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

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Message-ID: <20210128212130.6bda5d5a@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 28 Jan 2021 21:21:30 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Cong Wang <xiyou.wangcong@...il.com>
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, 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?