bpf/verifier: disallow pointer subtraction

commit dd066823db2ac4e22f721ec85190817b58059a54 upstream.

Subtraction of pointers was accidentally allowed for unpriv programs
by commit 82abbf8d2fc4. Revert that part of commit.

Fixes: 82abbf8d2fc4 ("bpf: do not allow root to mangle valid pointers")
Reported-by: Jann Horn <jannh@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
[fllinden@amazon.com: backport to 4.14]
Signed-off-by: Frank van der Linden <fllinden@amazon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b999e268ee2337a21d63dbb952ad2431344023d1..5c440398e0a170fa3fba7bc8416c16ac44ed7687 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2754,7 +2754,7 @@ 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(regs, insn->dst_reg);
                                        return 0;
                                }