linux-kernel - Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

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Date:	Mon, 16 Jan 2012 14:28:25 -0600
From:	Will Drewry <wad@...omium.org>
To:	Andrew Lutomirski <luto@....edu>
Cc:	linux-kernel@...r.kernel.org, keescook@...omium.org,
	john.johansen@...onical.com, serge.hallyn@...onical.com,
	coreyb@...ux.vnet.ibm.com, pmoore@...hat.com, eparis@...hat.com,
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	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com
Subject: Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

On Thu, Jan 12, 2012 at 11:02 AM, Andrew Lutomirski <luto@....edu> wrote:
> On Wed, Jan 11, 2012 at 9:25 AM, Will Drewry <wad@...omium.org> wrote:
>> This patch adds support for seccomp mode 2.  This mode enables dynamic
>> enforcement of system call filtering policy in the kernel as specified
>> by a userland task.  The policy is expressed in terms of a BPF program,
>> as is used for userland-exposed socket filtering.  Instead of network
>> data, the BPF program is evaluated over struct user_regs_struct at the
>> time of the system call (as retrieved using regviews).
>>
>https://www.google.com/calendar?tab=mc&authuser=1
> There's some seccomp-related code in the vsyscall emulation path in
> arch/x86/kernel/vsyscall_64.c.  How should time(), getcpu(), and
> gettimeofday() be handled?

Nice catch:
  lxr.linux.no/linux+v3.2.1/arch/x86/kernel/vsyscall_64.c#L180
I'd missed it.

> If you want filtering to work, there
> aren't any real syscall registers to inspect, but they could be
> synthesized.

Hrm, I wonder if making sure orig_eax is populated with the
vsyscall_nr would be enough.  Unless I'm misreading, args 0 and 1 are
correct, so there may be other noise, but performing a call to
__secure_computing() (either in the case or with a pre-validate
syscall nr: 0-2) should send the do_exit.  Does that sound reasonable?

I'll try to do the right thing in my next patch set.

> Preventing a malicious task from figuring out approximately what time
> it is is basically impossible because of the way that vvars work.  I
> don't know how to change that efficiently.

There are other ways to guess the time too, so I don't think it's that
bad.  For those that are really worried, they could disable or
otherwise attempt to limit vsyscall access from their sandbox.

thanks!
will
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