From cc2f39d6ac48e6e3cb2d6240bc0d6df839dd0828 Mon Sep 17 00:00:00 2001 From: Lianjie Wang Date: Fri, 30 Jan 2026 06:50:16 +0900 Subject: 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. Fixes: be4000bc4644 ("hwrng: create filler thread") Suggested-by: Herbert Xu Signed-off-by: Lianjie Wang Signed-off-by: Herbert Xu --- include/linux/hw_random.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'include/linux') diff --git a/include/linux/hw_random.h b/include/linux/hw_random.h index b424555753b1..b77bc55a4cf3 100644 --- a/include/linux/hw_random.h +++ b/include/linux/hw_random.h @@ -15,6 +15,7 @@ #include #include #include +#include /** * struct hwrng - Hardware Random Number Generator driver @@ -48,6 +49,7 @@ struct hwrng { /* internal. */ struct list_head list; struct kref ref; + struct work_struct cleanup_work; struct completion cleanup_done; struct completion dying; }; -- cgit v1.2.3