[PATCH] bpf: Call rcu_read_unlock() before copy_to_user()

[Michael Holzheu]

We must not hold locks when calling copy_to_user():

BUG: sleeping function called from invalid context at mm/memory.c:3732
in_atomic(): 0, irqs_disabled(): 0, pid: 671, name: test_maps
1 lock held by test_maps/671:
 #0:  (rcu_read_lock){......}, at: [<0000000000264190>] map_lookup_elem+0xe8/0x260
Preemption disabled at:[<00000000001be3b6>] vprintk_default+0x56/0x68

CPU: 0 PID: 671 Comm: test_maps Not tainted 3.19.0-rc5-00117-g5eb11d6-dirty #424
       000000001e447bb0 000000001e447c40 0000000000000002 0000000000000000 
       000000001e447ce0 000000001e447c58 000000001e447c58 0000000000115c8a 
       0000000000000000 0000000000c08246 0000000000c27e8a 000000000000000b 
       000000001e447ca0 000000001e447c40 0000000000000000 0000000000000000 
       0000000000000000 0000000000115c8a 000000001e447c40 000000001e447ca0 
Call Trace:
([<0000000000115b7e>] show_trace+0x12e/0x150)
 [<0000000000115c40>] show_stack+0xa0/0x100
 [<00000000009b163c>] dump_stack+0x74/0xc8
 [<000000000017424a>] ___might_sleep+0x23a/0x248
 [<00000000002b58e8>] might_fault+0x70/0xe8
 [<0000000000264230>] map_lookup_elem+0x188/0x260
 [<0000000000264716>] SyS_bpf+0x20e/0x840
 [<00000000009bbe3a>] system_call+0xd6/0x24c
 [<000003fffd15f566>] 0x3fffd15f566
1 lock held by test_maps/671:
 #0:  (rcu_read_lock){......}, at: [<0000000000264190>] map_lookup_elem+0xe8/0x260

So call rcu_read_unlock() before copy_to_user(). We can
release the lock earlier because it is not needed for copy_to_user().

Signed-off-by: Michael Holzheu <holzheu@xxxxxxxxxxxxxxxxxx>
---
 kernel/bpf/syscall.c |    7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -172,17 +172,16 @@ static int map_lookup_elem(union bpf_att
 	err = -ENOENT;
 	rcu_read_lock();
 	value = map->ops->map_lookup_elem(map, key);
+	rcu_read_unlock();
 	if (!value)
-		goto err_unlock;
+		goto free_key;
 
 	err = -EFAULT;
 	if (copy_to_user(uvalue, value, map->value_size) != 0)
-		goto err_unlock;
+		goto free_key;
 
 	err = 0;
 
-err_unlock:
-	rcu_read_unlock();
 free_key:
 	kfree(key);
 err_put:

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