perf: Fix race in perf_event_exit_task_context()
There is a race between perf_event_exit_task_context() and
orphans_remove_work() which results in a use-after-free.
We mark ctx->task with TASK_TOMBSTONE to indicate a context is
'dead', under ctx->lock. After which point event_function_call()
on any event of that context will NOP
A concurrent orphans_remove_work() will only hold ctx->mutex for
the list iteration and not serialize against this. Therefore its
possible that orphans_remove_work()'s perf_remove_from_context()
call will fail, but we'll continue to free the event, with the
result of free'd memory still being on lists and everything.
Once perf_event_exit_task_context() gets around to acquiring
ctx->mutex it too will iterate the event list, encounter the
already free'd event and proceed to free it _again_. This fails
with the WARN in free_event().
Plug the race by having perf_event_exit_task_context() hold
ctx::mutex over the whole tear-down, thereby 'naturally'
serializing against all other sites, including the orphan work.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: alexander.shishkin@linux.intel.com
Cc: dsahern@gmail.com
Cc: namhyung@kernel.org
Link: http://lkml.kernel.org/r/20160125130954.GY6357@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 6759f2a..1d243fa 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8748,14 +8748,40 @@
{
struct perf_event_context *child_ctx, *clone_ctx = NULL;
struct perf_event *child_event, *next;
- unsigned long flags;
WARN_ON_ONCE(child != current);
- child_ctx = perf_lock_task_context(child, ctxn, &flags);
+ child_ctx = perf_pin_task_context(child, ctxn);
if (!child_ctx)
return;
+ /*
+ * In order to reduce the amount of tricky in ctx tear-down, we hold
+ * ctx::mutex over the entire thing. This serializes against almost
+ * everything that wants to access the ctx.
+ *
+ * The exception is sys_perf_event_open() /
+ * perf_event_create_kernel_count() which does find_get_context()
+ * without ctx::mutex (it cannot because of the move_group double mutex
+ * lock thing). See the comments in perf_install_in_context().
+ *
+ * We can recurse on the same lock type through:
+ *
+ * __perf_event_exit_task()
+ * sync_child_event()
+ * put_event()
+ * mutex_lock(&ctx->mutex)
+ *
+ * But since its the parent context it won't be the same instance.
+ */
+ mutex_lock(&child_ctx->mutex);
+
+ /*
+ * In a single ctx::lock section, de-schedule the events and detach the
+ * context from the task such that we cannot ever get it scheduled back
+ * in.
+ */
+ raw_spin_lock_irq(&child_ctx->lock);
task_ctx_sched_out(__get_cpu_context(child_ctx), child_ctx);
/*
@@ -8767,14 +8793,8 @@
WRITE_ONCE(child_ctx->task, TASK_TOMBSTONE);
put_task_struct(current); /* cannot be last */
- /*
- * If this context is a clone; unclone it so it can't get
- * swapped to another process while we're removing all
- * the events from it.
- */
clone_ctx = unclone_ctx(child_ctx);
- update_context_time(child_ctx);
- raw_spin_unlock_irqrestore(&child_ctx->lock, flags);
+ raw_spin_unlock_irq(&child_ctx->lock);
if (clone_ctx)
put_ctx(clone_ctx);
@@ -8786,18 +8806,6 @@
*/
perf_event_task(child, child_ctx, 0);
- /*
- * We can recurse on the same lock type through:
- *
- * __perf_event_exit_task()
- * sync_child_event()
- * put_event()
- * mutex_lock(&ctx->mutex)
- *
- * But since its the parent context it won't be the same instance.
- */
- mutex_lock(&child_ctx->mutex);
-
list_for_each_entry_safe(child_event, next, &child_ctx->event_list, event_entry)
__perf_event_exit_task(child_event, child_ctx, child);