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 <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Reviewed-by: Daniel Borkmann <[email protected]>
Reviewed-by: Amery Hung <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: Alexei Starovoitov

kernel/bpf/verifier.c

@@ -19915,8 +19915,13 @@ 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.
 		 */