d_path() truncating excessive long path name vulnerability

[Wojciech Purczynski <cliph () isec pl>]

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Name:           Linux kernel
Version:        up to 2.2.20 and 2.4.18
Homepage:       http://www.kernel.org/
Author:         Wojciech Purczynski <cliph () isec pl>
Date:           March 26, 2002


Issue:
======

In case of excessively long path names d_path kernel internal function
returns truncated trailing components of a path name instead of an error
value. As this function is called by getcwd(2) system call and
do_proc_readlink() function, false information may be returned to
user-space processes.


Description:
============

Linux is a clone of the operating system Unix, written from scratch by
Linus Torvalds with assistance from a loosely-knit team of hackers across
the Net. It aims towards POSIX and Single UNIX Specification compliance.


Details:
========

d_path kernel function resolves a string of absolute path name of a dentry
passed as an argument to the function.

The path is a concatenation of subsequent path components starting from
trailing path component. The concatenated path name is stored into a
fixed-length buffer of PAGE_SIZE bytes.

If a dentry points to a path that exceeds PAGE_SIZE - 1 characters length,
leading path components are not written to the buffer and function returns
truncated path without an error value.

Because getcwd(2) system call uses d_path() function, it may return
invalid path to the user-space process. However, if a returned path is
longer than user-space buffer a correct error value is returned.

readlink(2) system call called on proc filesystem uses do_proc_readlink()
function which is also vulnerable to d_path() bug.


Impact:
=======

Privileged process may be tricked to think it is inside of arbitrary
directory. Other scenarios are possible if readlink() is used on files on
proc filesystem (like "/proc/self/exe").


PS: Please CC to security () isec pl as I may not be subscribed to the list.

- --
Wojciech Purczynski
iSEC Security Research
http://isec.pl/

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Attachment: dpathx.c
Description: proof-of-concept exploit