netdev - Re: [PATCH v1] bpf: Prevent infinite loops with bpf_redirect

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Message-ID: <CADKFtnS7va+Q5qSd6e+k2Vq+z6Oc+ba_zFoTdLhWmt03TMLu-Q@mail.gmail.com>
Date: Mon, 30 Sep 2024 07:09:28 -0700
From: Jordan Rife <jrife@...gle.com>
To: Daniel Borkmann <daniel@...earbox.net>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, 
	Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Kui-Feng Lee <thinker.li@...il.com>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	"David S. Miller" <davem@...emloft.net>, Paolo Abeni <pabeni@...hat.com>, stable@...r.kernel.org, 
	John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH v1] bpf: Prevent infinite loops with bpf_redirect_peer

> No, as you mentioned, there are plenty of other misconfiguration
> possibilities in and
> outside bpf where something can loop in the stack (or where you can lock
> yourself
> out e.g. drop-all).

I wasn't sure if it should be possible to lock up the kernel with such
a combination of BPF programs. If this is the view generally, then
fair enough I suppose. Maybe this is my ignorance showing, but my
understanding is that with BPF generally we go to great lengths to
make sure things don't block (e.g. making sure a BPF program
terminates eventually) to avoid locking up the kernel. By extension,
should it not also be impossible to block indefinitely in
__netif_receive_skb_core with a combination of two BPF programs that
create a cycle with bpf_redirect_peer? It seems like there are other
provisions in place to avoid misconfiguration or buggy combinations of
programs from breaking things too badly such as the
dev_xmit_recursion() check in __bpf_tx_skb().

> if (dev_xmit_recursion()) {
>   net_crit_ratelimited("bpf: recursion limit reached on datapath, buggy bpf program?\n");
>   kfree_skb(skb);
>   return -ENETDOWN;
> }

-Jordan