Subject: [PATCH net] af_key: free SKBs under RCU protection

From: Sean Tranchetti <[email-masked]>
Date: Wed, 19 Sep 2018 18:18:32 -0600

pfkey_broadcast() can make calls to pfkey_broadcast_one() which
will clone or copy the passed in SKB. This new SKB will be assigned
the sock_rfree() function as its destructor, which requires that
the socket reference the SKB contains is valid when the SKB is freed.

If this SKB is ever passed to pfkey_broadcast() again by some other
function (such as pkfey_dump() or pfkey_promisc) it will then be
freed there. However, since this free occurs outside of RCU protection,
it is possible that userspace could close the socket and trigger
pfkey_release() to free the socket before sock_rfree() can run, creating
the following race condition:

1: An SKB belonging to the pfkey socket is passed to pfkey_broadcast().
   It performs the broadcast to any other sockets, and calls
   rcu_read_unlock(), but does not yet reach kfree_skb().
2: Userspace closes the socket, triggering pfkey_realse(). Since no one
   holds the RCU lock, synchronize_rcu() returns and it is allowed to
   continue. It calls sock_put() to free the socket.
3: pfkey_broadcast() now calls kfree_skb() on the original SKB it was
   passed, triggering a call to sock_rfree(). This function now accesses
   the freed struct sock * via skb->sk, and attempts to update invalid
   memory.

By ensuring that the pfkey_broadcast() also frees the SKBs while it holds
the RCU lock, we can ensure that the socket will remain valid when the SKB
is freed, avoiding crashes like the following:

Unable to handle kernel paging request at virtual address 6b6b6b6b6b6c4b
[006b6b6b6b6b6c4b] address between user and kernel address ranges
Internal error: Oops: 96000004 [#1] PREEMPT SMP
task: fffffff78f65b380 task.stack: ffffff8049a88000
pc : sock_rfree+0x38/0x6c
lr : skb_release_head_state+0x6c/0xcc
Process repro (pid: 7117, stack limit = 0xffffff8049a88000)
Call trace:
	sock_rfree+0x38/0x6c
	skb_release_head_state+0x6c/0xcc
	skb_release_all+0x1c/0x38
	__kfree_skb+0x1c/0x30
	kfree_skb+0xd0/0xf4
	pfkey_broadcast+0x14c/0x18c
	pfkey_sendmsg+0x1d8/0x408
	sock_sendmsg+0x44/0x60
	___sys_sendmsg+0x1d0/0x2a8
	__sys_sendmsg+0x64/0xb4
	SyS_sendmsg+0x34/0x4c
	el0_svc_naked+0x34/0x38
Kernel panic - not syncing: Fatal exception

Signed-off-by: Sean Tranchetti <[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 9d61266..dd257c7 100644
--- a/net/key/af_key.c
+++ b/net/key/af_key.c
@@ -275,13 +275,13 @@ static int pfkey_broadcast(struct sk_buff *skb, gfp_t allocation,
 		if ((broadcast_flags & BROADCAST_REGISTERED) && err)
 			err = err2;
 	}
-	rcu_read_unlock();
 
 	if (one_sk != NULL)
 		err = pfkey_broadcast_one(skb, &skb2, allocation, one_sk);
 
 	kfree_skb(skb2);
 	kfree_skb(skb);
+	rcu_read_unlock();
 	return err;
 }
 
-- 
1.9.1