[PATCH 4.14 37/47] bpf/verifier: disallow pointer subtraction

[Greg Kroah-Hartman]

From: Alexei Starovoitov <ast@xxxxxxxxxx>

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@xxxxxxxxxx>
Acked-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
[fllinden@xxxxxxxxxx: backport to 4.14]
Signed-off-by: Frank van der Linden <fllinden@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/bpf/verifier.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2754,7 +2754,7 @@ static int adjust_reg_min_max_vals(struc
 				 * 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;
 				}