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

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