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Date: Fri, 26 Jul 2019 17:24:54 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-kernel@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
stable@...r.kernel.org,
syzbot+a24c397a29ad22d86c98@...kaller.appspotmail.com,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Arnaldo Carvalho de Melo <acme@...hat.com>,
Jiri Olsa <jolsa@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Stephane Eranian <eranian@...gle.com>,
Thomas Gleixner <tglx@...utronix.de>,
Vince Weaver <vincent.weaver@...ne.edu>,
Ingo Molnar <mingo@...nel.org>
Subject: [PATCH 5.2 55/66] perf/core: Fix race between close() and fork()
From: Peter Zijlstra <peterz@...radead.org>
commit 1cf8dfe8a661f0462925df943140e9f6d1ea5233 upstream.
Syzcaller reported the following Use-after-Free bug:
close() clone()
copy_process()
perf_event_init_task()
perf_event_init_context()
mutex_lock(parent_ctx->mutex)
inherit_task_group()
inherit_group()
inherit_event()
mutex_lock(event->child_mutex)
// expose event on child list
list_add_tail()
mutex_unlock(event->child_mutex)
mutex_unlock(parent_ctx->mutex)
...
goto bad_fork_*
bad_fork_cleanup_perf:
perf_event_free_task()
perf_release()
perf_event_release_kernel()
list_for_each_entry()
mutex_lock(ctx->mutex)
mutex_lock(event->child_mutex)
// event is from the failing inherit
// on the other CPU
perf_remove_from_context()
list_move()
mutex_unlock(event->child_mutex)
mutex_unlock(ctx->mutex)
mutex_lock(ctx->mutex)
list_for_each_entry_safe()
// event already stolen
mutex_unlock(ctx->mutex)
delayed_free_task()
free_task()
list_for_each_entry_safe()
list_del()
free_event()
_free_event()
// and so event->hw.target
// is the already freed failed clone()
if (event->hw.target)
put_task_struct(event->hw.target)
// WHOOPSIE, already quite dead
Which puts the lie to the the comment on perf_event_free_task():
'unexposed, unused context' not so much.
Which is a 'fun' confluence of fail; copy_process() doing an
unconditional free_task() and not respecting refcounts, and perf having
creative locking. In particular:
82d94856fa22 ("perf/core: Fix lock inversion between perf,trace,cpuhp")
seems to have overlooked this 'fun' parade.
Solve it by using the fact that detached events still have a reference
count on their (previous) context. With this perf_event_free_task()
can detect when events have escaped and wait for their destruction.
Debugged-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Reported-by: syzbot+a24c397a29ad22d86c98@...kaller.appspotmail.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Mark Rutland <mark.rutland@....com>
Cc: <stable@...r.kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Jiri Olsa <jolsa@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Stephane Eranian <eranian@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vince Weaver <vincent.weaver@...ne.edu>
Fixes: 82d94856fa22 ("perf/core: Fix lock inversion between perf,trace,cpuhp")
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
kernel/events/core.c | 49 +++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 41 insertions(+), 8 deletions(-)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4469,12 +4469,20 @@ static void _free_event(struct perf_even
if (event->destroy)
event->destroy(event);
- if (event->ctx)
- put_ctx(event->ctx);
-
+ /*
+ * Must be after ->destroy(), due to uprobe_perf_close() using
+ * hw.target.
+ */
if (event->hw.target)
put_task_struct(event->hw.target);
+ /*
+ * perf_event_free_task() relies on put_ctx() being 'last', in particular
+ * all task references must be cleaned up.
+ */
+ if (event->ctx)
+ put_ctx(event->ctx);
+
exclusive_event_destroy(event);
module_put(event->pmu->module);
@@ -4654,8 +4662,17 @@ again:
mutex_unlock(&event->child_mutex);
list_for_each_entry_safe(child, tmp, &free_list, child_list) {
+ void *var = &child->ctx->refcount;
+
list_del(&child->child_list);
free_event(child);
+
+ /*
+ * Wake any perf_event_free_task() waiting for this event to be
+ * freed.
+ */
+ smp_mb(); /* pairs with wait_var_event() */
+ wake_up_var(var);
}
no_ctx:
@@ -11529,11 +11546,11 @@ static void perf_free_event(struct perf_
}
/*
- * Free an unexposed, unused context as created by inheritance by
- * perf_event_init_task below, used by fork() in case of fail.
+ * Free a context as created by inheritance by perf_event_init_task() below,
+ * used by fork() in case of fail.
*
- * Not all locks are strictly required, but take them anyway to be nice and
- * help out with the lockdep assertions.
+ * Even though the task has never lived, the context and events have been
+ * exposed through the child_list, so we must take care tearing it all down.
*/
void perf_event_free_task(struct task_struct *task)
{
@@ -11563,7 +11580,23 @@ void perf_event_free_task(struct task_st
perf_free_event(event, ctx);
mutex_unlock(&ctx->mutex);
- put_ctx(ctx);
+
+ /*
+ * perf_event_release_kernel() could've stolen some of our
+ * child events and still have them on its free_list. In that
+ * case we must wait for these events to have been freed (in
+ * particular all their references to this task must've been
+ * dropped).
+ *
+ * Without this copy_process() will unconditionally free this
+ * task (irrespective of its reference count) and
+ * _free_event()'s put_task_struct(event->hw.target) will be a
+ * use-after-free.
+ *
+ * Wait for all events to drop their context reference.
+ */
+ wait_var_event(&ctx->refcount, refcount_read(&ctx->refcount) == 1);
+ put_ctx(ctx); /* must be last */
}
}
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