netdev - Re: [PATCH v11 06/12] seccomp: add system call filtering using BPF

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Date:	Mon, 27 Feb 2012 12:15:38 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Will Drewry <wad@...omium.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
	kernel-hardening@...ts.openwall.com, netdev@...r.kernel.org,
	x86@...nel.org, arnd@...db.de, davem@...emloft.net, hpa@...or.com,
	mingo@...hat.com, peterz@...radead.org, rdunlap@...otime.net,
	mcgrathr@...omium.org, tglx@...utronix.de, luto@....edu,
	eparis@...hat.com, serge.hallyn@...onical.com, djm@...drot.org,
	scarybeasts@...il.com, indan@....nu, pmoore@...hat.com,
	akpm@...ux-foundation.org, corbet@....net, eric.dumazet@...il.com,
	markus@...omium.org, coreyb@...ux.vnet.ibm.com
Subject: Re: [PATCH v11 06/12] seccomp: add system call filtering using BPF

On Mon, Feb 27, 2012 at 11:54 AM, Will Drewry <wad@...omium.org> wrote:
> On Mon, Feb 27, 2012 at 11:09 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>> On 02/24, Will Drewry wrote:
>>> +static struct seccomp_filter *get_seccomp_filter(struct seccomp_filter *orig)
>>> +{
>>> +     if (!orig)
>>> +             return NULL;
>>> +     /* Reference count is bounded by the number of total processes. */
>>> +     atomic_inc(&orig->usage);
>>> +     return orig;
>>> +}
>>> ...
>>> +void copy_seccomp(struct seccomp *child, const struct seccomp *parent)
>>> +{
>>> +     /* Other fields are handled by dup_task_struct. */
>>> +     child->filter = get_seccomp_filter(parent->filter);
>>> +}
>>
>> This is purely cosmetic, but imho looks a bit confusing.
>>
>> We do not copy seccomp->mode and this is correct, it was already copied
>> implicitely. So why do we copy ->filter? This is not "symmetrical", afaics
>> you can simply do
>>
>>        void copy_seccomp(struct seccomp *child)
>>        {
>>                if (child->filter)
>>                        atomic_inc(child->filter->usage);
>>
>> But once again, this is cosmetic, feel free to ignore.
>
> Right now get_seccomp_filter does the NULL check, so really this could
> be reduced to adding an external get_seccomp_filter(p->seccomp.filter)
> in place of copy_seccomp().
>
> As to removing the extra arg, that should be fine since the parent
> can't drop its refcount when copy_seccomp is called.  At the very
> least, I can make that change so it reads more cleanly.

I had various conflicting thoughts while looking over the refcounting:

- get_seccomp_filter is defined static, and has a single caller: copy_seccomp()
- put isn't static, and has a single caller: kernel/fork.c:free_task()
- having only get_/put_ touch ->usage seems cleaner to me
- seccomp_attach_filter touches ->usage without get_seccomp_filter
- having the initializing routine use atomic_set(..., 1) is a common pattern

In a fit of extreme bike-shedding, I can't decide which is more sensible:

- rename put_seccomp_filter to free_seccomp_filter and inline the
get_seccomp_filter logic into copy_seccomp().

or

- create a wrapper for put_seccomp_filter named free_seccomp_filter so
that get_/put_ can both be static.

-Kees

-- 
Kees Cook
ChromeOS Security
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