On April 25, 2020 1:15:05 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: > On Fri, Apr 24, 2020 at 11:51 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote: >> >> On Fri, Apr 24, 2020 at 4:27 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: >>> Hi SELinux maintainers, >>> >>> We've hit a case where a developer wasn't able to reproduce a kernel >>> bug, it turned out to be a difference in behavior between SELinux and >>> non-SELinux kernels. >>> Condensed version: a program does sendmmsg on netlink socket with 2 >>> mmsghdr's, first is completely empty/zeros, second contains some >>> actual payload. Without SELinux the first mmsghdr is treated as no-op >>> and the kernel processes the second one (triggers bug). However the >>> SELinux hook does: >>> >>> static int selinux_netlink_send(struct sock *sk, struct sk_buff *skb) >>> { >>> if (skb->len < NLMSG_HDRLEN) { >>> err = -EINVAL; >>> goto out; >>> } >>> >>> and fails processing on the first empty mmsghdr (does not happen >>> without SELinux). >>> >>> Is this difference in behavior intentional/acceptable/should be fixed? >> >> From a practical perspective, SELinux is always going to need to do a >> length check as it needs to peek into the netlink message header for >> the message type so it can map that to the associated SELinux >> permissions. So in that sense, the behavior is intentional and >> desired; however from a bug-for-bug compatibility perspective ... not >> so much. >> >> Ultimately, my it's-Friday-and-it's-been-a-long-week-ending-in-a-long-day >> thought is that this was a buggy operation to begin with and the bug >> was just caught in different parts of the kernel, depending on how it >> was configured. It may not be ideal, but I can think of worse things >> (and arguably SELinux is doing the Right Thing). > > +netlink maintainers for intended semantics of empty netlink messages > > If it's a bug, or intended behavior depends on the intended > behavior... which I assume is not documented anywhere officially. Your original email gave the impression that there was a big in the non-SELinux case; if that is not the case my response changes. > However, most of the netlink families use netlink_rcv_skb, which does: > > int netlink_rcv_skb(struct sk_buff *skb, int (*cb)(struct sk_buff *, > struct nlmsghdr *, > struct netlink_ext_ack *)) > { > ... > while (skb->len >= nlmsg_total_size(0)) { > ... > skb_pull(skb, msglen); > } > return 0; > } > > 1. How intentional is this while loop logic vs sloppy error checking? > 2. netlink_rcv_skb seems to be able to handle 2+ messages in the same > skb, while selinux_netlink_send only checks the first one... so can I > skip SELinux checks by putting a malicious message after a permitted > one?..