Fixed off-by-one writes (CVE-2018-14599).

The functions XGetFontPath, XListExtensions, and XListFonts are
vulnerable to an off-by-one override on malicious server responses.

The server replies consist of chunks consisting of a length byte
followed by actual string, which is not NUL-terminated.

While parsing the response, the length byte is overridden with '\0',
thus the memory area can be used as storage of C strings later on. To
be able to NUL-terminate the last string, the buffer is reserved with
an additional byte of space.

For a boundary check, the variable chend (end of ch) was introduced,
pointing at the end of the buffer which ch initially points to.
Unfortunately there is a difference in handling "the end of ch".

While chend points at the first byte that must not be written to,
the for-loop uses chend as the last byte that can be written to.

Therefore, an off-by-one can occur.

I have refactored the code so chend actually points to the last byte
that can be written to without an out of boundary access. As it is not
possible to achieve "ch + length < chend" and "ch + length + 1 > chend"
with the corrected chend meaning, I removed the inner if-check.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

3 files changed, 9 insertions, 21 deletions

diff --git a/src/FontNames.c b/src/FontNames.c
index 9ffdfd29..ec7d90fa 100644
--- a/src/FontNames.c
+++ b/src/FontNames.c
@@ -88,24 +88,16 @@ int *actualCount)	/* RETURN */
 	 * unpack into null terminated strings.
 	 */
 	chstart = ch;
-	chend = ch + (rlen + 1);
+	chend = ch + rlen;
 	length = *(unsigned char *)ch;
 	*ch = 1; /* make sure it is non-zero for XFreeFontNames */
 	for (i = 0; i < rep.nFonts; i++) {
 	    if (ch + length < chend) {
 		flist[i] = ch + 1;  /* skip over length */
 		ch += length + 1;  /* find next length ... */
-		if (ch <= chend) {
-		    length = *(unsigned char *)ch;
-		    *ch = '\0';  /* and replace with null-termination */
-		    count++;
-		} else {
-                    Xfree(chstart);
-                    Xfree(flist);
-                    flist = NULL;
-                    count = 0;
-                    break;
-		}
+		length = *(unsigned char *)ch;
+		*ch = '\0';  /* and replace with null-termination */
+		count++;
 	    } else {
                 Xfree(chstart);
                 Xfree(flist);
diff --git a/src/GetFPath.c b/src/GetFPath.c
index 3d87e4f6..7ad21e9a 100644
--- a/src/GetFPath.c
+++ b/src/GetFPath.c
@@ -69,7 +69,7 @@ char **XGetFontPath(
 	    /*
 	     * unpack into null terminated strings.
 	     */
-	    chend = ch + (nbytes + 1);
+	    chend = ch + nbytes;
 	    length = *ch;
 	    for (i = 0; i < rep.nPaths; i++) {
 		if (ch + length < chend) {
diff --git a/src/ListExt.c b/src/ListExt.c
index 7fdf9932..8f344ac0 100644
--- a/src/ListExt.c
+++ b/src/ListExt.c
@@ -74,19 +74,15 @@ char **XListExtensions(
 	    /*
 	     * unpack into null terminated strings.
 	     */
-	    chend = ch + (rlen + 1);
+	    chend = ch + rlen;
 	    length = *ch;
 	    for (i = 0; i < rep.nExtensions; i++) {
 		if (ch + length < chend) {
 		    list[i] = ch+1;  /* skip over length */
 		    ch += length + 1; /* find next length ... */
-		    if (ch <= chend) {
-			length = *ch;
-			*ch = '\0'; /* and replace with null-termination */
-			count++;
-		    } else {
-			list[i] = NULL;
-		    }
+		    length = *ch;
+		    *ch = '\0'; /* and replace with null-termination */
+		    count++;
 		} else
 		    list[i] = NULL;
 	    }