Message-ID: <Y4SGYr6VBkIMTEpj@nanopsycho>
Date:   Mon, 28 Nov 2022 10:58:58 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     Ido Schimmel <idosch@...sch.org>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Yang Yingliang <yangyingliang@...wei.com>,
        Leon Romanovsky <leon@...nel.org>, netdev@...r.kernel.org,
        jiri@...dia.com, davem@...emloft.net, edumazet@...gle.com,
        pabeni@...hat.com
Subject: Re: [PATCH net] net: devlink: fix UAF in
 devlink_compat_running_version()

Mon, Nov 28, 2022 at 10:20:53AM CET, idosch@...sch.org wrote:
>On Wed, Nov 23, 2022 at 06:18:00PM -0800, Jakub Kicinski wrote:
>> On Wed, 23 Nov 2022 21:18:14 +0200 Ido Schimmel wrote:
>> > > I used the fix code proposed by Jakub, but it didn't work correctly, so
>> > > I tried to correct and improve it, and need some devlink helper.
>> > > 
>> > > Anyway, it is a nsim problem, if we want fix this without touch devlink,
>> > > I think we can add a 'registered' field in struct nsim_dev, and it can be
>> > > checked in nsim_get_devlink_port() like this:  
>> > 
>> > I read the discussion and it's not clear to me why this is a netdevsim
>> > specific problem. The fundamental problem seems to be that it is
>> > possible to hold a reference on a devlink instance before it's
>> > registered and that devlink_free() will free the instance regardless of
>> > its current reference count because it expects devlink_unregister() to
>> > block. In this case, the instance was never registered, so
>> > devlink_unregister() is not called.
>> > 
>> > ethtool was able to get a reference on the devlink instance before it
>> > was registered because netdevsim registers its netdevs before
>> > registering its devlink instance. However, netdevsim is not the only one
>> > doing this: funeth, ice, prestera, mlx4, mlxsw, nfp and potentially
>> > others do the same thing.
>> > 
>> > When you think about it, it's strange that it's even possible for
>> > ethtool to reach the driver when the netdev used in the request is long
>> > gone, but it's not holding a reference on the netdev (it's holding a
>> > reference on the devlink instance instead) and
>> > devlink_compat_running_version() is called without RTNL.
>> 
>> Indeed. We did a bit of a flip-flop with the devlink locking rules
>> and the fact that the instance is reachable before it is registered 
>> is a leftover from a previous restructuring :(
>> 
>> Hence my preference to get rid of the ordering at the driver level 
>> than to try to patch it up in the code. Dunno if that's convincing.
>
>I don't have a good solution, but changing all the drivers to register
>their netdevs after the devlink instance is going to be quite painful
>and too big for 'net'. I feel like the main motivation for this is the
>ethtool compat stuff, which is not very convincing IMO. I'm quite happy
>with the current flow where drivers call devlink_register() at the end
>of their probe.
>
>Regarding a solution for the current crash, assuming we agree it's not a
>netdevsim specific problem, I think the current fix [1] is OK. Note that
>while it fixes the crash, it potentially creates other (less severe)
>problems. After user space receives RTM_NEWLINK notification it will
>need to wait for a certain period of time before issuing
>'ETHTOOL_GDRVINFO' as otherwise it will not get the firmware version. I
>guess it's not a big deal for drivers that only register one netdev
>since they will very quickly follow with devlink_register(), but the
>race window is larger for drivers that need to register many netdevs,
>for either physical switch or eswitch ports.
>
>Long term, we either need to find a way to make the ethtool compat stuff
>work correctly or just get rid of it and have affected drivers implement
>the relevant ethtool operations instead of relying on devlink.
>
>[1] https://lore.kernel.org/netdev/20221122121048.776643-1-yangyingliang@huawei.com/

I just had a call with Ido. We both think that this might be a good
solution for -net to avoid the use after free.

For net-next, we eventually should change driver init flows to register
devlink instance first and only after that register devlink_port and
related netdevice. The ordering is important for the userspace app. For
example the init flow:
<- RTnetlink new netdev event
app sees devlink_port handle in IFLA_DEVLINK_PORT
-> query devlink instance using this handle
<- ENODEV

The instance is not registered yet.

So we need to make sure all devlink_port_register() calls are happening
after devlink_register(). This is aligned with the original flow before
devlink_register() was moved by Leon. Also it is aligned with devlink
reload and devlink port split flows.

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