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Date: Mon, 13 Feb 2012 16:50:34 +0100
From: Djalal Harouni <tixxdz@...ndz.org>
To: kernel-hardening@...ts.openwall.com
Cc: "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: procfs: infoleaks and DAC permissions
On Sun, Feb 12, 2012 at 04:36:12PM +0100, Djalal Harouni wrote:
> On Sat, Feb 11, 2012 at 02:07:09PM +0400, Solar Designer wrote:
> > On Fri, Feb 10, 2012 at 03:06:58AM +0100, Djalal Harouni wrote:
> > > 1) Infoleaks via self-read by a suid/guid:
> > ...
> > > I believe to fix this we should just let 'mm_struct' point to the old
> > > one (as done by Linus' commit [2] for the /proc/self/mem)
> >
> > I dislike this approach because it introduces a resource limits bypass.
> > When you hold an mm, you hold all data of the old process in VM, right?
> I guess, but that code can be considered semantically correct since we did
> not release it.
>
> Oleg's patch [1] makes sure that 'mm->mm_users' holds the right value, if
> the process drops the 'fd' the next read/write call will fail, so it will
> not be visible to userspace if the original process exits. It will fail
> at:
> if (!atomic_inc_not_zero(&mm->mm_users))
> return NULL;
>
> It will be freed only by the release function.
>
>
> Can you elaborate more on the resource limits bypass please ?
>
>
> I guess we can try this:
> self_pid = getpid()
> open("/proc/self_pid/mem")
> execv("/proc/self_pid/comm",...)
>
> A quick slabtop shows that mm_struct objects keep increasing...
> But the 'fd' limit will kill the program, what about others ?
>
Ok it seems that mm_struct can explode, thanks to the VFS:
/proc/sys/fs/file-max (34869 on the tested system)
This will catch it...
Executing NR programs that keep /proc/self/mem open and re-exec or
whatever can show this behaviour:
slabtop:
OBJS ACTIVE USE OBJ SIZE SLABS OBJ/SLAB CACHE SIZE NAME
186585 186410 99% 0.38K 8885 21 71080K kmemleak_object
...
34233 31414 91% 1.25K 2858 12 45728K mm_struct
34944 32062 91% 0.38K 1664 21 13312K filp
44835 41973 93% 0.19K 2135 21 8540K kmalloc-192
...
294 163 55% 4.88K 49 6 1568K task_struct
See: task_struct.
Hope this will help. Someone who knows this, can tell more.
Thanks.
--
tixxdz
http://opendz.org
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