bpf: Fix regsafe() for pointers to packet

In case rold->reg->range == BEYOND_PKT_END && rcur->reg->range == N
regsafe() may return true which may lead to current state with
valid packet range not being explored. Fix the bug.

Fixes: 6d94e74 ("bpf: Support for pointers beyond pkt_end.")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Amery Hung <ameryhung@gmail.com>

Author: Alexei Starovoitov

kernel/bpf/verifier.c

@@ -19915,7 +19915,11 @@ static bool regsafe(struct bpf_verifier_env *env, struct bpf_reg_state *rold,
 		 * since someone could have accessed through (ptr - k), or
 		 * even done ptr -= k in a register, to get a safe access.
 		 */
-		if (rold->range > rcur->range)
+		if (rold->range < 0 || rcur->range < 0) {
+			/* special case for [BEYOND|AT]_PKT_END */
+			if (rold->range != rcur->range)
+				return false;
+		} else if (rold->range > rcur->range)
 			return false;
 		/* If the offsets don't match, we can't trust our alignment;
 		 * nor can we be sure that we won't fall out of range.