pf: Limit the fragment entry queue length to 64 per bucket.

So we have a global limit of 1024 fragments, but it is fine grained to
the region of the packet.  Smaller packets may have less fragments.
This costs another 16 bytes of memory per reassembly and devides the
worst case for searching by 8.

Obtained from:	OpenBSD
Differential Revision:	https://reviews.freebsd.org/D17734

(cherry picked from commit 790194cd472b1d17e08940e9f839322abcf14ec9)

2 files changed, 36 insertions, 5 deletions

diff --git a/sys/net/pfvar.h b/sys/net/pfvar.h
index d6c2bf9120e9..3a535d04f12f 100644
--- a/sys/net/pfvar.h
+++ b/sys/net/pfvar.h
@@ -1012,6 +1012,13 @@ struct pf_divert {
 #define PF_FRAG_ENTRY_POINTS		16
 
 /*
+ * The number of entries in the fragment queue must be limited
+ * to avoid DoS by linear seaching.  Instead of a global limit,
+ * use a limit per entry point.  For large packets these sum up.
+ */
+#define PF_FRAG_ENTRY_LIMIT		64
+
+/*
  * ioctl parameter structures
  */
 
diff --git a/sys/netpfil/pf/pf_norm.c b/sys/netpfil/pf/pf_norm.c
index 0e2fdca4c2ce..eb310e27b9ae 100644
--- a/sys/netpfil/pf/pf_norm.c
+++ b/sys/netpfil/pf/pf_norm.c
@@ -87,7 +87,10 @@ struct pf_fragment {
 #define fr_af	fr_key.frc_af
 #define fr_proto	fr_key.frc_proto
 
+	/* pointers to queue element */
 	struct pf_frent	*fr_firstoff[PF_FRAG_ENTRY_POINTS];
+	/* count entries between pointers */
+	uint8_t	fr_entries[PF_FRAG_ENTRY_POINTS];
 	RB_ENTRY(pf_fragment) fr_entry;
 	TAILQ_ENTRY(pf_fragment) frag_next;
 	uint32_t	fr_timeout;
@@ -138,7 +141,7 @@ static int	pf_frent_holes(struct pf_frent *frent);
 static struct pf_fragment *pf_find_fragment(struct pf_fragment_cmp *key,
 		    struct pf_frag_tree *tree);
 static inline int	pf_frent_index(struct pf_frent *);
-static void	pf_frent_insert(struct pf_fragment *,
+static int	pf_frent_insert(struct pf_fragment *,
 			    struct pf_frent *, struct pf_frent *);
 void			pf_frent_remove(struct pf_fragment *,
 			    struct pf_frent *);
@@ -392,12 +395,24 @@ pf_frent_index(struct pf_frent *frent)
 	return frent->fe_off / (0x10000 / PF_FRAG_ENTRY_POINTS);
 }
 
-static void
+static int
 pf_frent_insert(struct pf_fragment *frag, struct pf_frent *frent,
     struct pf_frent *prev)
 {
 	int index;
 
+	CTASSERT(PF_FRAG_ENTRY_LIMIT <= 0xff);
+
+	/*
+	 * A packet has at most 65536 octets.  With 16 entry points, each one
+	 * spawns 4096 octets.  We limit these to 64 fragments each, which
+	 * means on average every fragment must have at least 64 octets.
+	 */
+	index = pf_frent_index(frent);
+	if (frag->fr_entries[index] >= PF_FRAG_ENTRY_LIMIT)
+		return ENOBUFS;
+	frag->fr_entries[index]++;
+
 	if (prev == NULL) {
 		TAILQ_INSERT_HEAD(&frag->fr_queue, frent, fr_next);
 	} else {
@@ -406,7 +421,6 @@ pf_frent_insert(struct pf_fragment *frag, struct pf_frent *frent,
 		TAILQ_INSERT_AFTER(&frag->fr_queue, prev, frent, fr_next);
 	}
 
-	index = pf_frent_index(frent);
 	if (frag->fr_firstoff[index] == NULL) {
 		KASSERT(prev == NULL || pf_frent_index(prev) < index,
 		    ("prev == NULL || pf_frent_index(pref) < index"));
@@ -424,6 +438,8 @@ pf_frent_insert(struct pf_fragment *frag, struct pf_frent *frent,
 	}
 
 	frag->fr_holes += pf_frent_holes(frent);
+
+	return 0;
 }
 
 void
@@ -460,6 +476,9 @@ pf_frent_remove(struct pf_fragment *frag, struct pf_frent *frent)
 	}
 
 	TAILQ_REMOVE(&frag->fr_queue, frent, fr_next);
+
+	KASSERT(frag->fr_entries[index] > 0, ("No fragments remaining"));
+	frag->fr_entries[index]--;
 }
 
 struct pf_frent *
@@ -567,6 +586,7 @@ pf_fillup_fragment(struct pf_fragment_cmp *key, struct pf_frent *frent,
 
 		*(struct pf_fragment_cmp *)frag = *key;
 		memset(frag->fr_firstoff, 0, sizeof(frag->fr_firstoff));
+		memset(frag->fr_entries, 0, sizeof(frag->fr_entries));
 		frag->fr_timeout = time_uptime;
 		frag->fr_maxlen = frent->fe_len;
 		frag->fr_holes = 1;
@@ -575,7 +595,7 @@ pf_fillup_fragment(struct pf_fragment_cmp *key, struct pf_frent *frent,
 		RB_INSERT(pf_frag_tree, &V_pf_frag_tree, frag);
 		TAILQ_INSERT_HEAD(&V_pf_fragqueue, frag, frag_next);
 
-		/* We do not have a previous fragment. */
+		/* We do not have a previous fragment, cannot fail. */
 		pf_frent_insert(frag, frent, NULL);
 
 		return (frag);
@@ -646,7 +666,11 @@ pf_fillup_fragment(struct pf_fragment_cmp *key, struct pf_frent *frent,
 		uma_zfree(V_pf_frent_z, after);
 	}
 
-	pf_frent_insert(frag, frent, prev);
+	/* If part of the queue gets too long, there is not way to recover. */
+	if (pf_frent_insert(frag, frent, prev)) {
+		DPFPRINTF(("fragment queue limit exceeded"));
+		goto bad_fragment;
+	}
 
 	return (frag);