[PATCH AUTOSEL 4.9 81/98] perf/core: Fix locking for children siblings group read

From: Sasha Levin
Date: Thu Oct 25 2018 - 10:16:29 EST

From: Jiri Olsa <jolsa@xxxxxxxxxx>

[ 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:

lock event->ctx->mutex
lock leader->child_mutex
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:

lock ctx->mutex
list_for_each_entry_safe(child_event, next, &child_ctx->event_list,...
lock ctx->lock
unlock ctx->lock

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


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>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
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;