Re: [PATCH] tun: Fix use-after-free in tun_net_xmit

[Jason Wang <jasowang@redhat.com>]

On 2019/4/29 上午1:59, Cong Wang wrote:
> On Sun, Apr 28, 2019 at 12:51 AM Jason Wang <jasowang@redhat.com> wrote:
>>> tun_net_xmit() doesn't have the chance to
>>> access the change because it holding the rcu_read_lock().
>>
>>
>> The problem is the following codes:
>>
>>
>>          --tun->numqueues;
>>
>>          ...
>>
>>          synchronize_net();
>>
>> We need make sure the decrement of tun->numqueues be visible to readers
>> after synchronize_net(). And in tun_net_xmit():
>
> It doesn't matter at all. Readers are okay to read it even they still use the
> stale tun->numqueues, as long as the tfile is not freed readers can read
> whatever they want...

This is only true if we set SOCK_RCU_FREE, isn't it?

>
> The decrement of tun->numqueues is just how we unpublish the old
> tfile, it is still valid for readers to read it _after_ unpublish, we only need
> to worry about free, not about unpublish. This is the whole spirit of RCU.
>

The point is we don't convert tun->numqueues to RCU but use
synchronize_net().

> You need to rethink about my SOCK_RCU_FREE patch.

The code is wrote before SOCK_RCU_FREE is introduced and assume no
de-reference from device after synchronize_net(). It doesn't harm to
figure out the root cause which may give us more confidence to the fix
(e.g like SOCK_RCU_FREE).

I don't object to fix with SOCK_RCU_FREE, but then we should remove
the redundant synchronize_net(). But I still prefer to synchronize
everything explicitly like (completely untested):

From df91f77d35a6aa7943b6f2a7d4b329990896a0fe Mon Sep 17 00:00:00 2001
From: Jason Wang <jasowang@redhat.com>
Date: Mon, 29 Apr 2019 10:21:06 +0800
Subject: [PATCH] tuntap: synchronize through tfiles instead of numqueues

Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 drivers/net/tun.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 80bff1b4ec17..03715f605fb5 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -698,6 +698,7 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
 
 		rcu_assign_pointer(tun->tfiles[index],
 				   tun->tfiles[tun->numqueues - 1]);
+		rcu_assign_pointer(tun->tfiles[tun->numqueues], NULL);
 		ntfile = rtnl_dereference(tun->tfiles[index]);
 		ntfile->queue_index = index;
 
@@ -1082,7 +1083,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 	tfile = rcu_dereference(tun->tfiles[txq]);
 
 	/* Drop packet if interface is not attached */
-	if (txq >= tun->numqueues)
+	if (!tfile)
 		goto drop;
 
 	if (!rcu_dereference(tun->steering_prog))
@@ -1305,15 +1306,13 @@ static int tun_xdp_xmit(struct net_device *dev, int n,
 
 	rcu_read_lock();
 
-	numqueues = READ_ONCE(tun->numqueues);
-	if (!numqueues) {
+	tfile = rcu_dereference(tun->tfiles[smp_processor_id() %
+					    tun->numqueues]);
+	if (!tfile) {
 		rcu_read_unlock();
 		return -ENXIO; /* Caller will free/return all frames */
 	}
 
-	tfile = rcu_dereference(tun->tfiles[smp_processor_id() %
-					    numqueues]);
-
 	spin_lock(&tfile->tx_ring.producer_lock);
 	for (i = 0; i < n; i++) {
 		struct xdp_frame *xdp = frames[i];
-- 
2.19.1