Re: PROBLEM: Using BPF_PROG_TEST_RUN with data_out != NULL is unsafe

[Daniel Borkmann <daniel@iogearbox.net>]

On 04/04/2018 11:04 AM, Lorenz Bauer wrote:
> Hello,
> 
> I’ve encountered an issue when using BPF_PROG_TEST_RUN and capturing the output.
> The kernel copies data into user space without checking the length of
> the destination buffer.
> 
> In bpf_test_finish(), size is the amount of data in the XDP buffer /
> skb after the program is run. This can be larger than data_size_in due
> to bpf_xdp_adjust_head() and friends.
> bpf_test_finish doesn’t clamp size to data_size_out, which is what I
> was expecting.
> 
> What is the correct way to use this interface?

Yeah, so XDP has a headroom of XDP_PACKET_HEADROOM + NET_IP_ALIGN which in case
of x86 is 256 bytes (NET_IP_ALIGN being 0 there). This means that attr.test.data_out
buffer needs to be 256 bytes larger than attr.test.data_in in case the BPF prog
calls bpf_xdp_adjust_head() or bpf_xdp_adjust_meta(). In case you point data_in
and data_out to the same address, then the total buffer size therefore has to be
attr.test.data_size_in + 256 in order to not overrun anything while not being
aware of the BPF test program. The XDP_PACKET_HEADROOM is exposed to user space
in linux/bpf.h.