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

Overview

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.

Affected

• Linux >= be4000bc4644d027c519b6361f5ae3bbfc52c347 < 1cde96d5d53b9105b48617afbc4682a83db4ff04
• Linux >= be4000bc4644d027c519b6361f5ae3bbfc52c347 < d5b7730f06994499632026c30e38e0317c4569e2
• Linux >= be4000bc4644d027c519b6361f5ae3bbfc52c347 < dcf416eb88eafe1e3c0f920a14bdffd10bc4d259
• Linux >= be4000bc4644d027c519b6361f5ae3bbfc52c347 < ad38f2cdfef9a2f2899c30cad269baec5bfd4a5d
• Linux >= be4000bc4644d027c519b6361f5ae3bbfc52c347 < cc2f39d6ac48e6e3cb2d6240bc0d6df839dd0828
• Linux 3.17

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.