[PATCH] perf: Fix locking for children siblings group read

[Jiri Olsa]

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.

Tested-by: Andi Kleen <a...@linux.intel.com>
Signed-off-by: Jiri Olsa <jo...@kernel.org>
---
 kernel/events/core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 9747e422ab20..585955b41489 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4365,7 +4365,9 @@ EXPORT_SYMBOL_GPL(perf_event_read_value);
 static int __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 */
        int ret;
 
@@ -4395,12 +4397,15 @@ static int __perf_read_group_add(struct perf_event 
*leader,
        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_count(sub);
                if (read_format & PERF_FORMAT_ID)
                        values[n++] = primary_event_id(sub);
        }
 
+       raw_spin_unlock_irqrestore(&ctx->lock, flags);
        return 0;
 }
 
-- 
2.9.4