From a483b5c7724469309e3df427730cbb8b805b9c9f Mon Sep 17 00:00:00 2001
From: Peter Hutterer <peter.hutterer@who-t.net>
Date: Thu, 4 Jan 2024 10:01:24 +1000
Subject: [PATCH xserver] Xi: flush hierarchy events after adding/removing
 master devices

The `XISendDeviceHierarchyEvent()` function allocates space to store up
to `MAXDEVICES` (256) `xXIHierarchyInfo` structures in `info`.

If a device with a given ID was removed and a new device with the same
ID added both in the same operation, the single device ID will lead to
two info structures being written to `info`.

Since this case can occur for every device ID at once, a total of two
times `MAXDEVICES` info structures might be written to the allocation.

To avoid it, once one add/remove master is processed, send out the
device hierarchy event for the current state and continue. That event
thus only ever has exactly one of either added/removed in it (and
optionally slave attached/detached).

CVE-2024-21885, ZDI-CAN-22744

This vulnerability was discovered by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative
---
 Xi/xichangehierarchy.c | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/Xi/xichangehierarchy.c b/Xi/xichangehierarchy.c
index 01eb7a8af4..67eedddec6 100644
--- a/Xi/xichangehierarchy.c
+++ b/Xi/xichangehierarchy.c
@@ -340,6 +340,11 @@ ProcXIChangeHierarchy(ClientPtr client)
     size_t len;			/* length of data remaining in request */
     int rc = Success;
     int flags[MAXDEVICES] = { 0 };
+    enum {
+        NO_CHANGE,
+        FLUSH,
+        CHANGED,
+    } changes = NO_CHANGE;
 
     REQUEST(xXIChangeHierarchyReq);
     REQUEST_AT_LEAST_SIZE(xXIChangeHierarchyReq);
@@ -389,8 +394,9 @@ ProcXIChangeHierarchy(ClientPtr client)
             rc = add_master(client, c, flags);
             if (rc != Success)
                 goto unwind;
-        }
+            changes = FLUSH;
             break;
+        }
         case XIRemoveMaster:
         {
             xXIRemoveMasterInfo *r = (xXIRemoveMasterInfo *) any;
@@ -399,8 +405,9 @@ ProcXIChangeHierarchy(ClientPtr client)
             rc = remove_master(client, r, flags);
             if (rc != Success)
                 goto unwind;
-        }
+            changes = FLUSH;
             break;
+        }
         case XIDetachSlave:
         {
             xXIDetachSlaveInfo *c = (xXIDetachSlaveInfo *) any;
@@ -409,8 +416,9 @@ ProcXIChangeHierarchy(ClientPtr client)
             rc = detach_slave(client, c, flags);
             if (rc != Success)
                 goto unwind;
-        }
+            changes = CHANGED;
             break;
+        }
         case XIAttachSlave:
         {
             xXIAttachSlaveInfo *c = (xXIAttachSlaveInfo *) any;
@@ -495,16 +503,25 @@ ProcXIChangeHierarchy(ClientPtr client)
             rc = attach_slave(client, c, flags);
             if (rc != Success)
                 goto unwind;
+            changes = CHANGED;
+            break;
         }
+        default:
             break;
         }
 
+        if (changes == FLUSH) {
+            XISendDeviceHierarchyEvent(flags);
+            memset(flags, 0, sizeof(flags));
+            changes = NO_CHANGE;
+        }
+
         len -= any->length * 4;
         any = (xXIAnyHierarchyChangeInfo *) ((char *) any + any->length * 4);
     }
 
  unwind:
-
-    XISendDeviceHierarchyEvent(flags);
+    if (changes != NO_CHANGE)
+        XISendDeviceHierarchyEvent(flags);
     return rc;
 }
-- 
2.43.0