sunrpc: prevent use-after-free on clearing XPT_BUSY

When an xprt is created, it has a refcount of 1, and XPT_BUSY is set.
The refcount is *not* owned by the thread that created the xprt
(as is clear from the fact that creators never put the reference).
Rather, it is owned by the absence of XPT_DEAD.  Once XPT_DEAD is set,
(And XPT_BUSY is clear) that initial reference is dropped and the xprt
can be freed.

So when a creator clears XPT_BUSY it is dropping its only reference and
so must not touch the xprt again.

However svc_recv, after calling ->xpo_accept (and so getting an XPT_BUSY
reference on a new xprt), calls svc_xprt_recieved.  This clears
XPT_BUSY and then svc_xprt_enqueue - this last without owning a reference.
This is dangerous and has been seen to leave svc_xprt_enqueue working
with an xprt containing garbage.

So we need to hold an extra counted reference over that call to
svc_xprt_received.

For safety, any time we clear XPT_BUSY and then use the xprt again, we
first get a reference, and the put it again afterwards.

Note that svc_close_all does not need this extra protection as there are
no threads running, and the final free can only be called asynchronously
from such a thread.

Signed-off-by: NeilBrown <neilb@suse.de>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/svc_xprt.c

@@ -212,6 +212,7 @@ int svc_create_xprt(struct svc_serv *serv, const char *xprt_name,
 	spin_lock(&svc_xprt_class_lock);
 	list_for_each_entry(xcl, &svc_xprt_class_list, xcl_list) {
 		struct svc_xprt *newxprt;
+		unsigned short newport;
 
 		if (strcmp(xprt_name, xcl->xcl_name))
 			continue;
@@ -230,8 +231,9 @@ int svc_create_xprt(struct svc_serv *serv, const char *xprt_name,
 		spin_lock_bh(&serv->sv_lock);
 		list_add(&newxprt->xpt_list, &serv->sv_permsocks);
 		spin_unlock_bh(&serv->sv_lock);
+		newport = svc_xprt_local_port(newxprt);
 		clear_bit(XPT_BUSY, &newxprt->xpt_flags);
-		return svc_xprt_local_port(newxprt);
+		return newport;
 	}
  err:
 	spin_unlock(&svc_xprt_class_lock);
@@ -425,8 +427,13 @@ void svc_xprt_received(struct svc_xprt *xprt)
 {
 	BUG_ON(!test_bit(XPT_BUSY, &xprt->xpt_flags));
 	xprt->xpt_pool = NULL;
+	/* As soon as we clear busy, the xprt could be closed and
+	 * 'put', so we need a reference to call svc_xprt_enqueue with:
+	 */
+	svc_xprt_get(xprt);
 	clear_bit(XPT_BUSY, &xprt->xpt_flags);
 	svc_xprt_enqueue(xprt);
+	svc_xprt_put(xprt);
 }
 EXPORT_SYMBOL_GPL(svc_xprt_received);