Re: [PATCH] bridge: use promisc arg instead of skb flags

On 08/10/2024 17:30, Pablo Neira Ayuso wrote:

Hi Nikolay,

On Sat, Oct 05, 2024 at 05:06:56PM +0300, Nikolay Aleksandrov wrote:

quoted

On 05/10/2024 04:44, Amedeo Baragiola wrote:

quoted

Since commit 751de2012eaf ("netfilter: br_netfilter: skip conntrack input hook for promisc packets")
a second argument (promisc) has been added to br_pass_frame_up which
represents whether the interface is in promiscuous mode. However,
internally - in one remaining case - br_pass_frame_up checks the device
flags derived from skb instead of the argument being passed in.
This one-line changes addresses this inconsistency.

Signed-off-by: Amedeo Baragiola <redacted>
---
 net/bridge/br_input.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
index ceaa5a89b947..156c18f42fa3 100644
--- a/net/bridge/br_input.c
+++ b/net/bridge/br_input.c

@@ -50,8 +50,7 @@ static int br_pass_frame_up(struct sk_buff *skb, bool promisc)
 	 * packet is allowed except in promisc mode when someone
 	 * may be running packet capture.
 	 */
-	if (!(brdev->flags & IFF_PROMISC) &&
-	    !br_allowed_egress(vg, skb)) {
+	if (!promisc && !br_allowed_egress(vg, skb)) {
 		kfree_skb(skb);
 		return NET_RX_DROP;
 	}

This is subtle, but it does change behaviour when a BR_FDB_LOCAL dst
is found it will always drop the traffic after this patch (w/ promisc) if it
doesn't pass br_allowed_egress(). It would've been allowed before, but current
situation does make the patch promisc bit inconsistent, i.e. we get
there because of BR_FDB_LOCAL regardless of the promisc flag.

Because we can have a BR_FDB_LOCAL dst and still pass up such skb because of
the flag instead of local_rcv (see br_br_handle_frame_finish()).

CCing also Pablo for a second pair of eyes and as the original patch
author. :)

Pablo WDYT?

Just FYI we definitely want to see all traffic if promisc is set, so
this patch is a no-go.

promisc is always _false_ for BR_FDB_LOCAL dst:

        if (dst) {
                unsigned long now = jiffies;

                if (test_bit(BR_FDB_LOCAL, &dst->flags))
                        return br_pass_frame_up(skb, false);

                ...
        }

        if (local_rcv)
                return br_pass_frame_up(skb, promisc);

quoted

quoted

-	if (!(brdev->flags & IFF_PROMISC) &&
-	    !br_allowed_egress(vg, skb)) {
+	if (!promisc && !br_allowed_egress(vg, skb)) {

Then, this is not equivalent.

But, why is br_allowed_egress() skipped depending on brdev->flags & IFF_PROMISC?

I mean, how does this combination work?

BR_FDB_LOCAL dst AND (brdev->flags & IFF_PROMISC) AND BR_INPUT_SKB_CB(skb)->vlan_filtered

The bridge should see all packets come up if promisc flag is set, regardless if the
vlan exists or not, so br_allowed_egress() is skipped entirely. As I commented
separately the patch changes that behaviour and suddenly these packets
(BR_FDB_LOCAL fdb + promisc bit set on the bridge dev) won't be sent up to
the bridge. I think the current code should stay as-is, but wanted to get
your opinion if we can still hit the warning that was fixed because we can
still hit that code with a BR_FDB_LOCAL dst with promisc flag set and
the promisc flag will be == false in that case.