CVE-2014-0211: Integer overflow in fs_get_reply/_fs_start_read

fs_get_reply() would take any reply size, multiply it by 4 and pass to
_fs_start_read.  If that size was bigger than the current reply buffer
size, _fs_start_read would add it to the existing buffer size plus the
buffer size increment constant and realloc the buffer to that result.

This math could overflow, causing the code to allocate a smaller
buffer than the amount it was about to read into that buffer from
the network.  It could also succeed, allowing the remote font server
to cause massive allocations in the X server, possibly using up all
the address space in a 32-bit X server, allowing the triggering of
other bugs in code that fails to handle malloc failure properly.

This patch protects against both problems, by disconnecting any
font server trying to feed us more than (the somewhat arbitrary)
64 mb in a single reply.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Matthieu Herrb <matthieu@herrb.eu>

1 files changed, 18 insertions, 0 deletions

diff --git a/src/fc/fserve.c b/src/fc/fserve.c
index f08028f..3abbacf 100644
--- a/src/fc/fserve.c
+++ b/src/fc/fserve.c
@@ -97,6 +97,9 @@ in this Software without prior written authorization from The Open Group.
  */
 #define LENGTHOF(r)	(SIZEOF(r) >> 2)
 
+/* Somewhat arbitrary limit on maximum reply size we'll try to read. */
+#define MAX_REPLY_LENGTH	((64 * 1024 * 1024) >> 2)
+
 extern void ErrorF(const char *f, ...);
 
 static int fs_read_glyphs ( FontPathElementPtr fpe, FSBlockDataPtr blockrec );
@@ -619,6 +622,21 @@ fs_get_reply (FSFpePtr conn, int *error)
 
     rep = (fsGenericReply *) buf;
 
+    /*
+     * Refuse to accept replies longer than a maximum reasonable length,
+     * before we pass to _fs_start_read, since it will try to resize the
+     * incoming connection buffer to this size.  Also avoids integer overflow
+     * on 32-bit systems.
+     */
+    if (rep->length > MAX_REPLY_LENGTH)
+    {
+	ErrorF("fserve: reply length %d > MAX_REPLY_LENGTH, disconnecting"
+	       " from font server\n", rep->length);
+	_fs_connection_died (conn);
+	*error = FSIO_ERROR;
+	return 0;
+    }
+
     ret = _fs_start_read (conn, rep->length << 2, &buf);
     if (ret != FSIO_READY)
     {