Subject: Re: [PATCH] net af_key: Fix RCU splat

From: David Ahern <[email-masked]>
Date: Thu, 20 Aug 2015 15:57:34 -0700

On 8/20/15 9:51 AM, Eric Dumazet wrote:
> On Thu, 2015-08-20 at 08:51 -0700, David Ahern wrote:
>> Hit the following splat testing VRF change for ipsec:
>>
>> [  113.475692] ===============================
>> [  113.476194] [ INFO: suspicious RCU usage. ]
>> [  113.476667] 4.2.0-rc6-1+deb7u2+clUNRELEASED #3.2.65-1+deb7u2+clUNRELEASED Not tainted
>> [  113.477545] -------------------------------
>> [  113.478013] /work/monster-14/dsa/kernel.git/include/linux/rcupdate.h:568 Illegal context switch in RCU read-side critical section!
>> [  113.479288]
>> [  113.479288] other info that might help us debug this:
>> [  113.479288]
>> [  113.480207]
>> [  113.480207] rcu_scheduler_active = 1, debug_locks = 1
>> [  113.480931] 2 locks held by setkey/6829:
>> [  113.481371]  #0:  (&net->xfrm.xfrm_cfg_mutex){+.+.+.}, at: [<ffffffff814e9887>] pfkey_sendmsg+0xfb/0x213
>> [  113.482509]  #1:  (rcu_read_lock){......}, at: [<ffffffff814e767f>] rcu_read_lock+0x0/0x6e
>> [  113.483509]
>> [  113.483509] stack backtrace:
>> [  113.484041] CPU: 0 PID: 6829 Comm: setkey Not tainted 4.2.0-rc6-1+deb7u2+clUNRELEASED #3.2.65-1+deb7u2+clUNRELEASED
>> [  113.485422] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5.1-0-g8936dbb-20141113_115728-nilsson.home.kraxel.org 04/01/2014
>> [  113.486845]  0000000000000001 ffff88001d4c7a98 ffffffff81518af2 ffffffff81086962
>> [  113.487732]  ffff88001d538480 ffff88001d4c7ac8 ffffffff8107ae75 ffffffff8180a154
>> [  113.488628]  0000000000000b30 0000000000000000 00000000000000d0 ffff88001d4c7ad8
>> [  113.489525] Call Trace:
>> [  113.489813]  [<ffffffff81518af2>] dump_stack+0x4c/0x65
>> [  113.490389]  [<ffffffff81086962>] ? console_unlock+0x3d6/0x405
>> [  113.491039]  [<ffffffff8107ae75>] lockdep_rcu_suspicious+0xfa/0x103
>> [  113.491735]  [<ffffffff81064032>] rcu_preempt_sleep_check+0x45/0x47
>> [  113.492442]  [<ffffffff8106404d>] ___might_sleep+0x19/0x1c8
>> [  113.493077]  [<ffffffff81064268>] __might_sleep+0x6c/0x82
>> [  113.493681]  [<ffffffff81133190>] cache_alloc_debugcheck_before.isra.50+0x1d/0x24
>> [  113.494508]  [<ffffffff81134876>] kmem_cache_alloc+0x31/0x18f
>> [  113.495149]  [<ffffffff814012b5>] skb_clone+0x64/0x80
>> [  113.495712]  [<ffffffff814e6f71>] pfkey_broadcast_one+0x3d/0xff
>> [  113.496380]  [<ffffffff814e7b84>] pfkey_broadcast+0xb5/0x11e
>> [  113.497024]  [<ffffffff814e82d1>] pfkey_register+0x191/0x1b1
>> [  113.497653]  [<ffffffff814e9770>] pfkey_process+0x162/0x17e
>> [  113.498274]  [<ffffffff814e9895>] pfkey_sendmsg+0x109/0x213
>>
>> In pfkey_sendmsg the net mutex is taken and then pfkey_broadcast takes
>> the RCU lock. Fix by using GFP_ATOMIC for the allocation flag.
>>
>> Signed-off-by: David Ahern <[email-masked]>
>> ---
>>   net/key/af_key.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/key/af_key.c b/net/key/af_key.c
>> index b397f0aa9005..73527e7dd247 100644
>> --- a/net/key/af_key.c
>> +++ b/net/key/af_key.c
>> @@ -1670,7 +1670,7 @@ static int pfkey_register(struct sock *sk, struct sk_buff *skb, const struct sad
>>   		return -ENOBUFS;
>>   	}
>>
>> -	pfkey_broadcast(supp_skb, GFP_KERNEL, BROADCAST_REGISTERED, sk, sock_net(sk));
>> +	pfkey_broadcast(supp_skb, GFP_ATOMIC, BROADCAST_REGISTERED, sk, sock_net(sk));
>>
>>   	return 0;
>>   }
>
> I would rather remove the useless rcu locking from pfkey_broadcast() if
> a mutex properly protects the thing.

rcu_read_lock was added by Stephen with 7f6b9dbd5afbd. It does not 
appear the net->xfrm.xfrm_cfg_mutex mutex added by 283bc9f35bbbc 
properly covers the locking. ie., the rcu_read_lock is needed.

David