Check SetMap request length carefully.

Avoid out of bounds memory accesses on too short request.

ZDI-CAN 11572 /  CVE-2020-14360

This vulnerability was discovered by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative
Signed-off-by: Matthieu Herrb <matthieu@herrb.eu>

xkb/xkb.c

@@ -2382,6 +2382,93 @@ SetVirtualModMap(XkbSrvInfoPtr xkbi,
     return (char *) wire;
 }
 
+#define _add_check_len(new) \
+    if (len > UINT32_MAX - (new) || len > req_len - (new)) goto bad; \
+    else len += new
+
+/**
+ * Check the length of the SetMap request
+ */
+static int
+_XkbSetMapCheckLength(xkbSetMapReq *req)
+{
+    size_t len = sz_xkbSetMapReq, req_len = req->length << 2;
+    xkbKeyTypeWireDesc *keytype;
+    xkbSymMapWireDesc *symmap;
+    BOOL preserve;
+    int i, map_count, nSyms;
+
+    if (req_len < len)
+        goto bad;
+    /* types */
+    if (req->present & XkbKeyTypesMask) {
+        keytype = (xkbKeyTypeWireDesc *)(req + 1);
+        for (i = 0; i < req->nTypes; i++) {
+            _add_check_len(XkbPaddedSize(sz_xkbKeyTypeWireDesc));
+            if (req->flags & XkbSetMapResizeTypes) {
+                _add_check_len(keytype->nMapEntries
+                               * sz_xkbKTSetMapEntryWireDesc);
+                preserve = keytype->preserve;
+                map_count = keytype->nMapEntries;
+                if (preserve) {
+                    _add_check_len(map_count * sz_xkbModsWireDesc);
+                }
+                keytype += 1;
+                keytype = (xkbKeyTypeWireDesc *)
+                          ((xkbKTSetMapEntryWireDesc *)keytype + map_count);
+                if (preserve)
+                    keytype = (xkbKeyTypeWireDesc *)
+                              ((xkbModsWireDesc *)keytype + map_count);
+            }
+        }
+    }
+    /* syms */
+    if (req->present & XkbKeySymsMask) {
+        symmap = (xkbSymMapWireDesc *)((char *)req + len);
+        for (i = 0; i < req->nKeySyms; i++) {
+            _add_check_len(sz_xkbSymMapWireDesc);
+            nSyms = symmap->nSyms;
+            _add_check_len(nSyms*sizeof(CARD32));
+            symmap += 1;
+            symmap = (xkbSymMapWireDesc *)((CARD32 *)symmap + nSyms);
+        }
+    }
+    /* actions */
+    if (req->present & XkbKeyActionsMask) {
+        _add_check_len(req->totalActs * sz_xkbActionWireDesc 
+                       + XkbPaddedSize(req->nKeyActs));
+    }
+    /* behaviours */
+    if (req->present & XkbKeyBehaviorsMask) {
+        _add_check_len(req->totalKeyBehaviors * sz_xkbBehaviorWireDesc);
+    }
+    /* vmods */
+    if (req->present & XkbVirtualModsMask) {
+        _add_check_len(XkbPaddedSize(Ones(req->virtualMods)));
+    }
+    /* explicit */
+    if (req->present & XkbExplicitComponentsMask) {
+        /* two bytes per non-zero explicit componen */
+        _add_check_len(XkbPaddedSize(req->totalKeyExplicit * sizeof(CARD16)));
+    }
+    /* modmap */
+    if (req->present & XkbModifierMapMask) {
+         /* two bytes per non-zero modmap component */
+        _add_check_len(XkbPaddedSize(req->totalModMapKeys * sizeof(CARD16)));
+    }
+    /* vmodmap */
+    if (req->present & XkbVirtualModMapMask) {
+        _add_check_len(req->totalVModMapKeys * sz_xkbVModMapWireDesc);
+    }
+    if (len == req_len)
+        return Success;
+bad:
+    ErrorF("[xkb] BOGUS LENGTH in SetMap: expected %ld got %ld\n",
+           len, req_len);
+    return BadLength;
+}
+
+
 /**
  * Check if the given request can be applied to the given device but don't
  * actually do anything, except swap values when client->swapped and doswap are both true.
@@ -2642,6 +2729,11 @@ ProcXkbSetMap(ClientPtr client)
     CHK_KBD_DEVICE(dev, stuff->deviceSpec, client, DixManageAccess);
     CHK_MASK_LEGAL(0x01, stuff->present, XkbAllMapComponentsMask);
 
+    /* first verify the request length carefully */
+    rc = _XkbSetMapCheckLength(stuff);
+    if (rc != Success)
+        return rc;
+
     tmp = (char *) &stuff[1];
 
     /* Check if we can to the SetMap on the requested device. If this