait and schedules unconditionally, never re-testing the pool. zstd_put_workspace() publishes the workspace and then calls cond_wake_up(), which only wakes when a sleeper is already visible, so a workspace returned between the failed allocation and prepare_to_wait() wakes nobody. The window is wide: zstd_alloc_workspace() goes through kvmalloc() and may enter reclaim. Only a max level workspace triggers the wakeup and one is deliberately kept allocated as the fallback every waiter waits for, so once its wakeup is lost the writer stays in TASK_UNINTERRUPTIBLE until some other task happens to return one. Re-check the pool after prepare_to_wait() has published the waiter, and use the workspace if one turned up.

Timeline

1. Sep 25, 2026, 10:24 AM UTCCVE published
2. Oct 21, 2026, 12:54 PM UTCRecord last updated

Affected versions

• Linux Linux

  From 3f93aef535c8ea03e40cd8acf0753b3e6ed33e96 · fixed in 14d982762945d9945c60bbd48b77df194dd8e28d; From 3f93aef535c8ea03e40cd8acf0753b3e6ed33e96 · fixed in 844603cf842ee67640d33f350e3fa60f3de7fad3; From 3f93aef535c8ea03e40cd8acf0753b3e6ed33e96 · fixed in d8f57049521948df5f50797141472fbbed3b0723; From 3f93aef535c8ea03e40cd8acf0753b3e6ed33e96 · fixed in 0de9f31d447ae71ab08c7850a321682f8d2907c3; From 3f93aef535c8ea03e40cd8acf0753b3e6ed33e96 · fixed in 2acb9f3d1cc8f65dc81ed55e238cbf8e5b60bff7

• Linux Linux

  5.1

• KernelLinux

  From 5.1.0 · fixed in 6.6.160; From 6.7.0 · fixed in 6.12.114; From 6.13.0 · fixed in 6.18.53; From 6.19.0 · fixed in 7.2.7

References 5