ntdll: Check buffer for access in NtReadFile before performing I/O.

This also triggers page faults needed for write watches.

dlls/ntdll/file.c

@@ -568,6 +568,12 @@ NTSTATUS WINAPI NtReadFile(HANDLE hFile, HANDLE hEvent,
                                  &needs_close, &type, &options );
     if (status) return status;
 
+    if (!virtual_check_buffer_for_write( buffer, length ))
+    {
+        status = STATUS_ACCESS_VIOLATION;
+        goto done;
+    }
+
     if (type == FD_TYPE_FILE && offset && offset->QuadPart != (LONGLONG)-2 /* FILE_USE_FILE_POINTER_POSITION */ )
     {
         /* async I/O doesn't make sense on regular files */
@@ -615,14 +621,11 @@ NTSTATUS WINAPI NtReadFile(HANDLE hFile, HANDLE hEvent,
             }
             else if (type == FD_TYPE_FILE) continue;  /* no async I/O on regular files */
         }
-        else
+        else if (errno != EAGAIN)
         {
             if (errno == EINTR) continue;
-            if (errno != EAGAIN)
-            {
-                status = FILE_GetNtStatus();
-                goto done;
-            }
+            if (!total) status = FILE_GetNtStatus();
+            goto done;
         }
 
         if (!(options & (FILE_SYNCHRONOUS_IO_ALERT | FILE_SYNCHRONOUS_IO_NONALERT)))

dlls/ntdll/ntdll_misc.h

@@ -142,6 +142,7 @@ extern void virtual_clear_thread_stack(void);
 extern BOOL virtual_handle_stack_fault( void *addr );
 extern NTSTATUS virtual_handle_fault( LPCVOID addr, DWORD err );
 extern BOOL virtual_check_buffer_for_read( const void *ptr, SIZE_T size );
+extern BOOL virtual_check_buffer_for_write( void *ptr, SIZE_T size );
 extern void VIRTUAL_SetForceExec( BOOL enable );
 extern void VIRTUAL_UseLargeAddressSpace(void);
 extern struct _KUSER_SHARED_DATA *user_shared_data;

dlls/ntdll/virtual.c

@@ -1550,6 +1550,39 @@ BOOL virtual_check_buffer_for_read( const void *ptr, SIZE_T size )
 }
 
 
+/***********************************************************************
+ *           virtual_check_buffer_for_write
+ *
+ * Check if a memory buffer can be written to, triggering page faults if needed for write watches.
+ */
+BOOL virtual_check_buffer_for_write( void *ptr, SIZE_T size )
+{
+    if (!size) return TRUE;
+    if (!ptr) return FALSE;
+
+    __TRY
+    {
+        volatile char *p = ptr;
+        SIZE_T count = size;
+
+        while (count > page_size)
+        {
+            *p |= 0;
+            p += page_size;
+            count -= page_size;
+        }
+        p[0] |= 0;
+        p[count - 1] |= 0;
+    }
+    __EXCEPT_PAGE_FAULT
+    {
+        return FALSE;
+    }
+    __ENDTRY
+    return TRUE;
+}
+
+
 /***********************************************************************
  *           VIRTUAL_SetForceExec
  *