5.15-stable review patch. If anyone has any objections, please let me know.
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From: Ido Schimmel idosch@nvidia.com
[ Upstream commit 91b6dbced0ef1d680afdd69b14fc83d50ebafaf3 ]
When netfilter defrag hooks are loaded (due to the presence of conntrack rules, for example), fragmented packets entering the bridge will be defragged by the bridge's pre-routing hook (br_nf_pre_routing() -> ipv4_conntrack_defrag()).
Later on, in the bridge's post-routing hook, the defragged packet will be fragmented again. If the size of the largest fragment is larger than what the kernel has determined as the destination MTU (using ip_skb_dst_mtu()), the defragged packet will be dropped.
Before commit ac6627a28dbf ("net: ipv4: Consolidate ipv4_mtu and ip_dst_mtu_maybe_forward"), ip_skb_dst_mtu() would return dst_mtu() as the destination MTU. Assuming the dst entry attached to the packet is the bridge's fake rtable one, this would simply be the bridge's MTU (see fake_mtu()).
However, after above mentioned commit, ip_skb_dst_mtu() ends up returning the route's MTU stored in the dst entry's metrics. Ideally, in case the dst entry is the bridge's fake rtable one, this should be the bridge's MTU as the bridge takes care of updating this metric when its MTU changes (see br_change_mtu()).
Unfortunately, the last operation is a no-op given the metrics attached to the fake rtable entry are marked as read-only. Therefore, ip_skb_dst_mtu() ends up returning 1500 (the initial MTU value) and defragged packets are dropped during fragmentation when dealing with large fragments and high MTU (e.g., 9k).
Fix by moving the fake rtable entry's metrics to be per-bridge (in a similar fashion to the fake rtable entry itself) and marking them as writable, thereby allowing MTU changes to be reflected.
Fixes: 62fa8a846d7d ("net: Implement read-only protection and COW'ing of metrics.") Fixes: 33eb9873a283 ("bridge: initialize fake_rtable metrics") Reported-by: Venkat Venkatsubra venkat.x.venkatsubra@oracle.com Closes: https://lore.kernel.org/netdev/PH0PR10MB4504888284FF4CBA648197D0ACB82@PH0PR1... Tested-by: Venkat Venkatsubra venkat.x.venkatsubra@oracle.com Signed-off-by: Ido Schimmel idosch@nvidia.com Acked-by: Nikolay Aleksandrov razor@blackwall.org Link: https://patch.msgid.link/20250515084848.727706-1-idosch@nvidia.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- net/bridge/br_nf_core.c | 7 ++----- net/bridge/br_private.h | 1 + 2 files changed, 3 insertions(+), 5 deletions(-)
diff --git a/net/bridge/br_nf_core.c b/net/bridge/br_nf_core.c index 8c69f0c95a8ed..b8c8deb87407d 100644 --- a/net/bridge/br_nf_core.c +++ b/net/bridge/br_nf_core.c @@ -65,17 +65,14 @@ static struct dst_ops fake_dst_ops = { * ipt_REJECT needs it. Future netfilter modules might * require us to fill additional fields. */ -static const u32 br_dst_default_metrics[RTAX_MAX] = { - [RTAX_MTU - 1] = 1500, -}; - void br_netfilter_rtable_init(struct net_bridge *br) { struct rtable *rt = &br->fake_rtable;
atomic_set(&rt->dst.__refcnt, 1); rt->dst.dev = br->dev; - dst_init_metrics(&rt->dst, br_dst_default_metrics, true); + dst_init_metrics(&rt->dst, br->metrics, false); + dst_metric_set(&rt->dst, RTAX_MTU, br->dev->mtu); rt->dst.flags = DST_NOXFRM | DST_FAKE_RTABLE; rt->dst.ops = &fake_dst_ops; } diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h index fe61d3b8d0cc2..1718168bd927e 100644 --- a/net/bridge/br_private.h +++ b/net/bridge/br_private.h @@ -466,6 +466,7 @@ struct net_bridge { struct rtable fake_rtable; struct rt6_info fake_rt6_info; }; + u32 metrics[RTAX_MAX]; #endif u16 group_fwd_mask; u16 group_fwd_mask_required;