From: Jiri Olsa jolsa@kernel.org
[ Upstream commit 2aeb1883547626d82c597cce2c99f0b9c62e2425 ]
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@linux.intel.com Signed-off-by: Jiri Olsa jolsa@kernel.org Acked-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: Alexander Shishkin alexander.shishkin@linux.intel.com Cc: Arnaldo Carvalho de Melo acme@redhat.com Cc: Jiri Olsa jolsa@redhat.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Peter Zijlstra a.p.zijlstra@chello.nl Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Fixes: ba5213ae6b88 ("perf/core: Correct event creation with PERF_FORMAT_GROUP") Link: http://lkml.kernel.org/r/20170720141455.2106-1-jolsa@kernel.org Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Sasha Levin sashal@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 95bd00d9f2c3..06b359af4322 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -4331,7 +4331,9 @@ 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;
@@ -4361,12 +4363,15 @@ 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; }