win32u: Provide extra space in buffers used to dispatch string getter messages.

dlls/win32u/hook.c

@@ -241,6 +241,7 @@ static LRESULT call_hook( struct win_hook_params *info, const WCHAR *module, siz
         HHOOK prev = thread_info->hook;
         BOOL prev_unicode = thread_info->hook_unicode;
         struct win_hook_params *params = info;
+        size_t reply_size;
         void *ret_ptr;
         ULONG ret_len;
 
@@ -252,7 +253,7 @@ static LRESULT call_hook( struct win_hook_params *info, const WCHAR *module, siz
             {
                 CBT_CREATEWNDW *cbtc = (CBT_CREATEWNDW *)params->lparam;
                 message_size = user_message_size( (HWND)params->wparam, WM_NCCREATE,
-                                                  0, (LPARAM)cbtc->lpcs, TRUE, FALSE );
+                                                  0, (LPARAM)cbtc->lpcs, TRUE, FALSE, &reply_size );
                 lparam_size = lparam_ret_size = 0;
             }
 

dlls/win32u/message.c

@@ -1624,11 +1624,15 @@ static size_t copy_string( void *ptr, const void *str, BOOL ansi )
  * Calculate size of packed message buffer.
  */
 size_t user_message_size( HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam,
-                          BOOL other_process, BOOL ansi )
+                          BOOL other_process, BOOL ansi, size_t *reply_size )
 {
     const void *lparam_ptr = (const void *)lparam;
     size_t size = 0;
 
+    /* Windows provices space for at least 2048 bytes for string getters, which
+     * mitigates problems with buffer overflows. */
+    static const size_t min_string_buffer_size = 2048;
+
     switch (message)
     {
     case WM_NCCREATE:
@@ -1645,8 +1649,8 @@ size_t user_message_size( HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam,
         break;
     case WM_GETTEXT:
     case WM_ASKCBFORMATNAME:
-        size = wparam * char_size( ansi );
-        break;
+        *reply_size = wparam * char_size( ansi );
+        return max( *reply_size, min_string_buffer_size );
     case WM_WININICHANGE:
     case WM_SETTEXT:
     case WM_DEVMODECHANGE:
@@ -1730,13 +1734,17 @@ size_t user_message_size( HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam,
         size = wparam * sizeof(UINT);
         break;
     case CB_GETLBTEXT:
-        size = send_message_timeout( hwnd, CB_GETLBTEXTLEN, wparam, 0, SMTO_NORMAL, 0, ansi );
-        size = (size + 1) * char_size( ansi );
-        break;
+    {
+        size_t len = send_message_timeout( hwnd, CB_GETLBTEXTLEN, wparam, 0, SMTO_NORMAL, 0, ansi );
+        *reply_size = (len + 1) * char_size( ansi );
+        return max( *reply_size, min_string_buffer_size );
+    }
     case LB_GETTEXT:
-        size = send_message_timeout( hwnd, LB_GETTEXTLEN, wparam, 0, SMTO_NORMAL, 0, ansi );
-        size = (size + 1) * char_size( ansi );
-        break;
+    {
+        size_t len = send_message_timeout( hwnd, LB_GETTEXTLEN, wparam, 0, SMTO_NORMAL, 0, ansi );
+        *reply_size = (len + 1) * char_size( ansi );
+        return max( *reply_size, min_string_buffer_size );
+    }
     case LB_GETSELITEMS:
         size = wparam * sizeof(UINT);
         break;
@@ -1766,7 +1774,7 @@ size_t user_message_size( HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam,
         break;
     }
 
-    return size;
+    return *reply_size = size;
 }
 
 /***********************************************************************
@@ -1859,6 +1867,10 @@ void pack_user_message( void *buffer, size_t size, UINT message,
             return;
         }
         break;
+    case CB_GETLBTEXT:
+    case LB_GETTEXT:
+        if (size) memset( buffer, 0, size );
+        return;
     }
 
     if (size) memcpy( buffer, lparam_ptr, size );
@@ -2167,7 +2179,7 @@ static LRESULT call_window_proc( HWND hwnd, UINT msg, WPARAM wparam, LPARAM lpar
 {
     struct win_proc_params p, *params = &p;
     BOOL ansi = ansi_dst && type == MSG_ASCII;
-    size_t packed_size = 0, offset = sizeof(*params);
+    size_t packed_size = 0, offset = sizeof(*params), reply_size;
     LRESULT result = 0;
     CWPSTRUCT cwp;
     CWPRETSTRUCT cwpret;
@@ -2179,7 +2191,7 @@ static LRESULT call_window_proc( HWND hwnd, UINT msg, WPARAM wparam, LPARAM lpar
 
     if (!is_current_thread_window( hwnd )) return 0;
 
-    packed_size = user_message_size( hwnd, msg, wparam, lparam, type == MSG_OTHER_PROCESS, ansi );
+    packed_size = user_message_size( hwnd, msg, wparam, lparam, type == MSG_OTHER_PROCESS, ansi, &reply_size );
 
     /* first the WH_CALLWNDPROC hook */
     cwp.lParam  = lparam;
@@ -2208,7 +2220,7 @@ static LRESULT call_window_proc( HWND hwnd, UINT msg, WPARAM wparam, LPARAM lpar
     result = dispatch_win_proc_params( params, offset + packed_size, &ret_ptr, &ret_len );
     if (params != &p) free( params );
 
-    copy_user_result( ret_ptr, min( ret_len, packed_size ), result, msg, wparam, lparam, ansi );
+    copy_user_result( ret_ptr, min( ret_len, reply_size ), result, msg, wparam, lparam, ansi );
 
     /* and finally the WH_CALLWNDPROCRET hook */
     cwpret.lResult = result;

dlls/win32u/tests/win32u.c

@@ -1382,6 +1382,19 @@ static LPARAM callwnd_hook_lparam, callwnd_hook_lparam2, retwnd_hook_lparam, ret
 static LPARAM wndproc_lparam;
 static char lparam_buffer[521];
 
+static void check_zero_memory( const char *mem, size_t size )
+{
+    size_t i;
+    for (i = 0; i < size; i++)
+    {
+        if (mem[i])
+        {
+            ok( 0, "non-zero byte %x at offset %Iu\n", mem[i], i );
+            return;
+        }
+    }
+}
+
 static void check_params( const struct lparam_hook_test *test, UINT message,
                          WPARAM wparam, LPARAM lparam, BOOL is_ret )
 {
@@ -1428,8 +1441,14 @@ static void check_params( const struct lparam_hook_test *test, UINT message,
         }
         break;
 
+    case CB_GETLBTEXT:
+    case LB_GETTEXT:
+        check_zero_memory( (const char *)lparam, 2048 );
+        break;
+
     default:
-        if (test->check_size) {
+        if (test->check_size)
+        {
             const void *expected;
             if (is_ret && test->check_lparam)
                 expected = test->check_lparam;

dlls/win32u/win32u_private.h

@@ -141,7 +141,7 @@ extern BOOL send_notify_message( HWND hwnd, UINT msg, WPARAM wparam, LPARAM lpar
 extern LRESULT send_message_timeout( HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam,
                                      UINT flags, UINT timeout, BOOL ansi );
 extern size_t user_message_size( HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam,
-                                 BOOL other_process, BOOL ansi );
+                                 BOOL other_process, BOOL ansi, size_t *reply_size );
 extern void pack_user_message( void *buffer, size_t size, UINT message,
                                WPARAM wparam, LPARAM lparam, BOOL ansi );