ppp: fix race in ppp device destruction

Guillaume Nault · davem330 · commit 6151b8b37b11 · 2017-10-06T10:16:34.000-07:00
ppp_release() tries to ensure that netdevices are unregistered before decrementing the unit refcount and running ppp_destroy_interface(). This is all fine as long as the the device is unregistered by ppp_release(): the unregister_netdevice() call, followed by rtnl_unlock(), guarantee that the unregistration process completes before rtnl_unlock() returns. However, the device may be unregistered by other means (like ppp_nl_dellink()). If this happens right before ppp_release() calling rtnl_lock(), then ppp_release() has to wait for the concurrent unregistration code to release the lock. But rtnl_unlock() releases the lock before completing the device unregistration process. This allows ppp_release() to proceed and eventually call ppp_destroy_interface() before the unregistration process completes. Calling free_netdev() on this partially unregistered device will BUG(): ------------[ cut here ]------------ kernel BUG at net/core/dev.c:8141! invalid opcode: 0000 [#1] SMP CPU: 1 PID: 1557 Comm: pppd Not tainted 4.14.0-rc2+ #4 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1.fc26 04/01/2014 Call Trace: ppp_destroy_interface+0xd8/0xe0 [ppp_generic] ppp_disconnect_channel+0xda/0x110 [ppp_generic] ppp_unregister_channel+0x5e/0x110 [ppp_generic] pppox_unbind_sock+0x23/0x30 [pppox] pppoe_connect+0x130/0x440 [pppoe] SYSC_connect+0x98/0x110 ? do_fcntl+0x2c0/0x5d0 SyS_connect+0xe/0x10 entry_SYSCALL_64_fastpath+0x1a/0xa5 RIP: free_netdev+0x107/0x110 RSP: ffffc28a40573d88 ---[ end trace ed294ff0cc40eeff ]--- We could set the ->needs_free_netdev flag on PPP devices and move the ppp_destroy_interface() logic in the ->priv_destructor() callback. But that'd be quite intrusive as we'd first need to unlink from the other channels and units that depend on the device (the ones that used the PPPIOCCONNECT and PPPIOCATTACH ioctls). Instead, we can just let the netdevice hold a reference on its ppp_file. This reference is dropped in ->priv_destructor(), at the very end of the unregistration process, so that neither ppp_release() nor ppp_disconnect_channel() can call ppp_destroy_interface() in the interim. Reported-by: Beniamino Galvani <bgalvani@redhat.com> Fixes: 8cb775b ("ppp: fix device unregistration upon netns deletion") Signed-off-by: Guillaume Nault <g.nault@alphalink.fr> Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
@@ -1339,7 +1339,17 @@ ppp_get_stats64(struct net_device *dev, struct rtnl_link_stats64 *stats64)
 
 static int ppp_dev_init(struct net_device *dev)
 {
+	struct ppp *ppp;
+
 	netdev_lockdep_set_classes(dev);
+
+	ppp = netdev_priv(dev);
+	/* Let the netdevice take a reference on the ppp file. This ensures
+	 * that ppp_destroy_interface() won't run before the device gets
+	 * unregistered.
+	 */
+	atomic_inc(&ppp->file.refcnt);
+
 	return 0;
 }
 
@@ -1362,6 +1372,15 @@ static void ppp_dev_uninit(struct net_device *dev)
 	wake_up_interruptible(&ppp->file.rwait);
 }
 
+static void ppp_dev_priv_destructor(struct net_device *dev)
+{
+	struct ppp *ppp;
+
+	ppp = netdev_priv(dev);
+	if (atomic_dec_and_test(&ppp->file.refcnt))
+		ppp_destroy_interface(ppp);
+}
+
 static const struct net_device_ops ppp_netdev_ops = {
 	.ndo_init	 = ppp_dev_init,
 	.ndo_uninit      = ppp_dev_uninit,
@@ -1387,6 +1406,7 @@ static void ppp_setup(struct net_device *dev)
 	dev->tx_queue_len = 3;
 	dev->type = ARPHRD_PPP;
 	dev->flags = IFF_POINTOPOINT | IFF_NOARP | IFF_MULTICAST;
+	dev->priv_destructor = ppp_dev_priv_destructor;
 	netif_keep_dst(dev);
 }
 
