net: genl: fix error path memory leak in policy dumping

If construction of the array of policies fails when recording
non-first policy we need to unwind.

netlink_policy_dump_add_policy() itself also needs fixing as
it currently gives up on error without recording the allocated
pointer in the pstate pointer.

Reported-by: syzbot+dc54d9ba8153b216cae0@syzkaller.appspotmail.com
Fixes: 50a896c ("genetlink: properly support per-op policy dumping")
Link: https://lore.kernel.org/r/20220816161939.577583-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/netlink/genetlink.c

@@ -1174,13 +1174,17 @@ static int ctrl_dumppolicy_start(struct netlink_callback *cb)
 							     op.policy,
 							     op.maxattr);
 			if (err)
-				return err;
+				goto err_free_state;
 		}
 	}
 
 	if (!ctx->state)
 		return -ENODATA;
 	return 0;
+
+err_free_state:
+	netlink_policy_dump_free(ctx->state);
+	return err;
 }
 
 static void *ctrl_dumppolicy_prep(struct sk_buff *skb,

net/netlink/policy.c

@@ -144,7 +144,7 @@ int netlink_policy_dump_add_policy(struct netlink_policy_dump_state **pstate,
 
 	err = add_policy(&state, policy, maxtype);
 	if (err)
-		return err;
+		goto err_try_undo;
 
 	for (policy_idx = 0;
 	     policy_idx < state->n_alloc && state->policies[policy_idx].policy;
@@ -164,7 +164,7 @@ int netlink_policy_dump_add_policy(struct netlink_policy_dump_state **pstate,
 						 policy[type].nested_policy,
 						 policy[type].len);
 				if (err)
-					return err;
+					goto err_try_undo;
 				break;
 			default:
 				break;
@@ -174,6 +174,16 @@ int netlink_policy_dump_add_policy(struct netlink_policy_dump_state **pstate,
 
 	*pstate = state;
 	return 0;
+
+err_try_undo:
+	/* Try to preserve reasonable unwind semantics - if we're starting from
+	 * scratch clean up fully, otherwise record what we got and caller will.
+	 */
+	if (!*pstate)
+		netlink_policy_dump_free(state);
+	else
+		*pstate = state;
+	return err;
 }
 
 static bool