[PATCH v2] tracing/user_events: Clear copied tracing state before fork duplication

["Jérémy Jean" <Jeremy.Jean@oss.cyber.gouv.fr>]

dup_task_struct() copies user_event_mm from the parent into the child,
without grabbing a reference to it. user_event_mm_dup() should
replace it, but it leaves that copied pointer unmodified if
user_event_mm_alloc() fails.

When the child exits, user_event_mm_remove() decrements a reference
the child never owned, which ultimately frees user_event_mm, while
the parent still as a stale pointer to it. This creates a UAF, which
KASAN reports as:

    BUG: KASAN: slab-use-after-free in
    current_user_event_mm+0x51/0x1d0 Write of size 4 at addr
    ffff888005010d30 by task init/44

    Call Trace:
     <TASK>
     kasan_report+0xce/0x100
     kasan_check_range+0x10f/0x1e0
     current_user_event_mm+0x51/0x1d0
     user_events_ioctl+0x82e/0x15c0
     __x64_sys_ioctl+0x139/0x1c0
     do_syscall_64+0xce/0x450
     entry_SYSCALL_64_after_hwframe+0x77/0x7f

    Allocated by task 44:
     __kasan_kmalloc+0x8f/0xa0
     __kmalloc_cache_noprof+0x180/0x3a0
     user_event_mm_alloc+0x3c/0x1f0
     current_user_event_mm+0x88/0x1d0

    Freed by task 42:
     __kasan_slab_free+0x43/0x70
     kfree+0x13a/0x390
     process_one_work+0x696/0xf90
     worker_thread+0x420/0xba0

The fix simply clears the copied pointer before starting the
duplication, before any possible failure. In case of failure,
the child then has nothing to free.

Fixes: 7235759084a4 ("tracing/user_events: Use remote writes for event enablement")
Cc: stable@vger.kernel.org
Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
---

Change in v2:
 Move the pointer reset into user_event_mm_dup(), before the first
 allocation (suggestion by Steven Rostedt).

 kernel/trace/trace_events_user.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index 2bbc89d4a266..339e18085af3 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -865,9 +865,12 @@ void user_event_mm_remove(struct task_struct *t)
 
 void user_event_mm_dup(struct task_struct *t, struct user_event_mm *old_mm)
 {
-	struct user_event_mm *mm = user_event_mm_alloc(t);
+	struct user_event_mm *mm;
 	struct user_event_enabler *enabler;
 
+	t->user_event_mm = NULL;
+	mm = user_event_mm_alloc(t);
+
 	if (!mm)
 		return;
 
-- 
2.47.3