Load the Windows printing subsystem at run time.

The printing functions are split between winspool.drv and spoolss.dll
in a really weird way (who would have guessed that OpenPrinter and
ClosePrinter don't live in the same dynamic library?!), but _neither_
of those counts as a system 'known DLL', so linking against either one
of these at load time is again a potential DLL hijacking vector.

Recipe

@@ -274,7 +274,7 @@ CHARSET  = sbcsdat slookup sbcs utf8 toucs fromucs xenc mimeenc macenc localenc
 
 # Standard libraries.
 LIBS     = advapi32.lib user32.lib gdi32.lib comctl32.lib comdlg32.lib
-         + shell32.lib imm32.lib winspool.lib ole32.lib
+         + shell32.lib imm32.lib ole32.lib
 
 # Network backend sets. This also brings in the relevant attachment
 # to proxy.c depending on whether we're crypto-avoidant or not.

windows/winprint.c

@@ -18,11 +18,46 @@ struct printer_job_tag {
     HANDLE hprinter;
 };
 
+DECL_WINDOWS_FUNCTION(static, BOOL, EnumPrinters,
+                      (DWORD, LPTSTR, DWORD, LPBYTE, DWORD, LPDWORD, LPDWORD));
+DECL_WINDOWS_FUNCTION(static, BOOL, OpenPrinter,
+                      (LPTSTR, LPHANDLE, LPPRINTER_DEFAULTS));
+DECL_WINDOWS_FUNCTION(static, BOOL, ClosePrinter, (HANDLE));
+DECL_WINDOWS_FUNCTION(static, BOOL, StartDocPrinter, (HANDLE, DWORD, LPBYTE));
+DECL_WINDOWS_FUNCTION(static, BOOL, EndDocPrinter, (HANDLE));
+DECL_WINDOWS_FUNCTION(static, BOOL, StartPagePrinter, (HANDLE));
+DECL_WINDOWS_FUNCTION(static, BOOL, EndPagePrinter, (HANDLE));
+DECL_WINDOWS_FUNCTION(static, BOOL, WritePrinter,
+                      (HANDLE, LPVOID, DWORD, LPDWORD));
+
+static void init_winfuncs(void)
+{
+    static int initialised = FALSE;
+    char buf[4096];
+    if (initialised)
+        return;
+    {
+        HMODULE winspool_module = load_system32_dll("winspool.drv");
+        HMODULE spoolss_module = load_system32_dll("spoolss.dll");
+        GET_WINDOWS_FUNCTION_PP(winspool_module, EnumPrinters);
+        GET_WINDOWS_FUNCTION_PP(winspool_module, OpenPrinter);
+        GET_WINDOWS_FUNCTION_PP(spoolss_module, ClosePrinter);
+        GET_WINDOWS_FUNCTION_PP(winspool_module, StartDocPrinter);
+        GET_WINDOWS_FUNCTION_PP(spoolss_module, EndDocPrinter);
+        GET_WINDOWS_FUNCTION_PP(spoolss_module, StartPagePrinter);
+        GET_WINDOWS_FUNCTION_PP(spoolss_module, EndPagePrinter);
+        GET_WINDOWS_FUNCTION_PP(spoolss_module, WritePrinter);
+    }
+    initialised = TRUE;
+}
+
 static int printer_add_enum(int param, DWORD level, char **buffer,
                             int offset, int *nprinters_ptr)
 {
     DWORD needed = 0, nprinters = 0;
 
+    init_winfuncs();
+
     *buffer = sresize(*buffer, offset+512, char);
 
     /*
@@ -30,7 +65,7 @@ static int printer_add_enum(int param, DWORD level, char **buffer,
      * we'll need for the output. Discard the return value since it
      * will almost certainly be a failure due to lack of space.
      */
-    EnumPrinters(param, NULL, level, (LPBYTE)((*buffer)+offset), 512,
+    p_EnumPrinters(param, NULL, level, (LPBYTE)((*buffer)+offset), 512,
                    &needed, &nprinters);
 
     if (needed < 512)
@@ -38,7 +73,7 @@ static int printer_add_enum(int param, DWORD level, char **buffer,
 
     *buffer = sresize(*buffer, offset+needed, char);
 
-    if (EnumPrinters(param, NULL, level, (LPBYTE)((*buffer)+offset),
+    if (p_EnumPrinters(param, NULL, level, (LPBYTE)((*buffer)+offset),
                        needed, &needed, &nprinters) == 0)
         return FALSE;
 
@@ -131,19 +166,21 @@ printer_job *printer_start_job(char *printer)
     DOC_INFO_1 docinfo;
     int jobstarted = 0, pagestarted = 0;
 
+    init_winfuncs();
+
     ret->hprinter = NULL;
-    if (!OpenPrinter(printer, &ret->hprinter, NULL))
+    if (!p_OpenPrinter(printer, &ret->hprinter, NULL))
 	goto error;
 
     docinfo.pDocName = "PuTTY remote printer output";
     docinfo.pOutputFile = NULL;
     docinfo.pDatatype = "RAW";
 
-    if (!StartDocPrinter(ret->hprinter, 1, (LPBYTE)&docinfo))
+    if (!p_StartDocPrinter(ret->hprinter, 1, (LPBYTE)&docinfo))
 	goto error;
     jobstarted = 1;
 
-    if (!StartPagePrinter(ret->hprinter))
+    if (!p_StartPagePrinter(ret->hprinter))
 	goto error;
     pagestarted = 1;
 
@@ -151,11 +188,11 @@ printer_job *printer_start_job(char *printer)
 
     error:
     if (pagestarted)
-	EndPagePrinter(ret->hprinter);
+	p_EndPagePrinter(ret->hprinter);
     if (jobstarted)
-	EndDocPrinter(ret->hprinter);
+	p_EndDocPrinter(ret->hprinter);
     if (ret->hprinter)
-	ClosePrinter(ret->hprinter);
+	p_ClosePrinter(ret->hprinter);
     sfree(ret);
     return NULL;
 }
@@ -167,7 +204,7 @@ void printer_job_data(printer_job *pj, void *data, int len)
     if (!pj)
 	return;
 
-    WritePrinter(pj->hprinter, data, len, &written);
+    p_WritePrinter(pj->hprinter, data, len, &written);
 }
 
 void printer_finish_job(printer_job *pj)
@@ -175,8 +212,8 @@ void printer_finish_job(printer_job *pj)
     if (!pj)
 	return;
 
-    EndPagePrinter(pj->hprinter);
-    EndDocPrinter(pj->hprinter);
-    ClosePrinter(pj->hprinter);
+    p_EndPagePrinter(pj->hprinter);
+    p_EndDocPrinter(pj->hprinter);
+    p_ClosePrinter(pj->hprinter);
     sfree(pj);
 }