bpf: Check netfilter ctx accesses are aligned

Similarly to the previous patch fixing the flow_dissector ctx accesses,
nf_is_valid_access also doesn't check that ctx accesses are aligned.
Contrary to flow_dissector programs, netfilter programs don't have
context conversion. The unaligned ctx accesses are therefore allowed by
the verifier.

Fixes: fd9c663 ("bpf: minimal support for programs hooked into netfilter framework")
Signed-off-by: Paul Chaignon <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/853ae9ed5edaa5196e8472ff0f1bb1cc24059214.1754039605.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: pchaigno

net/netfilter/nf_bpf_link.c

@@ -296,6 +296,9 @@ static bool nf_is_valid_access(int off, int size, enum bpf_access_type type,
 	if (off < 0 || off >= sizeof(struct bpf_nf_ctx))
 		return false;
 
+	if (off % size != 0)
+		return false;
+
 	if (type == BPF_WRITE)
 		return false;