netfilter: br_netfilter: reread nf_conn from skb after confirm()

[PATCH net] netfilter: br_netfilter: reread nf_conn from skb after confirm()

Posted by Wang Liang 2 weeks, 2 days ago

Previous commit 2d72afb34065 ("netfilter: nf_conntrack: fix crash due to
removal of uninitialised entry") move the IPS_CONFIRMED assignment after
the hash table insertion.

When send a broadcast packet to a tap device, which was added to a bridge,
br_nf_local_in() is called to confirm the conntrack. If another conntrack
with the same hash value is added to the hash table, which can be
triggered by a normal packet to a non-bridge device, the below warning
may happen.

  ------------[ cut here ]------------
  WARNING: CPU: 1 PID: 96 at net/bridge/br_netfilter_hooks.c:632 br_nf_local_in+0x168/0x200
  CPU: 1 UID: 0 PID: 96 Comm: tap_send Not tainted 6.17.0-rc2-dirty #44 PREEMPT(voluntary)
  RIP: 0010:br_nf_local_in+0x168/0x200
  Call Trace:
   <TASK>
   nf_hook_slow+0x3e/0xf0
   br_pass_frame_up+0x103/0x180
   br_handle_frame_finish+0x2de/0x5b0
   br_nf_hook_thresh+0xc0/0x120
   br_nf_pre_routing_finish+0x168/0x3a0
   br_nf_pre_routing+0x237/0x5e0
   br_handle_frame+0x1ec/0x3c0
   __netif_receive_skb_core+0x225/0x1210
   __netif_receive_skb_one_core+0x37/0xa0
   netif_receive_skb+0x36/0x160
   tun_get_user+0xa54/0x10c0
   tun_chr_write_iter+0x65/0xb0
   vfs_write+0x305/0x410
   ksys_write+0x60/0xd0
   do_syscall_64+0xa4/0x260
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>
  ---[ end trace 0000000000000000 ]---

To solve the hash conflict, nf_ct_resolve_clash() try to merge the
conntracks, and update skb->_nfct. However, br_nf_local_in() still use the
old ct from local variable 'nfct' after confirm(), which leads to this
issue. Fix it by rereading nfct from skb.

Fixes: 62e7151ae3eb ("netfilter: bridge: confirm multicast packets before passing them up the stack")
Signed-off-by: Wang Liang <wangliang74@huawei.com>
---
 net/bridge/br_netfilter_hooks.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/bridge/br_netfilter_hooks.c b/net/bridge/br_netfilter_hooks.c
index 94cbe967d1c1..55b1b7dcb609 100644
--- a/net/bridge/br_netfilter_hooks.c
+++ b/net/bridge/br_netfilter_hooks.c
@@ -626,6 +626,7 @@ static unsigned int br_nf_local_in(void *priv,
 		break;
 	}
 
+	nfct = skb_nfct(skb);
 	ct = container_of(nfct, struct nf_conn, ct_general);
 	WARN_ON_ONCE(!nf_ct_is_confirmed(ct));
 
-- 
2.33.0

Re: [PATCH net] netfilter: br_netfilter: reread nf_conn from skb after confirm()

Posted by Florian Westphal 2 weeks, 2 days ago

Wang Liang <wangliang74@huawei.com> wrote:
> Previous commit 2d72afb34065 ("netfilter: nf_conntrack: fix crash due to
> removal of uninitialised entry") move the IPS_CONFIRMED assignment after
> the hash table insertion.

How is that related to this change?
As you write below, the bug came in with 62e7151ae3eb.

