From 7b21c4eabf1f3946d3f63cce1319c490caab8ecf Mon Sep 17 00:00:00 2001
From: Nick Mathewson <nickm@torproject.org>
Date: Sun, 5 Jan 2014 08:30:46 -0500
Subject: [PATCH] Fix CVE-2014-6272 in Libevent 1.4

For this fix, we need to make sure that passing too-large inputs to
the evbuffer functions can't make us do bad things with the heap.
---
 buffer.c | 34 ++++++++++++++++++++++++++--------
 1 file changed, 26 insertions(+), 8 deletions(-)

diff --git a/buffer.c b/buffer.c
index 09d2160d..64324bbd 100644
--- a/buffer.c
+++ b/buffer.c
@@ -144,7 +144,8 @@ evbuffer_add_vprintf(struct evbuffer *buf, const char *fmt, va_list ap)
 	va_list aq;
 
 	/* make sure that at least some space is available */
-	evbuffer_expand(buf, 64);
+	if (evbuffer_expand(buf, 64) < 0)
+		return (-1);
 	for (;;) {
 		size_t used = buf->misalign + buf->off;
 		buffer = (char *)buf->buffer + buf->off;
@@ -345,31 +346,48 @@ evbuffer_align(struct evbuffer *buf)
 	buf->misalign = 0;
 }
 
+#ifndef SIZE_MAX
+#define SIZE_MAX ((size_t)-1)
+#endif
+
 /* Expands the available space in the event buffer to at least datlen */
 
 int
 evbuffer_expand(struct evbuffer *buf, size_t datlen)
 {
-	size_t need = buf->misalign + buf->off + datlen;
+	size_t used = buf->misalign + buf->off;
+	size_t need;
+
+	assert(buf->totallen >= used);
 
 	/* If we can fit all the data, then we don't have to do anything */
-	if (buf->totallen >= need)
+	if (buf->totallen - used >= datlen)
 		return (0);
+	/* If we would need to overflow to fit this much data, we can't
+	 * do anything. */
+	if (datlen > SIZE_MAX - buf->off)
+		return (-1);
 
 	/*
 	 * If the misalignment fulfills our data needs, we just force an
 	 * alignment to happen.  Afterwards, we have enough space.
 	 */
-	if (buf->misalign >= datlen) {
+	if (buf->totallen - buf->off >= datlen) {
 		evbuffer_align(buf);
 	} else {
 		void *newbuf;
 		size_t length = buf->totallen;
+		size_t need = buf->off + datlen;
 
 		if (length < 256)
 			length = 256;
-		while (length < need)
-			length <<= 1;
+		if (need < SIZE_MAX / 2) {
+			while (length < need) {
+				length <<= 1;
+			}
+		} else {
+			length = need;
+		}
 
 		if (buf->orig_buffer != buf->buffer)
 			evbuffer_align(buf);
@@ -386,10 +404,10 @@ evbuffer_expand(struct evbuffer *buf, size_t datlen)
 int
 evbuffer_add(struct evbuffer *buf, const void *data, size_t datlen)
 {
-	size_t need = buf->misalign + buf->off + datlen;
+	size_t used = buf->misalign + buf->off;
 	size_t oldoff = buf->off;
 
-	if (buf->totallen < need) {
+	if (buf->totallen - used < datlen) {
 		if (evbuffer_expand(buf, datlen) == -1)
 			return (-1);
 	}