Re: [Qemu-devel] [RFC] [PATCHv2 2/2] Adding basic calls to libseccomp in vl.c

[Blue Swirl]

On Mon, Jun 18, 2012 at 8:13 PM, Eduardo Otubo <address@hidden> wrote:
> On Mon, Jun 18, 2012 at 02:55:35PM +0100, Daniel P. Berrange wrote:
>> On Mon, Jun 18, 2012 at 09:52:44AM -0400, Paul Moore wrote:
>> > On Monday, June 18, 2012 09:31:03 AM Daniel P. Berrange wrote:
>> > > On Fri, Jun 15, 2012 at 05:02:19PM -0400, Paul Moore wrote:
>> > > > On Friday, June 15, 2012 07:06:10 PM Blue Swirl wrote:
>> > > > > I think allowing execve() would render seccomp pretty much useless.
>> > > >
>> > > > Not necessarily.
>> > > >
>> > > > I'll agree that it does seem a bit odd to allow execve(), but there is
>> > > > still value in enabling seccomp to disable potentially 
>> > > > buggy/exploitable
>> > > > syscalls. Let's not forget that we have over 300 syscalls on x86_64, 
>> > > > not
>> > > > including the 32 bit versions, and even if we add all of the new 
>> > > > syscalls
>> > > > suggested in this thread we are still talking about a small subset of
>> > > > syscalls.  As far as security goes, the old adage of "less is more"
>> > > > applies.
>> > >
>> > > I can sort of see this argument, but *only* if the QEMU process is being
>> > > run under a dedicated, fully unprivileged (from a DAC pov) user, 
>> > > completely
>> > > separate from anything else on the system.
>> > >
>> > > Or, of course, for a QEMU already confined by SELinux.
>> >
>> > Agreed ... and considering at least one major distribution takes this 
>> > approach
>> > it seems like reasonable functionality to me.  Confining QEMU, either 
>> > through
>> > DAC and/or MAC, when faced with potentially malicious guests is just good
>> > sense.
>>
>> Good, I'm not missing anything then. I'd suggest that future iterations
>> of these patches explicitly mention the deployment scenarios in which
>> this technology is able to offer increases security, and also describe
>> the scenarios where it will not improve things.
>
> Please correct me if I'm wrong here, but I don't understand how exactly
> whitelisting execve() is odd. The white list is inherit and passed along
> the child processes so they also need to have their own syscalls filtered
> by BPF in the kernel as stated in the Will's commit log[1] - "Filter
> programs will be inherited across fork/clone and execve." - I wonder if
> this is main point of your concern. Whitelisting execve() or not should be
> no difference from the security pov.

The helper might behave strangely (read: according to attacker's
wishes) when some of the required syscalls are not working.

> However, I agree that a possible future feature could a customized
> whitelist for each child process spawned. But for a first instance, the
> default whitelist should be enough to start seccomp support in Qemu.

It's a good way. I just wish the list weren't so open and lame to begin with.

>
> Also, as far as I understand, seccomp never meant to replace any of the
> technologies above mentioned. Using more than one layer of protection
> (SELinux, AppArmor MAC policy and/or DAC) should always be a good practice
> for the defense in depth.
>
> [1] -
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commit;h=e2cfabdfd075648216f99c2c03821cf3f47c1727
>
> --
> Eduardo Otubo
> Software Engineer
> Linux Technology Center
> IBM Systems & Technology Group
> Mobile: +55 19 8135 0885
> address@hidden
>