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 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: [email protected]
Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <[email protected]>
Reviewed-by: Bradley Morgan <[email protected]>
---
Change in v3:
- Simplify the fix.
- Add Reviewed-by: tag as requested.
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 | 3 +++
1 file changed, 3 insertions(+)
diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index 8c82ecb735f4..6b89d225b189 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -868,6 +868,9 @@ 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_enabler *enabler;
+ /* On failure, do not free parent's copy */
+ t->user_event_mm = NULL;
+
if (!mm)
return;
--
2.47.3
