Re: [PATCH] tracing/user_events: Use kfree_rcu for enabler cleanup

From: Tristan Madani <redacted>

user_event_enabler_destroy() removes the enabler from an RCU-protected
list via list_del_rcu() and then immediately frees it with kfree(). This
can result in a concurrent reader in user_event_enabler_dup() accessing
stale memory during fork, since the enabler list is traversed under
rcu_read_lock().

The ENABLE_VAL_FREEING_BIT check in user_event_enabler_dup() is not
sufficient to prevent this, as the enabler can be freed between the bit
test and the subsequent pointer dereference.

Use kfree_rcu() to defer the free until after all RCU read-side critical
sections complete.

Fixes: 7235759084a4 ("tracing/user_events: Use remote writes for event enablement")
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <redacted>
---
 kernel/trace/trace_events_user.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index c4ba484f7b38b..72bcb429eb4f3 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c

@@ -109,6 +109,7 @@ struct user_event_enabler {
 
 	/* Track enable bit, flags, etc. Aligned for bitops. */
 	unsigned long		values;
+	struct rcu_head		rcu;
 };
 
 /* Bits 0-5 are for the bit to update upon enable/disable (0-63 allowed) */

@@ -404,7 +405,7 @@ static void user_event_enabler_destroy(struct user_event_enabler *enabler,
 	/* No longer tracking the event via the enabler */
 	user_event_put(enabler->event, locked);
 
-	kfree(enabler);
+	kfree_rcu(enabler, rcu);
 }
 
 static int user_event_mm_fault_in(struct user_event_mm *mm, unsigned long uaddr,

-- 
2.47.3