user_event_mm_dup() walks the parent mm's enabler list locklessly under
rcu_read_lock() during fork() (from copy_process()); it does not take
event_mutex:
rcu_read_lock();
list_for_each_entry_rcu(enabler, &old_mm->enablers, mm_enablers_link)
enabler->event = user_event_get(orig->event);
user_event_enabler_destroy() removes an enabler from that list with
list_del_rcu() and then, without waiting for a grace period, drops the
enabler's user_event reference with user_event_put() and frees the enabler
with kfree(). A reader that loaded the enabler before the list_del_rcu()
can still be walking it, which leads to two use-after-frees:
- kfree(enabler) frees the enabler while that reader dereferences
enabler->event.
- user_event_put() may drop the last reference to the user_event, which
is then freed (via delayed_destroy_user_event() on a work queue), while
the same reader does user_event_get(orig->event) on it.
Both are reachable by an unprivileged task that can open user_events_data:
one multithreaded process that registers an enabler and then concurrently
unregisters it and calls fork() triggers the race. KASAN reports a
slab-use-after-free in user_event_mm_dup() during clone(), with a
"refcount_t: addition on 0" warning when the user_event is freed.
The enabler use-after-free was found first; the user_event one was reported
by XIAO WU, and the earlier enabler-only fix did not address it.
Defer both the user_event_put() and the kfree(enabler) to a work item
queued with queue_rcu_work(), so they run only after an RCU grace period,
once all readers walking the enabler list have finished. The put must run
in process context because user_event_put() takes event_mutex on the last
reference, so a work queue is used rather than call_rcu(). The now-unlocked
put lets the locked argument of user_event_enabler_destroy() be removed;
all callers are updated.
Fixes: 7235759084a4 ("tracing/user_events: Use remote writes for event
enablement")
Cc: [email protected]
Reported-by: XIAO WU <[email protected]>
Closes:
https://lore.kernel.org/all/[email protected]/
Suggested-by: Beau Belgrave <[email protected]>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <[email protected]>
---
Since v1 (the enabler-only fix "tracing/user_events: fix use-after-free
of enabler in user_event_mm_dup()", being dropped from for-linus):
- also fix the user_event-object UAF reported by XIAO WU; defer both the
put and the free with queue_rcu_work() instead of kfree_rcu().
- drop the locked argument now that the put runs from the work item.
- add patch 2 for the selftest teardown timing.
kernel/trace/trace_events_user.c | 39 ++++++++++++++++++++++++++------
1 file changed, 32 insertions(+), 7 deletions(-)
diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index c4ba484f7b38b..8c82ecb735f41 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -109,6 +109,9 @@ struct user_event_enabler {
/* Track enable bit, flags, etc. Aligned for bitops. */
unsigned long values;
+
+ /* Defer the event put and enabler free past an RCU grace period. */
+ struct rcu_work put_rwork;
};
/* Bits 0-5 are for the bit to update upon enable/disable (0-63 allowed) */
@@ -396,17 +399,39 @@ static struct user_event_group
*user_event_group_create(void)
return NULL;
};
-static void user_event_enabler_destroy(struct user_event_enabler *enabler,
- bool locked)
+static void delayed_user_event_enabler_put(struct work_struct *work)
{
- list_del_rcu(&enabler->mm_enablers_link);
+ struct user_event_enabler *enabler = container_of(to_rcu_work(work),
+ struct user_event_enabler, put_rwork);
/* No longer tracking the event via the enabler */
- user_event_put(enabler->event, locked);
+ user_event_put(enabler->event, false);
+ /* Run from queue_rcu_work(), the RCU grace period has elapsed */
kfree(enabler);
}
+static void user_event_enabler_destroy(struct user_event_enabler *enabler)
+{
+ list_del_rcu(&enabler->mm_enablers_link);
+
+ /*
+ * The enabler is removed from an RCU-traversed list
+ * (user_event_mm_dup() walks mm->enablers under rcu_read_lock() only),
+ * and readers there dereference enabler->event and take a new ref on
+ * it. Both the put of that event reference and the free of the enabler
+ * therefore have to wait for a grace period so no reader can be looking
+ * at the enabler or racing the last put of its event.
+ *
+ * The put itself must not run in RCU context: when it drops the last
+ * reference user_event_put() takes event_mutex, which cannot be taken
+ * from a softirq/RCU callback. Defer both to a work item scheduled
+ * after a grace period via queue_rcu_work().
+ */
+ INIT_RCU_WORK(&enabler->put_rwork, delayed_user_event_enabler_put);
+ queue_rcu_work(system_percpu_wq, &enabler->put_rwork);
+}
+
static int user_event_mm_fault_in(struct user_event_mm *mm, unsigned long
uaddr,
int attempt)
{
@@ -464,7 +489,7 @@ static void user_event_enabler_fault_fixup(struct
work_struct *work)
/* User asked for enabler to be removed during fault */
if (test_bit(ENABLE_VAL_FREEING_BIT, ENABLE_BITOPS(enabler))) {
- user_event_enabler_destroy(enabler, true);
+ user_event_enabler_destroy(enabler);
goto out;
}
@@ -764,7 +789,7 @@ static void user_event_mm_destroy(struct user_event_mm *mm)
struct user_event_enabler *enabler, *next;
list_for_each_entry_safe(enabler, next, &mm->enablers, mm_enablers_link)
- user_event_enabler_destroy(enabler, false);
+ user_event_enabler_destroy(enabler);
mmdrop(mm->mm);
kfree(mm);
@@ -2645,7 +2670,7 @@ static long user_events_ioctl_unreg(unsigned long uarg)
flags |= enabler->values & ENABLE_VAL_COMPAT_MASK;
if (!test_bit(ENABLE_VAL_FAULTING_BIT,
ENABLE_BITOPS(enabler)))
- user_event_enabler_destroy(enabler, true);
+ user_event_enabler_destroy(enabler);
/* Removed at least one */
ret = 0;
--
2.53.0
