linux-kernel - Re: [PATCH][RFC] v2 exec: move core_pattern pipe helper into the crashing namespace

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Date:	Mon, 20 Sep 2010 15:26:18 -0500
From:	Will Drewry <wad@...omium.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Roland McGrath <roland@...hat.com>,
	Neil Horman <nhorman@...driver.com>,
	containers@...ts.linux-foundation.org,
	Eugene Teo <eteo@...hat.com>, Tejun Heo <tj@...nel.org>,
	Serge Hallyn <serue@...ibm.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH][RFC] v2 exec: move core_pattern pipe helper into the
 crashing namespace

On Mon, Sep 20, 2010 at 2:12 PM, Andi Kleen <andi@...stfloor.org> wrote:
>> The pipe process needs to run in the namespaces of the process who set
>> the core pattern, not in the namespaces of the dumping process.
>> Otherwise it is possible to trigger a privileged process to run in a
>> context where it's reality that it expected, causing it to misuse
>> it's privileges.  Even if we don't have a privilege problem I think
>> we will have a case of mismatched functionality where the core pattern
>> will not work as expected.
>
> For me it seems rather the other way around: running the helper in some
> highly priviledged namespace is more dangerous. If it runs in the
> same context as the crasher it can do the least amount of damage
> relative to the crash process.
>
> And as Will pointed out it's the only sane way to deal with net namespaces.

I think you're both right.  How it is implemented right now is an
escape from the linux container. If you allow the root user in a
container to mount proc and update core_pattern, they can escape.
(core_pattern = |/well/known/binary_or_scripting_lang)  I'm sure there
are other escapes too (and any umh call is likely an escape like this
one -- e.g., modprobe_path).  That said, using my patch above might
let you traverse a path otherwise blocked by an LSM enforcement (E.g.,
root user runs a process which sets up a vfs namespace with an
encrypted mount and the lsm blocks access to the /proc/[pid]/root -
but core_pattern still runs and with access).

That said, using the setters namespace makes sense to me as a consumer
of core_pattern too. You can set the core_pattern outside of a
chroot/container and collect core dumps there _or_ you can let a
"root" user in a container have their own core collector without
providing a simple escape.  Making format_corename use the correct pid
namespace for translation would make these cases even more seamless.

Unfortunately, I haven't yet looked at doing it that way yet.  The
namespace-transition patch posted is what occurred to me initially.
Perhaps it won't be so hard.  I'll take a look at what it'd take to do
move core_pattern since it'd resolve both the escape/lsm-bypass
scenarios and the mismatch between the arbitrary namespace and the
core_pattern values.

Thanks!
will
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