Re: [PATCH bpf-next] xsk: proper socket state check in xsk_poll

[Daniel Borkmann <daniel@iogearbox.net>]

On 8/20/19 5:29 PM, Björn Töpel wrote:
> On Tue, 20 Aug 2019 at 16:30, Daniel Borkmann <daniel@iogearbox.net> wrote:
>> On 8/20/19 12:04 PM, Björn Töpel wrote:
>>> From: Björn Töpel <bjorn.topel@intel.com>
>>>
>>> The poll() implementation for AF_XDP sockets did not perform the
>>> proper state checks, prior accessing the socket umem. This patch fixes
>>> that by performing a xsk_is_bound() check.
>>>
>>> Suggested-by: Hillf Danton <hdanton@sina.com>
>>> Reported-by: syzbot+c82697e3043781e08802@syzkaller.appspotmail.com
>>> Fixes: 77cd0d7b3f25 ("xsk: add support for need_wakeup flag in AF_XDP rings")
>>> Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
>>> ---
>>>    net/xdp/xsk.c | 14 ++++++++++++--
>>>    1 file changed, 12 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
>>> index ee4428a892fa..08bed5e92af4 100644
>>> --- a/net/xdp/xsk.c
>>> +++ b/net/xdp/xsk.c
>>> @@ -356,13 +356,20 @@ static int xsk_generic_xmit(struct sock *sk, struct msghdr *m,
>>>        return err;
>>>    }
>>>
>>> +static bool xsk_is_bound(struct xdp_sock *xs)
>>> +{
>>> +     struct net_device *dev = READ_ONCE(xs->dev);
>>> +
>>> +     return dev && xs->state == XSK_BOUND;
>>> +}
>>> +
>>>    static int xsk_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len)
>>>    {
>>>        bool need_wait = !(m->msg_flags & MSG_DONTWAIT);
>>>        struct sock *sk = sock->sk;
>>>        struct xdp_sock *xs = xdp_sk(sk);
>>>
>>> -     if (unlikely(!xs->dev))
>>> +     if (unlikely(!xsk_is_bound(xs)))
>>>                return -ENXIO;
>>>        if (unlikely(!(xs->dev->flags & IFF_UP)))
>>>                return -ENETDOWN;
>>> @@ -383,6 +390,9 @@ static unsigned int xsk_poll(struct file *file, struct socket *sock,
>>>        struct net_device *dev = xs->dev;
>>>        struct xdp_umem *umem = xs->umem;
>>>
>>> +     if (unlikely(!xsk_is_bound(xs)))
>>> +             return mask;
>>> +
>>>        if (umem->need_wakeup)
>>>                dev->netdev_ops->ndo_xsk_wakeup(dev, xs->queue_id,
>>>                                                umem->need_wakeup);
>>> @@ -417,7 +427,7 @@ static void xsk_unbind_dev(struct xdp_sock *xs)
>>>    {
>>>        struct net_device *dev = xs->dev;
>>>
>>> -     if (!dev || xs->state != XSK_BOUND)
>>> +     if (!xsk_is_bound(xs))
>>>                return;
>>
>> I think I'm a bit confused by your READ_ONCE() usage. ;-/ I can see why you're
>> using it in xsk_is_bound() above, but then at the same time all the other callbacks
>> like xsk_poll() or xsk_unbind_dev() above have a struct net_device *dev = xs->dev
>> right before the test. Could you elaborate?
> 
> Yes, now I'm confused as well! Digging deeper... I believe there are a
> couple of places in xsk.c that do not have
> READ_ONCE/WRITE_ONCE-correctness. Various xdp_sock members are read
> lock-less outside the control plane mutex (mutex member of struct
> xdp_sock). This needs some re-work. I'll look into using the newly

Right, so even in above two cases, the compiler could have refetched, e.g.
dev variable could have first been NULL, but xsk_is_bound() later returns
true.

> introduced state member (with corresponding read/write barriers) for
> this.
> 
> I'll cook some patch(es) that address this, but first it sounds like I
> need to reread [1] two, or three times. At least. ;-)
> 
> 
> Thanks,
> Björn
> 
> 
> [1] https://lwn.net/Articles/793253/
> 
> 
>> Thanks,
>> Daniel