hwrng: core - use RCU and work_struct to fix race condition

Summary

In the Linux kernel, the following vulnerability has been resolved: hwrng: core - use RCU and work_struct to fix race condition Currently, hwrng_fill is not cleared until the hwrng_fillfn() thread exits. Since hwrng_unregister() reads hwrng_fill outside the rng_mutex lock, a concurrent hwrng_unregister() may call kthread_stop() again on the same task. Additionally, if hwrng_unregister() is called immediately after hwrng_register(), the stopped thread may have never been executed. Thus, hwrng_fill remains dirty even after hwrng_unregister() returns. In this case, subsequent calls to hwrng_register() will fail to start new threads, and hwrng_unregister() will call kthread_stop() on the same freed task. In both cases, a use-after-free occurs: refcount_t: addition on 0; use-after-free. WARNING: ... at lib/refcount.c:25 refcount_warn_saturate+0xec/0x1c0 Call Trace: kthread_stop+0x181/0x360 hwrng_unregister+0x288/0x380 virtrng_remove+0xe3/0x200 This patch fixes the race by protecting the global hwrng_fill pointer inside the rng_mutex lock, so that hwrng_fillfn() thread is stopped only once, and calls to kthread_run() and kthread_stop() are serialized with the lock held. To avoid deadlock in hwrng_fillfn() while being stopped with the lock held, we convert current_rng to RCU, so that get_current_rng() can read current_rng without holding the lock. To remove the lock from put_rng(), we also delay the actual cleanup into a work_struct. Since get_current_rng() no longer returns ERR_PTR values, the IS_ERR() checks are removed from its callers. With hwrng_fill protected by the rng_mutex lock, hwrng_fillfn() can no longer clear hwrng_fill itself. Therefore, if hwrng_fillfn() returns directly after current_rng is dropped, kthread_stop() would be called on a freed task_struct later. To fix this, hwrng_fillfn() calls schedule() now to keep the task alive until being stopped. The kthread_stop() call is also moved from hwrng_unregister() to drop_current_rng(), ensuring kthread_stop() is called on all possible paths where current_rng becomes NULL, so that the thread would not wait forever.

Timeline

1. May 27, 2026, 12:18 PM UTCCVE published
2. Oct 21, 2026, 12:41 PM UTCRecord last updated

Affected versions

• Linux Linux

  From be4000bc4644d027c519b6361f5ae3bbfc52c347 · fixed in 1cde96d5d53b9105b48617afbc4682a83db4ff04; From be4000bc4644d027c519b6361f5ae3bbfc52c347 · fixed in d5b7730f06994499632026c30e38e0317c4569e2; From be4000bc4644d027c519b6361f5ae3bbfc52c347 · fixed in dcf416eb88eafe1e3c0f920a14bdffd10bc4d259; From be4000bc4644d027c519b6361f5ae3bbfc52c347 · fixed in ad38f2cdfef9a2f2899c30cad269baec5bfd4a5d; From be4000bc4644d027c519b6361f5ae3bbfc52c347 · fixed in cc2f39d6ac48e6e3cb2d6240bc0d6df839dd0828

• Linux Linux

  3.17

• KernelLinux

  From 3.17.0 · fixed in 6.6.160; From 6.7.0 · fixed in 6.12.75; From 6.13.0 · fixed in 6.18.14; From 6.19.0 · fixed in 6.19.4

References 5