buffer_list_aggregate_separator(): prevent 0-byte malloc

As pointed out in finding OVPN-05 of the cryptograpy engineering audit
(funded by Private Internet Access), buffer_list_aggregate_separator()
could perform a 0-byte malloc when called with a list of 0-length buffers
and a "" separator.  If other could would later try to access that buffer
memory, this would result in undefined behaviour.  To prevent this, always
malloc() 1 byte.

To simplify as we go, use alloc_buf() to allocate the buffer.  This has
the additional benefit that the actual buffer data (not the contents) is
zero-terminated, because alloc_buf() calls calloc() and we have 1 extra
byte of data.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <1514541240-19536-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16106.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 748902f46260fe11cb25726d2bf93bb06ad338f2)

diff --git a/src/openvpn/buffer.c b/src/openvpn/buffer.c
index 7d43d07af24624cee59981ce22f8ace8409b1a09..0b75edef90a73cad9e7a528561d1e87e9cff7cc5 100644 (file)
--- a/src/openvpn/buffer.c
+++ b/src/openvpn/buffer.c
@@ -1259,8 +1259,7 @@ buffer_list_aggregate_separator(struct buffer_list *bl, const size_t max_len,
             struct buffer_entry *e = bl->head, *f;
 
             ALLOC_OBJ_CLEAR(f, struct buffer_entry);
-            f->buf.data = malloc(size);
-            check_malloc_return(f->buf.data);
+            f->buf = alloc_buf(size + 1); /* prevent 0-byte malloc */
             f->buf.capacity = size;
             for (i = 0; e && i < count; ++i)
             {