netdev - Re: [Patch net] net: fix dev_ifsioc

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Message-ID: <CAM_iQpWxTVQ01NqR8CGuWK6H_YKPS7FNWHgs=07XQEkw=Wu7KQ@mail.gmail.com>
Date:   Fri, 29 Jan 2021 15:12:59 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Cong Wang <cong.wang@...edance.com>,
        "Gong, Sishuai" <sishuai@...due.edu>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [Patch net] net: fix dev_ifsioc_locked() race condition

On Fri, Jan 29, 2021 at 10:00 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu, 28 Jan 2021 21:47:04 -0800 Cong Wang wrote:
> > On Thu, Jan 28, 2021 at 9:21 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > > On Thu, 28 Jan 2021 21:08:05 -0800 Cong Wang wrote:
> > > > On Thu, Jan 28, 2021 at 12:55 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > > > >
> > > > > On Sat, 23 Jan 2021 17:30:49 -0800 Cong Wang wrote:
> > > > > > From: Cong Wang <cong.wang@...edance.com>
> > > > > >
> > > > > > dev_ifsioc_locked() is called with only RCU read lock, so when
> > > > > > there is a parallel writer changing the mac address, it could
> > > > > > get a partially updated mac address, as shown below:
> > > > > >
> > > > > > Thread 1                      Thread 2
> > > > > > // eth_commit_mac_addr_change()
> > > > > > memcpy(dev->dev_addr, addr->sa_data, ETH_ALEN);
> > > > > >                               // dev_ifsioc_locked()
> > > > > >                               memcpy(ifr->ifr_hwaddr.sa_data,
> > > > > >                                       dev->dev_addr,...);
> > > > > >
> > > > > > Close this race condition by guarding them with a RW semaphore,
> > > > > > like netdev_get_name(). The writers take RTNL anyway, so this
> > > > > > will not affect the slow path.
> > > > > >
> > > > > > Fixes: 3710becf8a58 ("net: RCU locking for simple ioctl()")
> > > > > > Reported-by: "Gong, Sishuai" <sishuai@...due.edu>
> > > > > > Cc: Eric Dumazet <eric.dumazet@...il.com>
> > > > > > Signed-off-by: Cong Wang <cong.wang@...edance.com>
> > > > >
> > > > > The addition of the write lock scares me a little for a fix, there's a
> > > > > lot of code which can potentially run under the callbacks and notifiers
> > > > > there.
> > > > >
> > > > > What about using a seqlock?
> > > >
> > > > Actually I did use seqlock in my initial version (not posted), it does not
> > > > allow blocking inside write_seqlock() protection, so I have to change
> > > > to rwsem.
> > >
> > > Argh, you're right. No way we can construct something that tries to
> > > read once and if it fails falls back to waiting for RTNL?
> >
> > I don't think there is any way to tell whether the read fails, a partially
> > updated address can not be detected without additional flags etc..
>
> Let me pseudo code it, I can't English that well:
>
> void reader(obj)
> {
>         unsigned int seq;
>
>         seq = READ_ONCE(seqcnt);
>         if (seq & 1)
>                 goto slow_path;
>         smb_rmb();
>
>         obj = read_the_thing();
>
>         smb_rmb();
>         if (seq == READ_ONCE(seqcnt))
>                 return;
>
> slow_path:
>         rtnl_lock();
>         obj = read_the_thing();
>         rtnl_unlock();
> }
>
> void writer()
> {
>         ASSERT_RNTL();
>
>         seqcnt++;
>         smb_wmb();
>
>         modify_the_thing();
>
>         smb_wmb();
>         seqcnt++;
> }
>
>
> I think raw_seqcount helpers should do here?

Not sure why you want to kinda rewrite seqlock here. Please see below.

>
> > And devnet_rename_sem is already there, pretty much similar to this
> > one.
>
> Ack, I don't see rename triggering cascading notifications, tho.
> I think you've seen the recent patch for locking in team, that's
> pretty much what I'm afraid will happen here.

Right, I should make only user-facing callers (ioctl, rtnetlink) use this
writer semaphore, and leave other callers of dev_set_mac_address()
untouched. Something like:

dev_set_mac_address_locked(...)
{
  down_write(&dev_addr_sem);
  dev_set_mac_address(...);
  up_write(&dev_addr_sem);
  ...
}

With this, we don't need to reinvent seqlock.

>
> But if I'm missing something about the seqcount or you strongly prefer
> the rwlock, we can do that, too. Although I'd rather take this patch
> to net-next in that case.

I have no problem with applying to net-next. I will send v2 targeting
net-next instead.

Thanks.