Re: [PATCH] bpf: Don't redirect packets with pkt_len 0

On Fri, Jun 17, 2022 at 9:19 AM Di Zhu [off-list ref] wrote:

quoted

Syzbot found an issue [1]: fq_codel_drop() try to drop a flow whitout any
skbs, that is, the flow->head is null.
The root cause, as the [2] says, is because that bpf_prog_test_run_skb()
run a bpf prog which redirects empty skbs.
So we should determine whether the length of the packet modified by bpf
prog or others like bpf_prog_test is 0 before forwarding it directly.

LINK: [1]

https://syzkaller.appspot.com/bug?id=0b84da80c2917757915afa89f7738a9d16e
c96c5

quoted

LINK: [2] https://www.spinics.net/lists/netdev/msg777503.html

Reported-by: syzbot+7a12909485b94426aceb@syzkaller.appspotmail.com
Signed-off-by: Di Zhu <redacted>
---
 net/core/filter.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 5af58eb48587..c7fbfa90898a 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -2156,6 +2156,9 @@ static int __bpf_redirect_common(struct sk_buff

*skb, struct net_device *dev,

quoted

 static int __bpf_redirect(struct sk_buff *skb, struct net_device *dev,
                          u32 flags)
 {
+       if (unlikely(skb->len == 0))
+               return -EINVAL;
+

You focus again on fq_codel, but we have a more generic issue at hand.

I said that most drivers will assume packets are Ethernet ones, having
at least an Ethernet header in them.

I only focused on this particular test case...
the submission information should be modified to describe a general problem.
 

Also returning -EINVAL will leak the skb :/

Yes... 

I think a better fix would be to make sure the skb carries an expected
packet length,
and this probably differs in __bpf_redirect_common() and
__bpf_redirect_no_mac() ?

Does the no mac device need to check the length of the package?

Current test in __bpf_redirect_common() seems not good enough.

+       /* Verify that a link layer header is carried */
+       if (unlikely(skb->mac_header >= skb->network_header)) {
+               kfree_skb(skb);
+               return -ERANGE;
+       }
+

It should check that the link layer header size is >= dev->min_header_len

It is only valid for device with mac header. such as ip tunnel device, dev->min_header_len
has not set.

quoted

        if (dev_is_mac_header_xmit(dev))
                return __bpf_redirect_common(skb, dev, flags);
        else
--
2.27.0