vsprintf: Prevent crash when dereferencing invalid pointers for %pD

Commit 3e5903eb ("vsprintf: Prevent crash when dereferencing invalid
pointers") prevents most crash except for %pD.
There is an additional pointer dereferencing before dentry_name.

At least, vma->file can be NULL and be passed to printk %pD in
print_bad_pte, which can cause crash.

This patch fixes it with introducing a new file_dentry_name.

Link: http://lkml.kernel.org/r/20190809012457.56685-1-justin.he@arm.com
Fixes: 3e5903eb ("vsprintf: Prevent crash when dereferencing invalid pointers")
To: Geert Uytterhoeven <geert+renesas@glider.be>
To: Thomas Gleixner <tglx@linutronix.de>
To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: linux-kernel@vger.kernel.org
Cc: Kees Cook <keescook@chromium.org>
Cc: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: "Tobin C. Harding" <tobin@kernel.org>
Signed-off-by: Jia He <justin.he@arm.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

lib/vsprintf.c

@@ -869,6 +869,15 @@ char *dentry_name(char *buf, char *end, const struct dentry *d, struct printf_sp
 	return widen_string(buf, n, end, spec);
 }
 
+static noinline_for_stack
+char *file_dentry_name(char *buf, char *end, const struct file *f,
+			struct printf_spec spec, const char *fmt)
+{
+	if (check_pointer(&buf, end, f, spec))
+		return buf;
+
+	return dentry_name(buf, end, f->f_path.dentry, spec, fmt);
+}
 #ifdef CONFIG_BLOCK
 static noinline_for_stack
 char *bdev_name(char *buf, char *end, struct block_device *bdev,
@@ -2166,9 +2175,7 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
 	case 'C':
 		return clock(buf, end, ptr, spec, fmt);
 	case 'D':
-		return dentry_name(buf, end,
-				   ((const struct file *)ptr)->f_path.dentry,
-				   spec, fmt);
+		return file_dentry_name(buf, end, ptr, spec, fmt);
 #ifdef CONFIG_BLOCK
 	case 'g':
 		return bdev_name(buf, end, ptr, spec, fmt);