netdev - [PATCH bpf] bpf: fix a rcu usage warning in bpf_prog_array_copy

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Date:   Tue, 14 Aug 2018 11:01:12 -0700
From:   Yonghong Song <yhs@...com>
To:     <ast@...com>, <daniel@...earbox.net>, <netdev@...r.kernel.org>
CC:     <kernel-team@...com>, Roman Gushchin <guro@...com>
Subject: [PATCH bpf] bpf: fix a rcu usage warning in bpf_prog_array_copy_core()

Commit 394e40a29788 ("bpf: extend bpf_prog_array to store pointers
to the cgroup storage") refactored the bpf_prog_array_copy_core()
to accommodate new structure bpf_prog_array_item which contains
bpf_prog array itself.

In the old code, we had
   perf_event_query_prog_array():
     mutex_lock(...)
     bpf_prog_array_copy_call():
       prog = rcu_dereference_check(array, 1)->progs
       bpf_prog_array_copy_core(prog, ...)
     mutex_unlock(...)

With the above commit, we had
   perf_event_query_prog_array():
     mutex_lock(...)
     bpf_prog_array_copy_call():
       bpf_prog_array_copy_core(array, ...):
         item = rcu_dereference(array)->items;
         ...
     mutex_unlock(...)

The new code will trigger a lockdep rcu checking warning.
The fix is to change rcu_dereference() to rcu_dereference_check()
to prevent such a warning.

Reported-by: syzbot+6e72317008eef84a216b@...kaller.appspotmail.com
Fixes: 394e40a29788 ("bpf: extend bpf_prog_array to store pointers to the cgroup storage")
Cc: Roman Gushchin <guro@...com>
Signed-off-by: Yonghong Song <yhs@...com>
---
 kernel/bpf/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 4d09e610777f..3f5bf1af0826 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1579,7 +1579,7 @@ static bool bpf_prog_array_copy_core(struct bpf_prog_array __rcu *array,
 	struct bpf_prog_array_item *item;
 	int i = 0;
 
-	item = rcu_dereference(array)->items;
+	item = rcu_dereference_check(array, 1)->items;
 	for (; item->prog; item++) {
 		if (item->prog == &dummy_bpf_prog.prog)
 			continue;
-- 
2.17.1