Re: [PATCH v2] netfilter: xt_bpf: Fix XT_BPF_MODE_FD_PINNED mode of 'xt_bpf_info_v1'

[Daniel Borkmann <daniel@iogearbox.net>]

On 10/09/2017 02:27 PM, Shmulik Ladkani wrote:
> From: Shmulik Ladkani <shmulik.ladkani@gmail.com>
>
> Commit 2c16d6033264 ("netfilter: xt_bpf: support ebpf") introduced
> support for attaching an eBPF object by an fd, with the
> 'bpf_mt_check_v1' ABI expecting the '.fd' to be specified upon each
> IPT_SO_SET_REPLACE call.
>
> However this breaks subsequent iptables calls:
>
>   # iptables -A INPUT -m bpf --object-pinned /sys/fs/bpf/xxx -j ACCEPT
>   # iptables -A INPUT -s 5.6.7.8 -j ACCEPT
>   iptables: Invalid argument. Run `dmesg' for more information.
>
> That's because iptables works by loading exising rules using
> IPT_SO_GET_ENTRIES to userspace, then issuing IPT_SO_SET_REPLACE with
> the replacement set.
>
> However, the loaded 'xt_bpf_info_v1' has an arbitrary '.fd' number
> (from the initial "iptables -m bpf" invocation) - so when 2nd invocation
> occurs, userspace passes a bogus fd number, which leads to
> 'bpf_mt_check_v1' to fail.
>
> One suggested solution [1] was to hack iptables userspace, to perform a
> "entries fixup" immediatley after IPT_SO_GET_ENTRIES, by opening a new,
> process-local fd per every 'xt_bpf_info_v1' entry seen.
>
> However, in [2] both Pablo Neira Ayuso and Willem de Bruijn suggested to
> depricate the xt_bpf_info_v1 ABI dealing with pinned ebpf objects.
>
> This fix changes the XT_BPF_MODE_FD_PINNED behavior to ignore the given
> '.fd' and instead perform an in-kernel lookup for the bpf object given
> the provided '.path'.
>
> It also defines an alias for the XT_BPF_MODE_FD_PINNED mode, named
> XT_BPF_MODE_PATH_PINNED, to better reflect the fact that the user is
> expected to provide the path of the pinned object.
>
> Existing XT_BPF_MODE_FD_ELF behavior (non-pinned fd mode) is preserved.
>
> References: [1] https://marc.info/?l=netfilter-devel&m=150564724607440&w=2
>              [2] https://marc.info/?l=netfilter-devel&m=150575727129880&w=2
>
> Cc: Pablo Neira Ayuso <pablo@netfilter.org>
> Cc: Willem de Bruijn <willemb@google.com>
> Reported-by: Rafael Buchbinder <rafi@rbk.ms>
> Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>

Acked-by: Daniel Borkmann <daniel@iogearbox.net>