linux-kernel - Re: [PATCH v8 3/8] seccomp: add system call filtering using BPF

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Message-ID: <CABqD9haWxRLac1w3m9rSmyDvcg8jxYcxdmAAsgB7pEEBMHr=2g@mail.gmail.com>
Date:	Thu, 16 Feb 2012 15:51:23 -0600
From:	Will Drewry <wad@...omium.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Markus Gutschke <markus@...omium.org>,
	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, 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,
	indan@....nu, pmoore@...hat.com, akpm@...ux-foundation.org,
	corbet@....net, eric.dumazet@...il.com, keescook@...omium.org
Subject: Re: [PATCH v8 3/8] seccomp: add system call filtering using BPF

On Thu, Feb 16, 2012 at 3:34 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 02/16/2012 01:28 PM, Markus Gutschke wrote:
>>
>> I think, the documentation said that as soon as prctl() is used to set
>> a bpf filter for system calls, it automatically disallows system calls
>> using an entry point other than the one used by this particular
>> prctl().
>>
>> I was trying to come up with scenarios where this particular approach
>> causes problem, but I can't think of any off the top of my head. So,
>> it might actually turn out to be a very elegant way to reduce the
>> attack surface of the kernel. If we are really worried about userspace
>> compatibility, we could make the kernel send a signal instead of
>> terminating the program, if the wrong entry point was used; not sure
>> if that is needed, though.
>>
>
> Let's see... we're building an entire pattern-matching engine and then
> randomly disallowing its use because we didn't build in the right bits?
>
> Sorry, that's asinine.
>
> Put the bloody bit in there and let the pattern program make that decision.

Easy enough to add a bit for the mode: 32-bit or 64-bit.  It seemed
like a waste of cycles for every 32-bit program or every 64-bit
program to check to see that its calling convention hadn't changed,
but it does take away a valid decision the pattern program should be
making.

I'll add a flag for 32bit/64bit while cleaning up seccomp_data. I
think that will properly encapsulate the is_compat_task() behavior in
a way that is stable for compat and non-compat tasks to use.  If
there's a more obvious way, I'm all ears.

thanks!
will
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