[PATCH v4] tracing: Fix race between update_event_fields and, event_define_fields

[Michael Wu <michael@allwinnertech.com>]

event_define_fields() (pri=1 MODULE_STATE_COMING notifier, locked by
event_mutex) populates class->fields via list_add(), while
update_event_fields() (called from the pri=0 notifier path via
trace_event_update_all) traverses class->fields protected only by
trace_event_sem.  These are two different locks guarding the same
data structure, so during cross-module loading a reader on one CPU can
observe partially initialized list nodes being concurrently added by a
writer on another CPU.

On arm64 with weak memory ordering, __list_add() writes to two
different cache lines:

        next->prev = new;                // (1) ordinary store
        new->next = next;                // (2) ordinary store
        new->prev = prev;                // (3) ordinary store
        WRITE_ONCE(prev->next, new);    // (4) release store

The store buffer can drain (2) and (4) independently since they target
different cache lines.  A remote CPU may observe (4) before (2): it
sees prev->next pointing to the new node, but the new node's link.next
is still zero (kmem_cache_alloc zero-initialized via KMEM_CACHE with
SLAB_PANIC).  Since offsetof(struct ftrace_event_field, link) == 0,
list_for_each_entry() derives field == NULL from link.next == 0 and
crashes at field->type (offset 0x18):

  Unable to handle kernel access ... at virtual address 0000000000000018
  pc : update_event_fields+0xf8/0x368
  Call trace:
   update_event_fields+0xf8/0x368
   trace_event_update_all+0x7c/0x2b4
   trace_module_notify+0x4c/0x1dc
   notifier_call_chain+0x84/0x168
   blocking_notifier_call_chain_robust+0x64/0xd4
   load_module+0x10c8/0x123c
   __arm64_sys_finit_module+0x230/0x31c

Fix by holding event_mutex in trace_event_update_all() before
down_write(&trace_event_sem).  This serializes the reader against
event_define_fields() which takes event_mutex for the entire
write-side critical section.  The lock ordering event_mutex ->
trace_event_sem is already established at three other sites
(trace_remove_event_call, event_trace_add_tracer,
event_trace_del_tracer), so this introduces no ordering conflict.

An alternative of placing mutex_lock inside update_event_fields() was
considered but rejected -- it would invert the established lock
ordering (trace_event_sem -> event_mutex) and risk ABBA deadlock.

Fixes: b3bc8547d3be ("tracing: Have TRACE_DEFINE_ENUM affect trace event types as well")
Cc: stable@vger.kernel.org
Signed-off-by: Michael Wu <michael@allwinnertech.com>
---
 kernel/trace/trace_events.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 956692856fa8..9632788da5af 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -3566,6 +3566,7 @@ void trace_event_update_all(struct trace_eval_map **map, int len)
 	int last_i;
 	int i;
 
+	mutex_lock(&event_mutex);
 	down_write(&trace_event_sem);
 	list_for_each_entry_safe(call, p, &ftrace_events, list) {
 		/* events are usually grouped together with systems */
@@ -3604,6 +3605,7 @@ void trace_event_update_all(struct trace_eval_map **map, int len)
 		cond_resched();
 	}
 	up_write(&trace_event_sem);
+	mutex_unlock(&event_mutex);
 }
 
 static bool event_in_systems(struct trace_event_call *call,
-- 
2.29.0