CVE-2026-98079btrfs: zstd: fix lost wakeup when waiting for a workspace

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/14d982762945d9945c60bbd48b77df194dd8e28d
• https://git.kernel.org/stable/c/844603cf842ee67640d33f350e3fa60f3de7fad3
• https://git.kernel.org/stable/c/d8f57049521948df5f50797141472fbbed3b0723
• https://git.kernel.org/stable/c/0de9f31d447ae71ab08c7850a321682f8d2907c3
• https://git.kernel.org/stable/c/2acb9f3d1cc8f65dc81ed55e238cbf8e5b60bff7