Properly detect loops by recording the interface pointer in an mtag.

For now, preserve the gif_called functionality to limit the nesting
level because uncontrolled nesting can easily cause the kernel stack
exhaustion.  Rumors are it should be shot to allow people to easily
shoot themselves in the foot, but I have ran out of cartridges.  ;)

2 files changed, 33 insertions, 20 deletions

diff --git a/sys/net/if_gif.c b/sys/net/if_gif.c
index 69d8aaa..2519192 100644
--- a/sys/net/if_gif.c
+++ b/sys/net/if_gif.c
@@ -84,22 +84,13 @@
 #define GIFNAME		"gif"
 
 /*
- * gif_mtx protects the global gif_softc_list, and access to gif_called.
- * XXX: See comment blow on gif_called.
+ * gif_mtx protects the global gif_softc_list.
  * XXX: Per-softc locking is still required.
  */
 static struct mtx gif_mtx;
 static MALLOC_DEFINE(M_GIF, "gif", "Generic Tunnel Interface");
 static LIST_HEAD(, gif_softc) gif_softc_list;
 
-/*
- * XXX: gif_called is a recursion counter to prevent misconfiguration to
- * cause unbounded looping in the network stack.  However, this is a flawed
- * approach as it assumes non-reentrance in the stack.  This should be
- * changed to use packet tags to track recusion..
- */
-static int gif_called = 0;
-
 void	(*ng_gif_input_p)(struct ifnet *ifp, struct mbuf **mp, int af);
 void	(*ng_gif_input_orphan_p)(struct ifnet *ifp, struct mbuf *m, int af);
 void	(*ng_gif_attach_p)(struct ifnet *ifp);
@@ -347,7 +338,9 @@ gif_output(ifp, m, dst, rt)
 	struct rtentry *rt;	/* added in net2 */
 {
 	struct gif_softc *sc = (struct gif_softc*)ifp;
+	struct m_tag *mtag;
 	int error = 0;
+	int gif_called;
 
 #ifdef MAC
 	error = mac_check_ifnet_transmit(ifp, m);
@@ -359,14 +352,26 @@ gif_output(ifp, m, dst, rt)
 
 	/*
 	 * gif may cause infinite recursion calls when misconfigured.
+	 * We'll prevent this by detecting loops.
+	 *
+	 * High nesting level may cause stack exhaustion.
 	 * We'll prevent this by introducing upper limit.
-	 * XXX: this mechanism may introduce another problem about
-	 *      mutual exclusion of the variable CALLED, especially if we
-	 *      use kernel thread.
 	 */
-	mtx_lock(&gif_mtx);
-	if (++gif_called > max_gif_nesting) {
-		mtx_unlock(&gif_mtx);
+	gif_called = 1;
+	mtag = m_tag_locate(m, MTAG_GIF, MTAG_GIF_CALLED, NULL);
+	while (mtag != NULL) {
+		if (*(struct ifnet **)(mtag + 1) == ifp) {
+			log(LOG_NOTICE,
+			    "gif_output: loop detected on %s\n",
+			    (*(struct ifnet **)(mtag + 1))->if_xname);
+			m_freem(m);
+			error = EIO;	/* is there better errno? */
+			goto end;
+		}
+		mtag = m_tag_locate(m, MTAG_GIF, MTAG_GIF_CALLED, mtag);
+		gif_called++;
+	}
+	if (gif_called > max_gif_nesting) {
 		log(LOG_NOTICE,
 		    "gif_output: recursively called too many times(%d)\n",
 		    gif_called);
@@ -374,7 +379,15 @@ gif_output(ifp, m, dst, rt)
 		error = EIO;	/* is there better errno? */
 		goto end;
 	}
-	mtx_unlock(&gif_mtx);
+	mtag = m_tag_alloc(MTAG_GIF, MTAG_GIF_CALLED, sizeof(struct ifnet *),
+	    M_NOWAIT);
+	if (mtag == NULL) {
+		m_freem(m);
+		error = ENOMEM;
+		goto end;
+	}
+	*(struct ifnet **)(mtag + 1) = ifp;
+	m_tag_prepend(m, mtag);
 
 	m->m_flags &= ~(M_BCAST|M_MCAST);
 	if (!(ifp->if_flags & IFF_UP) ||
@@ -414,9 +427,6 @@ gif_output(ifp, m, dst, rt)
 	}
 
   end:
-	mtx_lock(&gif_mtx);
-	gif_called = 0;		/* reset recursion counter */
-	mtx_unlock(&gif_mtx);
 	if (error)
 		ifp->if_oerrors++;
 	return error;
diff --git a/sys/net/if_gif.h b/sys/net/if_gif.h
index 8a0ac56..c3a9df2 100644
--- a/sys/net/if_gif.h
+++ b/sys/net/if_gif.h
@@ -81,6 +81,9 @@ struct gif_softc {
 #define	GIF_MTU_MIN	(1280)	/* Minimum MTU */
 #define	GIF_MTU_MAX	(8192)	/* Maximum MTU */
 
+#define	MTAG_GIF	1080679712
+#define	MTAG_GIF_CALLED	0
+
 /* Prototypes */
 void gifattach0(struct gif_softc *);
 void gif_input(struct mbuf *, int, struct ifnet *);