bpf/verifier: disallow pointer subtraction

Subtraction of pointers was accidentally allowed for unpriv programs
by commit 82abbf8d. Revert that part of commit.

Fixes: 82abbf8d ("bpf: do not allow root to mangle valid pointers")
Reported-by: NJann Horn <jannh@google.com>
Acked-by: NDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

kernel/bpf/verifier.c

@@ -3163,7 +3163,7 @@ static int adjust_reg_min_max_vals(struct bpf_verifier_env *env,
 				 * an arbitrary scalar. Disallow all math except
 				 * pointer subtraction
 				 */
-				if (opcode == BPF_SUB){
+				if (opcode == BPF_SUB && env->allow_ptr_leaks) {
 					mark_reg_unknown(env, regs, insn->dst_reg);
 					return 0;
 				}