From c285798984c6bb99e454a33772cde23d394d3dcd Mon Sep 17 00:00:00 2001
From: Olivier Fourdan <ofourdan@redhat.com>
Date: Mon, 20 Jan 2025 17:10:31 +0100
Subject: [PATCH] sync: Apply changes last in SyncChangeAlarmAttributes()

SyncChangeAlarmAttributes() would apply the various changes while
checking for errors.

If one of the changes triggers an error, the changes for the trigger,
counter or delta value would remain, possibly leading to inconsistent
changes.

Postpone the actual changes until we're sure nothing else can go wrong.

Related to CVE-2025-26601, ZDI-CAN-25870

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1828>
---
 Xext/sync.c | 42 +++++++++++++++++++++++++++---------------
 1 file changed, 27 insertions(+), 15 deletions(-)

diff --git a/Xext/sync.c b/Xext/sync.c
index 65f2d4378..cab73be92 100644
--- a/Xext/sync.c
+++ b/Xext/sync.c
@@ -830,8 +830,14 @@ SyncChangeAlarmAttributes(ClientPtr client, SyncAlarm * pAlarm, Mask mask,
     int status;
     XSyncCounter counter;
     Mask origmask = mask;
+    SyncTrigger trigger;
+    Bool select_events_changed = FALSE;
+    Bool select_events_value = FALSE;
+    int64_t delta;
 
-    counter = pAlarm->trigger.pSync ? pAlarm->trigger.pSync->id : None;
+    trigger = pAlarm->trigger;
+    delta = pAlarm->delta;
+    counter = trigger.pSync ? trigger.pSync->id : None;
 
     while (mask) {
         int index2 = lowbit(mask);
@@ -847,24 +853,24 @@ SyncChangeAlarmAttributes(ClientPtr client, SyncAlarm * pAlarm, Mask mask,
         case XSyncCAValueType:
             mask &= ~XSyncCAValueType;
             /* sanity check in SyncInitTrigger */
-            pAlarm->trigger.value_type = *values++;
+            trigger.value_type = *values++;
             break;
 
         case XSyncCAValue:
             mask &= ~XSyncCAValue;
-            pAlarm->trigger.wait_value = ((int64_t)values[0] << 32) | values[1];
+            trigger.wait_value = ((int64_t)values[0] << 32) | values[1];
             values += 2;
             break;
 
         case XSyncCATestType:
             mask &= ~XSyncCATestType;
             /* sanity check in SyncInitTrigger */
-            pAlarm->trigger.test_type = *values++;
+            trigger.test_type = *values++;
             break;
 
         case XSyncCADelta:
             mask &= ~XSyncCADelta;
-            pAlarm->delta = ((int64_t)values[0] << 32) | values[1];
+            delta = ((int64_t)values[0] << 32) | values[1];
             values += 2;
             break;
 
@@ -874,10 +880,8 @@ SyncChangeAlarmAttributes(ClientPtr client, SyncAlarm * pAlarm, Mask mask,
                 client->errorValue = *values;
                 return BadValue;
             }
-            status = SyncEventSelectForAlarm(pAlarm, client,
-                                             (Bool) (*values++));
-            if (status != Success)
-                return status;
+            select_events_value = (Bool) (*values++);
+            select_events_changed = TRUE;
             break;
 
         default:
@@ -886,25 +890,33 @@ SyncChangeAlarmAttributes(ClientPtr client, SyncAlarm * pAlarm, Mask mask,
         }
     }
 
+    if (select_events_changed) {
+        status = SyncEventSelectForAlarm(pAlarm, client, select_events_value);
+        if (status != Success)
+            return status;
+    }
+
     /* "If the test-type is PositiveComparison or PositiveTransition
      *  and delta is less than zero, or if the test-type is
      *  NegativeComparison or NegativeTransition and delta is
      *  greater than zero, a Match error is generated."
      */
     if (origmask & (XSyncCADelta | XSyncCATestType)) {
-        if ((((pAlarm->trigger.test_type == XSyncPositiveComparison) ||
-              (pAlarm->trigger.test_type == XSyncPositiveTransition))
-             && pAlarm->delta < 0)
+        if ((((trigger.test_type == XSyncPositiveComparison) ||
+              (trigger.test_type == XSyncPositiveTransition))
+             && delta < 0)
             ||
-            (((pAlarm->trigger.test_type == XSyncNegativeComparison) ||
-              (pAlarm->trigger.test_type == XSyncNegativeTransition))
-             && pAlarm->delta > 0)
+            (((trigger.test_type == XSyncNegativeComparison) ||
+              (trigger.test_type == XSyncNegativeTransition))
+             && delta > 0)
             ) {
             return BadMatch;
         }
     }
 
     /* postpone this until now, when we're sure nothing else can go wrong */
+    pAlarm->delta = delta;
+    pAlarm->trigger = trigger;
     if ((status = SyncInitTrigger(client, &pAlarm->trigger, counter, RTCounter,
                                   origmask & XSyncCAAllTrigger)) != Success)
         return status;