> To solve the hash conflict, nf_ct_resolve_clash() try to merge the
> conntracks, and update skb->_nfct. However, br_nf_local_in() still use the
> old ct from local variable 'nfct' after confirm(), which leads to this
> issue. Fix it by rereading nfct from skb.
> 
> Fixes: 62e7151ae3eb ("netfilter: bridge: confirm multicast packets before passing them up the stack")
> Signed-off-by: Wang Liang <wangliang74@huawei.com>
> ---
>  net/bridge/br_netfilter_hooks.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/bridge/br_netfilter_hooks.c b/net/bridge/br_netfilter_hooks.c
> index 94cbe967d1c1..55b1b7dcb609 100644
> --- a/net/bridge/br_netfilter_hooks.c
> +++ b/net/bridge/br_netfilter_hooks.c
> @@ -626,6 +626,7 @@ static unsigned int br_nf_local_in(void *priv,
>  		break;
>  	}
>  
> +	nfct = skb_nfct(skb);
>  	ct = container_of(nfct, struct nf_conn, ct_general);
>  	WARN_ON_ONCE(!nf_ct_is_confirmed(ct));

There is a second bug here, confirm can return NF_DROP and
nfct will be NULL.

Can you make this change too? (or something similar)?

diff --git a/net/bridge/br_netfilter_hooks.c b/net/bridge/br_netfilter_hooks.c
index 94cbe967d1c1..69b7b7c7565e 100644
--- a/net/bridge/br_netfilter_hooks.c
+++ b/net/bridge/br_netfilter_hooks.c
@@ -619,8 +619,9 @@ static unsigned int br_nf_local_in(void *priv,
        nf_bridge_pull_encap_header(skb);
        ret = ct_hook->confirm(skb);
        switch (ret & NF_VERDICT_MASK) {
+       case NF_DROP:
        case NF_STOLEN:
-               return NF_STOLEN;
+               return ret;


nfct reload seems correct, thanks for catching this.

Re: [PATCH net] netfilter: br_netfilter: reread nf_conn from skb after confirm()

Posted by Wang Liang 2 weeks, 1 day ago

在 2025/8/20 19:31, Florian Westphal 写道:
> Wang Liang <wangliang74@huawei.com> wrote:
>> Previous commit 2d72afb34065 ("netfilter: nf_conntrack: fix crash due to
>> removal of uninitialised entry") move the IPS_CONFIRMED assignment after
>> the hash table insertion.
> How is that related to this change?
> As you write below, the bug came in with 62e7151ae3eb.


Before the commit 2d72afb34065, __nf_conntrack_confirm() set
'ct->status |= IPS_CONFIRMED;' before check hash, the warning will not
happen, so I put it here.

As you say, the bug came in with 62e7151ae3eb. I will delete this paragraph
in next patch.

>> To solve the hash conflict, nf_ct_resolve_clash() try to merge the
>> conntracks, and update skb->_nfct. However, br_nf_local_in() still use the
>> old ct from local variable 'nfct' after confirm(), which leads to this
>> issue. Fix it by rereading nfct from skb.
>>
>> Fixes: 62e7151ae3eb ("netfilter: bridge: confirm multicast packets before passing them up the stack")
>> Signed-off-by: Wang Liang <wangliang74@huawei.com>
>> ---
>>   net/bridge/br_netfilter_hooks.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/net/bridge/br_netfilter_hooks.c b/net/bridge/br_netfilter_hooks.c
>> index 94cbe967d1c1..55b1b7dcb609 100644
>> --- a/net/bridge/br_netfilter_hooks.c
>> +++ b/net/bridge/br_netfilter_hooks.c
>> @@ -626,6 +626,7 @@ static unsigned int br_nf_local_in(void *priv,
>>   		break;
>>   	}
>>   
>> +	nfct = skb_nfct(skb);
>>   	ct = container_of(nfct, struct nf_conn, ct_general);
>>   	WARN_ON_ONCE(!nf_ct_is_confirmed(ct));
> There is a second bug here, confirm can return NF_DROP and
> nfct will be NULL.


Thanks for your suggestion!

Do you mean that ct may be deleted in confirm and return NF_DROP, so we can
not visit it in br_nf_local_in() and need to add 'case NF_DROP:' here?

I cannot find somewhere set skb->_nfct to NULL and return NF_DROP. Can you
give some hints?

