kthread: consolidate kthread exit paths to prevent use-after-free

Guillaume reported crashes via corrupted RCU callback function pointers
during KUnit testing. The crash was traced back to the pidfs rhashtable
conversion which replaced the 24-byte rb_node with an 8-byte rhash_head
in struct pid, shrinking it from 160 to 144 bytes.

struct kthread (without CONFIG_BLK_CGROUP) is also 144 bytes. With
CONFIG_SLAB_MERGE_DEFAULT and SLAB_HWCACHE_ALIGN both round up to
192 bytes and share the same slab cache. struct pid.rcu.func and
struct kthread.affinity_node both sit at offset 0x78.

When a kthread exits via make_task_dead() it bypasses kthread_exit() and
misses the affinity_node cleanup. free_kthread_struct() frees the memory
while the node is still linked into the global kthread_affinity_list. A
subsequent list_del() by another kthread writes through dangling list
pointers into the freed and reused memory, corrupting the pid's
rcu.func pointer.

Instead of patching free_kthread_struct() to handle the missed cleanup,
consolidate all kthread exit paths. Turn kthread_exit() into a macro
that calls do_exit() and add kthread_do_exit() which is called from
do_exit() for any task with PF_KTHREAD set. This guarantees that
kthread-specific cleanup always happens regardless of the exit path -
make_task_dead(), direct do_exit(), or kthread_exit().

Replace __to_kthread() with a new tsk_is_kthread() accessor in the
public header. Export do_exit() since module code using the
kthread_exit() macro now needs it directly.

Reported-by: Guillaume Tucker <[email protected]>
Tested-by: Guillaume Tucker <[email protected]>
Tested-by: Mark Brown <[email protected]>
Tested-by: David Gow <[email protected]>
Cc: <[email protected]>
Link: https://lore.kernel.org/all/20260224-mittlerweile-besessen-2738831ae7f6@brauner
Co-developed-by: Linus Torvalds <[email protected]>
Fixes: 4d13f43 ("kthread: Implement preferred affinity")
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: brauner

include/linux/kthread.h

@@ -7,6 +7,24 @@
 
 struct mm_struct;
 
+/* opaque kthread data */
+struct kthread;
+
+/*
+ * When "(p->flags & PF_KTHREAD)" is set the task is a kthread and will
+ * always remain a kthread.  For kthreads p->worker_private always
+ * points to a struct kthread.  For tasks that are not kthreads
+ * p->worker_private is used to point to other things.
+ *
+ * Return NULL for any task that is not a kthread.
+ */
+static inline struct kthread *tsk_is_kthread(struct task_struct *p)
+{
+	if (p->flags & PF_KTHREAD)
+		return p->worker_private;
+	return NULL;
+}
+
 __printf(4, 5)
 struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
 					   void *data,
@@ -98,9 +116,10 @@ void *kthread_probe_data(struct task_struct *k);
 int kthread_park(struct task_struct *k);
 void kthread_unpark(struct task_struct *k);
 void kthread_parkme(void);
-void kthread_exit(long result) __noreturn;
+#define kthread_exit(result) do_exit(result)
 void kthread_complete_and_exit(struct completion *, long) __noreturn;
 int kthreads_update_housekeeping(void);
+void kthread_do_exit(struct kthread *, long);
 
 int kthreadd(void *unused);
 extern struct task_struct *kthreadd_task;

kernel/exit.c

@@ -896,11 +896,16 @@ static void synchronize_group_exit(struct task_struct *tsk, long code)
 void __noreturn do_exit(long code)
 {
 	struct task_struct *tsk = current;
+	struct kthread *kthread;
 	int group_dead;
 
 	WARN_ON(irqs_disabled());
 	WARN_ON(tsk->plug);
 
+	kthread = tsk_is_kthread(tsk);
+	if (unlikely(kthread))
+		kthread_do_exit(kthread, code);
+
 	kcov_task_exit(tsk);
 	kmsan_task_exit(tsk);
 
@@ -1013,6 +1018,7 @@ void __noreturn do_exit(long code)
 	lockdep_free_task(tsk);
 	do_task_dead();
 }
+EXPORT_SYMBOL(do_exit);
 
 void __noreturn make_task_dead(int signr)
 {

kernel/kthread.c

@@ -85,24 +85,6 @@ static inline struct kthread *to_kthread(struct task_struct *k)
 	return k->worker_private;
 }
 
-/*
- * Variant of to_kthread() that doesn't assume @p is a kthread.
- *
- * When "(p->flags & PF_KTHREAD)" is set the task is a kthread and will
- * always remain a kthread.  For kthreads p->worker_private always
- * points to a struct kthread.  For tasks that are not kthreads
- * p->worker_private is used to point to other things.
- *
- * Return NULL for any task that is not a kthread.
- */
-static inline struct kthread *__to_kthread(struct task_struct *p)
-{
-	void *kthread = p->worker_private;
-	if (kthread && !(p->flags & PF_KTHREAD))
-		kthread = NULL;
-	return kthread;
-}
-
 void get_kthread_comm(char *buf, size_t buf_size, struct task_struct *tsk)
 {
 	struct kthread *kthread = to_kthread(tsk);
@@ -193,7 +175,7 @@ EXPORT_SYMBOL_GPL(kthread_should_park);
 
 bool kthread_should_stop_or_park(void)
 {
-	struct kthread *kthread = __to_kthread(current);
+	struct kthread *kthread = tsk_is_kthread(current);
 
 	if (!kthread)
 		return false;
@@ -234,7 +216,7 @@ EXPORT_SYMBOL_GPL(kthread_freezable_should_stop);
  */
 void *kthread_func(struct task_struct *task)
 {
-	struct kthread *kthread = __to_kthread(task);
+	struct kthread *kthread = tsk_is_kthread(task);
 	if (kthread)
 		return kthread->threadfn;
 	return NULL;
@@ -266,7 +248,7 @@ EXPORT_SYMBOL_GPL(kthread_data);
  */
 void *kthread_probe_data(struct task_struct *task)
 {
-	struct kthread *kthread = __to_kthread(task);
+	struct kthread *kthread = tsk_is_kthread(task);
 	void *data = NULL;
 
 	if (kthread)
@@ -309,19 +291,8 @@ void kthread_parkme(void)
 }
 EXPORT_SYMBOL_GPL(kthread_parkme);
 
-/**
- * kthread_exit - Cause the current kthread return @result to kthread_stop().
- * @result: The integer value to return to kthread_stop().
- *
- * While kthread_exit can be called directly, it exists so that
- * functions which do some additional work in non-modular code such as
- * module_put_and_kthread_exit can be implemented.
- *
- * Does not return.
- */
-void __noreturn kthread_exit(long result)
+void kthread_do_exit(struct kthread *kthread, long result)
 {
-	struct kthread *kthread = to_kthread(current);
 	kthread->result = result;
 	if (!list_empty(&kthread->affinity_node)) {
 		mutex_lock(&kthread_affinity_lock);
@@ -333,9 +304,7 @@ void __noreturn kthread_exit(long result)
 			kthread->preferred_affinity = NULL;
 		}
 	}
-	do_exit(0);
 }
-EXPORT_SYMBOL(kthread_exit);
 
 /**
  * kthread_complete_and_exit - Exit the current kthread.
@@ -683,7 +652,7 @@ void kthread_set_per_cpu(struct task_struct *k, int cpu)
 
 bool kthread_is_per_cpu(struct task_struct *p)
 {
-	struct kthread *kthread = __to_kthread(p);
+	struct kthread *kthread = tsk_is_kthread(p);
 	if (!kthread)
 		return false;