CVE-2025-68807

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

block: fix race between wbt_enable_default and IO submission

When wbt_enable_default() is moved out of queue freezing in elevator_change(), it can cause the wbt inflight counter to become negative (-1), leading to hung tasks in the writeback path. Tasks get stuck in wbt_wait() because the counter is in an inconsistent state.

The issue occurs because wbt_enable_default() could race with IO submission, allowing the counter to be decremented before proper initialization. This manifests as:

rq_wait[0]: inflight: -1 has_waiters: True

rwb_enabled() checks the state, which can be updated exactly between wbt_wait() (rq_qos_throttle()) and wbt_track()(rq_qos_track()), then the inflight counter will become negative.

And results in hung task warnings like: task:kworker/u24:39 state:D stack:0 pid:14767 Call Trace: rq_qos_wait+0xb4/0x150 wbt_wait+0xa9/0x100 __rq_qos_throttle+0x24/0x40 blk_mq_submit_bio+0x672/0x7b0 …

Fix this by:

1. Splitting wbt_enable_default() into:

   • __wbt_enable_default(): Returns true if wbt_init() should be called
   • wbt_enable_default(): Wrapper for existing callers (no init)
   • wbt_init_enable_default(): New function that checks and inits WBT

2. Using wbt_init_enable_default() in blk_register_queue() to ensure proper initialization during queue registration

3. Move wbt_init() out of wbt_enable_default() which is only for enabling disabled wbt from bfq and iocost, and wbt_init() isnt needed. Then the original lock warning can be avoided.

4. Removing the ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT flag and its handling code since its no longer needed

This ensures WBT is properly initialized before any IO can be submitted, preventing the counter from going negative.

Affected Software

References

• https://git.kernel.org/stable/c/9869d3a6fed381f3b98404e26e1afc75d680cbf9
• https://git.kernel.org/stable/c/f55201fb3becff6a903fd29f4d1147cc7e91eb0c