------
Best regards
Wang Liang

>
> Can you make this change too? (or something similar)?
>
> diff --git a/net/bridge/br_netfilter_hooks.c b/net/bridge/br_netfilter_hooks.c
> index 94cbe967d1c1..69b7b7c7565e 100644
> --- a/net/bridge/br_netfilter_hooks.c
> +++ b/net/bridge/br_netfilter_hooks.c
> @@ -619,8 +619,9 @@ static unsigned int br_nf_local_in(void *priv,
>          nf_bridge_pull_encap_header(skb);
>          ret = ct_hook->confirm(skb);
>          switch (ret & NF_VERDICT_MASK) {
> +       case NF_DROP:
>          case NF_STOLEN:
> -               return NF_STOLEN;
> +               return ret;
>
>
> nfct reload seems correct, thanks for catching this.

Re: [PATCH net] netfilter: br_netfilter: reread nf_conn from skb after confirm()

Posted by Florian Westphal 2 weeks, 1 day ago

Wang Liang <wangliang74@huawei.com> wrote:
> 
> 在 2025/8/20 19:31, Florian Westphal 写道:
> > Wang Liang <wangliang74@huawei.com> wrote:
> > > Previous commit 2d72afb34065 ("netfilter: nf_conntrack: fix crash due to
> > > removal of uninitialised entry") move the IPS_CONFIRMED assignment after
> > > the hash table insertion.
> > How is that related to this change?
> > As you write below, the bug came in with 62e7151ae3eb.
> 
> Before the commit 2d72afb34065, __nf_conntrack_confirm() set
> 'ct->status |= IPS_CONFIRMED;' before check hash, the warning will not
> happen, so I put it here.

Oh, right, the problem was concealed before this.

> > There is a second bug here, confirm can return NF_DROP and
> > nfct will be NULL.
> 
> Thanks for your suggestion!
> 
> Do you mean that ct may be deleted in confirm and return NF_DROP, so we can
> not visit it in br_nf_local_in() and need to add 'case NF_DROP:' here?
>
> I cannot find somewhere set skb->_nfct to NULL and return NF_DROP. Can you
> give some hints?

You are right, skb->_nfct isn't set to NULL in case NF_DROP is returned.
However, the warning will trigger as we did not insert the conntrack
entry in that case.

I suggest to remove the warning, I don't think it buys anything.

Thanks.

Re: [PATCH net] netfilter: br_netfilter: reread nf_conn from skb after confirm()

Posted by Wang Liang 2 weeks ago

在 2025/8/21 14:57, Florian Westphal 写道:
> Wang Liang <wangliang74@huawei.com> wrote:
>> 在 2025/8/20 19:31, Florian Westphal 写道:
>>> Wang Liang <wangliang74@huawei.com> wrote:
>>>> Previous commit 2d72afb34065 ("netfilter: nf_conntrack: fix crash due to
>>>> removal of uninitialised entry") move the IPS_CONFIRMED assignment after
>>>> the hash table insertion.
>>> How is that related to this change?
>>> As you write below, the bug came in with 62e7151ae3eb.
>> Before the commit 2d72afb34065, __nf_conntrack_confirm() set
>> 'ct->status |= IPS_CONFIRMED;' before check hash, the warning will not
>> happen, so I put it here.
> Oh, right, the problem was concealed before this.
>
>>> There is a second bug here, confirm can return NF_DROP and
>>> nfct will be NULL.
>> Thanks for your suggestion!
>>
>> Do you mean that ct may be deleted in confirm and return NF_DROP, so we can
>> not visit it in br_nf_local_in() and need to add 'case NF_DROP:' here?
>>
>> I cannot find somewhere set skb->_nfct to NULL and return NF_DROP. Can you
>> give some hints?
> You are right, skb->_nfct isn't set to NULL in case NF_DROP is returned.
> However, the warning will trigger as we did not insert the conntrack
> entry in that case.
>
> I suggest to remove the warning, I don't think it buys anything.
>
> Thanks.


Yes, remove the warning is a good a choice. I will remove the two lines in
v2 patch later, please check it.

------
Best regards
Wang Liang