[PATCH v2 1/2] sctp: export sctp_endpoint_{hold,put}() and return incremented endpoint

[Lee Jones <lee.jones@linaro.org>]

net/sctp/diag.c for instance is built into its own separate module
(sctp_diag.ko) and requires the use of sctp_endpoint_{hold,put}() in
order to prevent a recently found use-after-free issue.

In order to prevent data corruption of the pointer used to take a
reference on a specific endpoint, between the time of calling
sctp_endpoint_hold() and it returning, the API now returns a pointer
to the exact endpoint that was incremented.

For example, in sctp_sock_dump(), we could have the following hunk:

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

It is possible for this task to be swapped out immediately following
the call into sctp_endpoint_hold() that would change the address of
tsp->asoc->ep to point to a completely different endpoint.  This means
a reference could be taken to the old endpoint and the new one would
be processed without a reference taken, moreover the new endpoint
could then be freed whilst still processing as a result, causing a
use-after-free.

If we return the exact pointer that was held, we ensure this task
processes only the endpoint we have taken a reference to.  The
resultant hunk now looks like this:

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

Cc: Vlad Yasevich <vyasevich@gmail.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: lksctp developers <linux-sctp@vger.kernel.org>
Cc: "H.P. Yarroll" <piggy@acm.org>
Cc: Karl Knutson <karl@athena.chicago.il.us>
Cc: Jon Grimm <jgrimm@us.ibm.com>
Cc: Xingang Guo <xingang.guo@intel.com>
Cc: Hui Huang <hui.huang@nokia.com>
Cc: Sridhar Samudrala <sri@us.ibm.com>
Cc: Daisy Chang <daisyc@us.ibm.com>
Cc: Ryan Layer <rmlayer@us.ibm.com>
Cc: Kevin Gao <kevin.gao@intel.com>
Cc: linux-sctp@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: stable@vger.kernel.org
Fixes: 8f840e47f190c ("sctp: add the sctp_diag.c file")
Signed-off-by: Lee Jones <lee.jones@linaro.org>
---
 include/net/sctp/structs.h | 2 +-
 net/sctp/endpointola.c     | 5 ++++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
index 651bba654d77d..78d71ca56452b 100644
--- a/include/net/sctp/structs.h
+++ b/include/net/sctp/structs.h
@@ -1380,7 +1380,7 @@ static inline struct sctp_endpoint *sctp_ep(struct sctp_ep_common *base)
 struct sctp_endpoint *sctp_endpoint_new(struct sock *, gfp_t);
 void sctp_endpoint_free(struct sctp_endpoint *);
 void sctp_endpoint_put(struct sctp_endpoint *);
-void sctp_endpoint_hold(struct sctp_endpoint *);
+struct sctp_endpoint *sctp_endpoint_hold(struct sctp_endpoint *);
 void sctp_endpoint_add_asoc(struct sctp_endpoint *, struct sctp_association *);
 struct sctp_association *sctp_endpoint_lookup_assoc(
 	const struct sctp_endpoint *ep,
diff --git a/net/sctp/endpointola.c b/net/sctp/endpointola.c
index 48c9c2c7602f7..bdbf74fc7eb4c 100644
--- a/net/sctp/endpointola.c
+++ b/net/sctp/endpointola.c
@@ -222,10 +222,12 @@ static void sctp_endpoint_destroy(struct sctp_endpoint *ep)
 }
 
 /* Hold a reference to an endpoint. */
-void sctp_endpoint_hold(struct sctp_endpoint *ep)
+struct sctp_endpoint *sctp_endpoint_hold(struct sctp_endpoint *ep)
 {
 	refcount_inc(&ep->base.refcnt);
+	return ep;
 }
+EXPORT_SYMBOL_GPL(sctp_endpoint_hold);
 
 /* Release a reference to an endpoint and clean up if there are
  * no more references.
@@ -235,6 +237,7 @@ void sctp_endpoint_put(struct sctp_endpoint *ep)
 	if (refcount_dec_and_test(&ep->base.refcnt))
 		sctp_endpoint_destroy(ep);
 }
+EXPORT_SYMBOL_GPL(sctp_endpoint_put);
 
 /* Is this the endpoint we are looking for?  */
 struct sctp_endpoint *sctp_endpoint_is_match(struct sctp_endpoint *ep,
-- 
2.34.1.173.g76aa8bc2d0-goog