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

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Date:   Thu, 28 Jan 2021 21:08:05 -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 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.

Thanks.