bpf: Check flow_dissector ctx accesses are aligned

flow_dissector_is_valid_access doesn't check that the context access is
aligned. As a consequence, an unaligned access within one of the exposed
field is considered valid and later rejected by
flow_dissector_convert_ctx_access when we try to convert it.

The later rejection is problematic because it's reported as a verifier
bug with a kernel warning and doesn't point to the right instruction in
verifier logs.

Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=ccac90e482b2a81d74aa
Signed-off-by: Paul Chaignon <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Acked-by: Eduard Zingerman <[e-mail]>
Link: https://lore.kernel.org/r/cc1b0[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

net/core/filter.c

···
 	if (off < 0 || off >= sizeof(struct __sk_buff))
 		return false;
 
+	if (off % size != 0)
+		return false;
+
 	if (type == BPF_WRITE)
 		return false;