linux-kernel - Re: [RESEND 2/2] sctp: hold cached endpoints to prevent possible UAF

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Message-ID: <Ybtzr5ZmD/IKjycz@google.com>
Date:   Thu, 16 Dec 2021 17:13:19 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     Xin Long <lucien.xin@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vlad Yasevich <vyasevich@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        lksctp developers <linux-sctp@...r.kernel.org>,
        "H.P. Yarroll" <piggy@....org>,
        Karl Knutson <karl@...ena.chicago.il.us>,
        Jon Grimm <jgrimm@...ibm.com>,
        Xingang Guo <xingang.guo@...el.com>,
        Hui Huang <hui.huang@...ia.com>,
        Sridhar Samudrala <sri@...ibm.com>,
        Daisy Chang <daisyc@...ibm.com>,
        Ryan Layer <rmlayer@...ibm.com>,
        Kevin Gao <kevin.gao@...el.com>,
        network dev <netdev@...r.kernel.org>
Subject: Re: [RESEND 2/2] sctp: hold cached endpoints to prevent possible UAF

On Thu, 16 Dec 2021, Xin Long wrote:

> On Thu, Dec 16, 2021 at 11:39 AM Lee Jones <lee.jones@...aro.org> wrote:
> >
> > On Thu, 16 Dec 2021, Xin Long wrote:
> >
> > > On Wed, Dec 15, 2021 at 8:48 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > > >
> > > > On Tue, 14 Dec 2021 21:57:32 +0000 Lee Jones wrote:
> > > > > The cause of the resultant dump_stack() reported below is a
> > > > > dereference of a freed pointer to 'struct sctp_endpoint' in
> > > > > sctp_sock_dump().
> > > > >
> > > > > This race condition occurs when a transport is cached into its
> > > > > associated hash table followed by an endpoint/sock migration to a new
> > > > > association in sctp_assoc_migrate() prior to their subsequent use in
> > > > > sctp_diag_dump() which uses sctp_for_each_transport() to walk the hash
> > > > > table calling into sctp_sock_dump() where the dereference occurs.
> >
> > > in sctp_sock_dump():
> > >         struct sock *sk = ep->base.sk;
> > >         ... <--[1]
> > >         lock_sock(sk);
> > >
> > > Do you mean in [1], the sk is peeled off and gets freed elsewhere?
> >
> > 'ep' and 'sk' are both switched out for new ones in sctp_sock_migrate().
> >
> > > if that's true, it's still late to do sock_hold(sk) in your this patch.
> >
> > No, that's not right.
> >
> > The schedule happens *inside* the lock_sock() call.
> Sorry, I don't follow this.
> We can't expect when the schedule happens, why do you think this
> can never be scheduled before the lock_sock() call?

True, but I've had this running for hours and it hasn't reproduced.

Without this patch, I can reproduce this in around 2 seconds.

The C-repro for this is pretty intense!

If you want to be *sure* that a schedule will never happen, we can
take a reference directly with:

     ep = sctp_endpoint_hold(tsp->asoc->ep);
     sk = sock_hold(ep->base.sk);

Which was my original plan before I soak tested this submitted patch
for hours without any sign of reproducing the issue.

> If the sock is peeled off or is being freed, we shouldn't dump this sock,
> and it's better to skip it.

I guess we can do that too.

Are you suggesting sctp_sock_migrate() as the call site?

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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