[PATCH 3.2 018/147] perf/core: Fix locking for children siblings group read

[Ben Hutchings]

3.2.95-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jiri Olsa <jolsa@xxxxxxxxxx>

commit 2aeb1883547626d82c597cce2c99f0b9c62e2425 upstream.

We're missing ctx lock when iterating children siblings
within the perf_read path for group reading. Following
race and crash can happen:

User space doing read syscall on event group leader:

T1:
  perf_read
    lock event->ctx->mutex
    perf_read_group
      lock leader->child_mutex
      __perf_read_group_add(child)
        list_for_each_entry(sub, &leader->sibling_list, group_entry)

---->   sub might be invalid at this point, because it could
        get removed via perf_event_exit_task_context in T2

Child exiting and cleaning up its events:

T2:
  perf_event_exit_task_context
    lock ctx->mutex
    list_for_each_entry_safe(child_event, next, &child_ctx->event_list,...
      perf_event_exit_event(child)
        lock ctx->lock
        perf_group_detach(child)
        unlock ctx->lock

---->   child is removed from sibling_list without any sync
        with T1 path above

        ...
        free_event(child)

Before the child is removed from the leader's child_list,
(and thus is omitted from perf_read_group processing), we
need to ensure that perf_read_group touches child's
siblings under its ctx->lock.

Peter further notes:

| One additional note; this bug got exposed by commit:
|
|   ba5213ae6b88 ("perf/core: Correct event creation with PERF_FORMAT_GROUP")
|
| which made it possible to actually trigger this code-path.

Tested-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Fixes: ba5213ae6b88 ("perf/core: Correct event creation with PERF_FORMAT_GROUP")
Link: http://lkml.kernel.org/r/20170720141455.2106-1-jolsa@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3280,7 +3280,9 @@ EXPORT_SYMBOL_GPL(perf_event_read_value)
 static void __perf_read_group_add(struct perf_event *leader,
 					u64 read_format, u64 *values)
 {
+	struct perf_event_context *ctx = leader->ctx;
 	struct perf_event *sub;
+	unsigned long flags;
 	int n = 1; /* skip @nr */
 	u64 count, enabled, running;
 
@@ -3303,11 +3305,15 @@ static void __perf_read_group_add(struct
 	if (read_format & PERF_FORMAT_ID)
 		values[n++] = primary_event_id(leader);
 
+	raw_spin_lock_irqsave(&ctx->lock, flags);
+
 	list_for_each_entry(sub, &leader->sibling_list, group_entry) {
 		values[n++] = perf_event_read_value(sub, &enabled, &running);
 		if (read_format & PERF_FORMAT_ID)
 			values[n++] = primary_event_id(sub);
 	}
+
+	raw_spin_unlock_irqrestore(&ctx->lock, flags);
 }
 
 static int perf_event_read_group(struct perf_event *event,