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Message-ID: <CABqD9ha5bEEAQKw9w6EwPfmEYPQJ9khtqon8pKu1VedDWyAsZg@mail.gmail.com>
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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