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

[Xin Long]

On Thu, Dec 16, 2021 at 12:14 PM Lee Jones <lee.jones@xxxxxxxxxx> wrote:
>
> On Thu, 16 Dec 2021, Lee Jones wrote:
>
> > On Thu, 16 Dec 2021, Xin Long wrote:
> >
> > > On Thu, Dec 16, 2021 at 11:39 AM Lee Jones <lee.jones@xxxxxxxxxx> wrote:
> > > >
> > > > On Thu, 16 Dec 2021, Xin Long wrote:
> > > >
> > > > > On Wed, Dec 15, 2021 at 8:48 PM Jakub Kicinski <kuba@xxxxxxxxxx> 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.
I understand, but it's a crash, we shouldn't take any risk that it
will never happen.
you may try to add a usleep() before the lock_sock call to reproduce it.

> >
> > 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.
we tried to not export sctp_obj_hold/put(), that's why we had
sctp_for_each_transport().

ep itself holds a reference of sk when it's alive, so it's weird to do
these 2 together.

> >
> > > 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?
diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index 85ac2e901ffc..56ea7a0e2add 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -9868,6 +9868,7 @@ static int sctp_sock_migrate(struct sock *oldsk,
struct sock *newsk,
                inet_sk_set_state(newsk, SCTP_SS_ESTABLISHED);
        }

+       sock_set_flag(oldsk, SOCK_RCU_FREE);
        release_sock(newsk);

        return 0;

SOCK_RCU_FREE is set to the previous sk, so that this sk will not
be freed between rcu_read_lock() and rcu_read_unlock().

>
> Also, when are you planning on testing the flag?
SOCK_RCU_FREE flag is used when freeing sk in sk_destruct(),
and if it's set, it will be freed in the next grace period of RCU.

>
> Won't that suffer with the same issue(s)?
diff --git a/net/sctp/diag.c b/net/sctp/diag.c
index 7970d786c4a2..b4c4acd9e67e 100644
--- a/net/sctp/diag.c
+++ b/net/sctp/diag.c
@@ -309,16 +309,21 @@ static int sctp_tsp_dump_one(struct
sctp_transport *tsp, void *p)

 static int sctp_sock_dump(struct sctp_transport *tsp, void *p)
 {
-       struct sctp_endpoint *ep = tsp->asoc->ep;
        struct sctp_comm_param *commp = p;
-       struct sock *sk = ep->base.sk;
        struct sk_buff *skb = commp->skb;
        struct netlink_callback *cb = commp->cb;
        const struct inet_diag_req_v2 *r = commp->r;
        struct sctp_association *assoc;
+       struct sctp_endpoint *ep;
+       struct sock *sk;
        int err = 0;

+       rcu_read_lock();
+       ep = tsp->asoc->ep;
+       sk = ep->base.sk;
        lock_sock(sk);
+       if (tsp->asoc->ep != ep)
+               goto release;
        list_for_each_entry(assoc, &ep->asocs, asocs) {
                if (cb->args[4] < cb->args[1])
                        goto next;
@@ -358,6 +363,7 @@ static int sctp_sock_dump(struct sctp_transport
*tsp, void *p)
        cb->args[4] = 0;
 release:
        release_sock(sk);
+       rcu_read_unlock();
        return err;
 }

rcu_read_lock() will make sure sk from tsp->asoc->ep->base.sk will not
be freed until rcu_read_unlock().

That's all I have. Do you see any other way to fix this?

Thanks.

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