dbe: unvalidated lengths in DbeSwapBuffers calls [CVE-2014-8097]

ProcDbeSwapBuffers() has a 32bit (n) length value that it uses to read
from a buffer. The length is never validated, which can lead to out of
bound reads, and possibly returning the data read from out of bounds to
the misbehaving client via an X Error packet.

SProcDbeSwapBuffers() swaps data (for correct endianness) before
handing it off to the real proc.  While doing the swapping, the
length field is not validated, which can cause memory corruption.

v2: reorder checks to avoid compilers optimizing out checks for overflow
that happen after we'd already have done the overflowing multiplications.

Reported-by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>

1 files changed, 8 insertions, 3 deletions

diff --git a/dbe/dbe.c b/dbe/dbe.c
index 527588c3c..df2ad5c51 100644
--- a/dbe/dbe.c
+++ b/dbe/dbe.c
@@ -450,18 +450,20 @@ ProcDbeSwapBuffers(ClientPtr client)
     DbeSwapInfoPtr swapInfo;
     xDbeSwapInfo *dbeSwapInfo;
     int error;
-    register int i, j;
-    int nStuff;
+    unsigned int i, j;
+    unsigned int nStuff;
 
     REQUEST_AT_LEAST_SIZE(xDbeSwapBuffersReq);
     nStuff = stuff->n;          /* use local variable for performance. */
 
     if (nStuff == 0) {
+        REQUEST_SIZE_MATCH(xDbeSwapBuffersReq);
         return Success;
     }
 
     if (nStuff > UINT32_MAX / sizeof(DbeSwapInfoRec))
         return BadAlloc;
+    REQUEST_FIXED_SIZE(xDbeSwapBuffersReq, nStuff * sizeof(xDbeSwapInfo));
 
     /* Get to the swap info appended to the end of the request. */
     dbeSwapInfo = (xDbeSwapInfo *) &stuff[1];
@@ -914,13 +916,16 @@ static int
 SProcDbeSwapBuffers(ClientPtr client)
 {
     REQUEST(xDbeSwapBuffersReq);
-    register int i;
+    unsigned int i;
     xDbeSwapInfo *pSwapInfo;
 
     swaps(&stuff->length);
     REQUEST_AT_LEAST_SIZE(xDbeSwapBuffersReq);
 
     swapl(&stuff->n);
+    if (stuff->n > UINT32_MAX / sizeof(DbeSwapInfoRec))
+        return BadAlloc;
+    REQUEST_FIXED_SIZE(xDbeSwapBuffersReq, stuff->n * sizeof(xDbeSwapInfo));
 
     if (stuff->n != 0) {
         pSwapInfo = (xDbeSwapInfo *) stuff + 1;