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    SubjectRe: [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@linaro.org> wrote:
    > >
    > > On Thu, 16 Dec 2021, Xin Long wrote:
    > >
    > > > On Wed, Dec 15, 2021 at 8:48 PM Jakub Kicinski <kuba@kernel.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
    Follow Linaro: Facebook | Twitter | Blog

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