netdev - Re: [PATCH v17 08/15] seccomp: add system call filtering using BPF

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Date:	Tue, 10 Apr 2012 15:15:50 -0500
From:	Will Drewry <wad@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Indan Zupancic <indan@....nu>, linux-kernel@...r.kernel.org,
	linux-security-module@...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,
	oleg@...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, pmoore@...hat.com, corbet@....net,
	eric.dumazet@...il.com, markus@...omium.org,
	coreyb@...ux.vnet.ibm.com, keescook@...omium.org, jmorris@...ei.org
Subject: Re: [PATCH v17 08/15] seccomp: add system call filtering using BPF

On Tue, Apr 10, 2012 at 2:54 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Mon, 9 Apr 2012 04:22:40 +1000
> "Indan Zupancic" <indan@....nu> wrote:
>
>> On Sat, April 7, 2012 06:23, Andrew Morton wrote:
>> > hm, I'm surprised that we don't have a zero-returning implementation of
>> > is_compat_task() when CONFIG_COMPAT=n.  Seems silly.  Blames Arnd.
>>
>> It's sneakily hidden at the end of compat.h.
>>
>> >> +/**
>> >> + * get_u32 - returns a u32 offset into data
>> >> + * @data: a unsigned 64 bit value
>> >> + * @index: 0 or 1 to return the first or second 32-bits
>> >> + *
>> >> + * This inline exists to hide the length of unsigned long.
>> >> + * If a 32-bit unsigned long is passed in, it will be extended
>> >> + * and the top 32-bits will be 0. If it is a 64-bit unsigned
>> >> + * long, then whatever data is resident will be properly returned.
>> >> + */
>> >> +static inline u32 get_u32(u64 data, int index)
>> >> +{
>> >> +  return ((u32 *)&data)[index];
>> >> +}
>> >
>> > This seems utterly broken on big-endian machines.  If so: fix.  If not:
>> > add comment explaining why?
>>
>> It's not a bug, it's intentional.
>
> Well it looks like a bug, which is why I suggest that it be clearly
> commented.

I've added a comment indicating it is intentionally ugly.

>> >
>> >> +  if (total_insns > MAX_INSNS_PER_PATH)
>> >> +          return -ENOMEM;
>> >> +
>> >> +  /*
>> >> +   * Installing a seccomp filter requires that the task have
>> >> +   * CAP_SYS_ADMIN in its namespace or be running with no_new_privs.
>> >> +   * This avoids scenarios where unprivileged tasks can affect the
>> >> +   * behavior of privileged children.
>> >> +   */
>> >> +  if (!current->no_new_privs &&
>> >> +      security_capable_noaudit(current_cred(), current_user_ns(),
>> >> +                               CAP_SYS_ADMIN) != 0)
>> >> +          return -EACCES;
>> >> +
>> >> +  /* Allocate a new seccomp_filter */
>> >> +  filter = kzalloc(sizeof(struct seccomp_filter) + fp_size, GFP_KERNEL);
>> >
>> > I think this gives userspace an easy way of causing page allocation
>> > failure warnings, by permitting large kmalloc() attempts.  Add
>> > __GFP_NOWARN?
>>
>> Max is 32kb. sk_attach_filter() in net/core/filter.c is worse,
>> it allocates up to 512kb before even checking the length.
>
> An order-3 allocation attempt is pretty fragile.  This will sometimes
> fail.
>
>> What about using GFP_USER (and adding __GFP_NOWARN to GFP_USER) instead?
>
> Let's be conventional and use the open-coded __GFP_NOWARN.
> __GFP_NOWARN says "this is a big allocation which will sometimes fail
> and I have carefully reviewed the failure paths and runtime tested
> them".
>
> Please carefully review the failure paths and runtime test them ;)

Thankfully the failure path is simple in this case.  Additional
runtime testing in progress :)

>> >> +  /* Check and rewrite the fprog via the skb checker */
>> >> +  ret = sk_chk_filter(filter->insns, filter->len);
>> >> +  if (ret)
>> >> +          goto fail;
>> >> +
>> >> +  /* Check and rewrite the fprog for seccomp use */
>> >> +  ret = seccomp_chk_filter(filter->insns, filter->len);
>> >
>> > "check" is spelled "check"!
>>
>> Yes, it is and he did spell "check" as "Check".
>>
>> seccomp_chk_filter() mirrors sk_chk_filter(). So it refers to
>> "chk", not "check".
>
> bah.  Two poor identifiers isn't better than one.  Whatever.

As per James's comment, reducing it to one poor identifier.

thanks!
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