linux-kernel - selinux_netlink_send changes program behavior

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Date:   Fri, 24 Apr 2020 10:27:25 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Paul Moore <paul@...l-moore.com>, stephen.smalley.work@...il.com,
        Eric Paris <eparis@...isplace.org>, selinux@...r.kernel.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: selinux_netlink_send changes program behavior

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?

Thanks

FTR, the C program is:
https://gist.githubusercontent.com/dvyukov/dda1c547ca9121817159d29afa72aea2/raw/41b021d722947df4d8c48e2fc783591b44671ceb/gistfile1.txt
kernel config:
https://gist.githubusercontent.com/dvyukov/08bf2c2fd873a84a2c4c771740716183/raw/78fb3b1063b7ae37625468f32868869edbd1bd19/gistfile1.txt
on upstream commit 50cc09c1 it triggers a KASAN bug without SELinux,
but does not with SELinux.