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

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

[Di Zhu]

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=0b84da80c2917757915afa89f7738a9d16ec96c5
LINK: [2] https://www.spinics.net/lists/netdev/msg777503.html

Reported-by: syzbot+7a12909485b94426aceb@syzkaller.appspotmail.com
Signed-off-by: Di Zhu <zhudi2@huawei.com>
---
 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,
 static int __bpf_redirect(struct sk_buff *skb, struct net_device *dev,
 			  u32 flags)
 {
+	if (unlikely(skb->len == 0))
+		return -EINVAL;
+
 	if (dev_is_mac_header_xmit(dev))
 		return __bpf_redirect_common(skb, dev, flags);
 	else
-- 
2.27.0

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

[Eric Dumazet]

On Fri, Jun 17, 2022 at 9:19 AM Di Zhu <zhudi2@huawei.com> wrote:
>
> 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=0b84da80c2917757915afa89f7738a9d16ec96c5
> LINK: [2] https://www.spinics.net/lists/netdev/msg777503.html
>
> Reported-by: syzbot+7a12909485b94426aceb@syzkaller.appspotmail.com
> Signed-off-by: Di Zhu <zhudi2@huawei.com>
> ---
>  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,
>  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.

Also returning -EINVAL will leak the skb :/

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() ?

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


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

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

[zhudi (E)]

> On Fri, Jun 17, 2022 at 9:19 AM Di Zhu <zhudi2@huawei.com> wrote:
> >
> > 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
> > LINK: [2] https://www.spinics.net/lists/netdev/msg777503.html
> >
> > Reported-by: syzbot+7a12909485b94426aceb@syzkaller.appspotmail.com
> > Signed-off-by: Di Zhu <zhudi2@huawei.com>
> > ---
> >  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,
> >  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.

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