netdev - Re: [PATCH bpf] bpf: Fix metadata_dst leak __bpf_redirect_neigh

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Message-ID: <20251003094247.GE2878334@horms.kernel.org>
Date: Fri, 3 Oct 2025 10:42:47 +0100
From: Simon Horman <horms@...nel.org>
To: Daniel Borkmann <daniel@...earbox.net>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org,
	Yusuke Suzuki <yusuke.suzuki@...valent.com>,
	Julian Wiedmann <jwi@...valent.com>,
	Martin KaFai Lau <martin.lau@...nel.org>,
	Jakub Kicinski <kuba@...nel.org>, Jordan Rife <jrife@...gle.com>
Subject: Re: [PATCH bpf] bpf: Fix metadata_dst leak
 __bpf_redirect_neigh_v{4,6}

On Fri, Oct 03, 2025 at 09:34:18AM +0200, Daniel Borkmann wrote:
> Cilium has a BPF egress gateway feature which forces outgoing K8s Pod
> traffic to pass through dedicated egress gateways which then SNAT the
> traffic in order to interact with stable IPs outside the cluster.
> 
> The traffic is directed to the gateway via vxlan tunnel in collect md
> mode. A recent BPF change utilized the bpf_redirect_neigh() helper to
> forward packets after the arrival and decap on vxlan, which turned out
> over time that the kmalloc-256 slab usage in kernel was ever-increasing.
> 
> The issue was that vxlan allocates the metadata_dst object and attaches
> it through a fake dst entry to the skb. The latter was never released
> though given bpf_redirect_neigh() was merely setting the new dst entry
> via skb_dst_set() without dropping an existing one first.
> 
> Fixes: b4ab31414970 ("bpf: Add redirect_neigh helper as redirect drop-in")
> Reported-by: Yusuke Suzuki <yusuke.suzuki@...valent.com>
> Reported-by: Julian Wiedmann <jwi@...valent.com>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> Cc: Martin KaFai Lau <martin.lau@...nel.org>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: Jordan Rife <jrife@...gle.com>

Reviewed-by: Simon Horman <horms@...nel.org>