Openwall GNU/*/Linux - a small security-enhanced Linux distro for servers
[<prev] [next>] [thread-next>] [day] [month] [year] [list]
Date: Wed, 25 Sep 2013 21:14:33 +0100
From: Djalal Harouni <tixxdz@...ndz.org>
To: "Eric W. Biederman" <ebiederm@...ssion.com>,
	Kees Cook <keescook@...omium.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	<kernel-hardening@...ts.openwall.com>
Cc: tixxdz@...il.com,
	Djalal Harouni <tixxdz@...ndz.org>
Subject: [PATCH 0/12] procfs: protect /proc/<pid>/* files with file->f_cred


/proc/<pid>/* entries varies at runtime, appropriate permission checks
need to happen during each system call.

Currently some of these sensitive entries are protected by performing
the ptrace_may_access() check. However even with that the /proc file
descriptors can be passed to a more privileged process
(e.g. a suid-exec) which will pass the classic ptrace_may_access()
check. In general the ->open() call will be issued by an unprivileged
process while the ->read(),->write() calls by a more privileged one.

Example of these files are:
/proc/*/syscall, /proc/*/stack etc.

And any open(/proc/self/*) then suid-exec to read()/write() /proc/self/*


These files are protected during read() by the ptrace_may_access(),
however the file descriptor can be passed to a suid-exec which can be
used to read data and bypass ASLR. Of course this was discussed several
times on LKML.


Solution:
Here we propose a clean solution that uses available mechanisms. No
additions, nor new structs/memory allocation...

After a discussion about some /proc/<pid>/* file permissions [1],
Eric W. Biederman proposed to use the file->f_cred to check if current's
cred have changed [2], actually he said that Linus was looking
on using the file->f_cred to implement a similar thing! But run in
problems with Chromes sandbox ? a link please ?


So here are my experiments:
The idea is to track the cred of current. If the cred change between
->open() and read()/write() this means that current has lost or gained
some X privileges. If so, in addition to the classic ptrace_may_access()
check, perform a second check using the file's opener cred. This means,
if an unprivileged process that tries to use a privileged one
(e.g. suid-exec) to read privileged files will get caught. The original
process that opened the file does not have the appropriate permissions
to read()/write() the target /proc/<pid>/* entry.

This second check is done of course during read(),write()...


Advantages of the proposed solution:
* It uses available mechanisms: file->f_cred which is a const cred
  that reference the cred of the opener.

* The file->f_cred can be easily referenced  and it will live until
  fput()

* Does not pin or grab any counter on any extra struct.

* It allows to pass file descriptors under certain conditions:
  (1) current at open time may have enough permissions
  (2) current does a suid-exec or change its ruid/euid (new cred)
  (3) current or suid-exec tries to read from the task /proc entry
     Allow the ->read() only if the file's opener cred at (1)
     have enough permissions on *this* task /proc entry during
     *this* ->read() moment. Otherwise fail.

  IOW allow it if the opener does not need the help of a suid-exec to
  read/write data.


Disadvantage:
* Currently only /proc/*/{stack,syscall,stat,personality} are handled
  If the solution is accepted I'll continue with other files, taking
  care to not break userspace. All the facilities are provided.
* Your feedback


The following series tries to implement what I describe.

1) Add the proc_same_open_cred() helper
This function will check if the file's opener cred matches the current
cred. The match concerns the cred that are used in the
ptrace_may_access() check to allow /proc task access. These cred are:
uid,gid and cap_permitted.

2) Add the proc_allow_access() helper
Check if the file's opener had enough permissions to access the target
process.

3) Make seq_file struct able to access the file's opener cred.
Since seq_file struct is used by procfs entries, embed a pointer to the
file->f_cred into the seq_file struct.

4) Add permission checks on the file's opener of /proc/*/stack

5) Add permission checks on the file's opener of /proc/*/personality

6) Add permission checks on the file's opener of /proc/*/stat

7) Improve permission checks on /proc/*/syscall

8) Finally the last patch is user_ns and seq_file cleaning.


Thanks!

[1] https://lkml.org/lkml/2013/8/26/354
[2] https://lkml.org/lkml/2013/8/31/209


Djalal Harouni (12):
 procfs: add proc_same_open_cred() to check if the cred have changed
 procfs: add proc_allow_access() to check if file's opener may access task
 procfs: Document the proposed solution to protect procfs entries
 seq_file: Make seq_file able to access the file's opener cred
 seq_file: set the seq_file->f_cred during seq_open()
 procfs: make /proc/*/stack 0400
 procfs: add permission checks on the file's opener of /proc/*/stack
 procfs: add permission checks on the file's opener of /proc/*/personality
 procfs: add permission checks on the file's opener of /proc/*/stat
 procfs: move PROC_BLOCK_SIZE declaration up to make it visible
 procfs: improve permission checks on /proc/*/syscall
 user_ns: seq_file: use the user_ns that is embedded in the f_cred struct

 fs/proc/array.c          |  14 ++++++-
 fs/proc/base.c           | 233 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++----------
 fs/proc/internal.h       |   3 ++
 fs/seq_file.c            |   4 +-
 include/linux/seq_file.h |  13 +++++--
 5 files changed, 238 insertions(+), 29 deletions(-)

Powered by blists - more mailing lists

Your e-mail address:

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.