BACKPORT: bpf: bpf_fib_lookup should not return neigh in NUD_FAILED state

commit 1fe4850b34ab512ff911e2c035c75fb6438f7307 upstream.

The bpf_fib_lookup() helper does not only look up the fib (ie. route)
but it also looks up the neigh. Before returning the neigh, the helper
does not check for NUD_VALID. When a neigh state (neigh->nud_state)
is in NUD_FAILED, its dmac (neigh->ha) could be all zeros. The helper
still returns SUCCESS instead of NO_NEIGH in this case. Because of the
SUCCESS return value, the bpf prog directly uses the returned dmac
and ends up filling all zero in the eth header.

This patch checks for NUD_VALID and returns NO_NEIGH if the neigh is
not valid.

Change-Id: I1d81fbe4e026d2572099c684582a30a0e62e5f03
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230217004150.2980689-3-martin.lau@linux.dev


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>