Re: [RFC] cgroup: syscalls limiting subsystem

[Åukasz Sowa]

First of all, thanks a lot for your replay.

> On Tue, Oct 18, 2011 at 5:21 PM, Åukasz Sowa <luksow@xxxxxxxxx> wrote:
>> Hi,
>>
>> Currently, I'm writing BSc thesis about security in modern Linux.
>> Together with my thesis mentor, I decided that as practical part of my
>> work I'll implement cgroup subsystem that allows to limit particular
>> (defined by number) syscalls for groups of processes. The patch is ready
>> for first review and we decided that I may try to push it to the
>> mainline - so here it is.
>>
> 
> The major problem with this approach is that denying/allowing system
> calls based purely on the syscall number is very coarse. I'd guess
> that most vulnerabilities in a system can be exploited just using
> system calls that almost all applications need in order to get regular
> work done (open, write, exec ,mmap, etc) which limits the utility of
> only being able to turn them off by syscall number. So overall I don't
> think you'll find very much support for this patch.

I have to disagree. Have you read through the link to the article at LWN
I gave? If not, please do so. There are a few people being interested in
ability to limit access to some system calls, just like seccomp does.
Moreover, as stated in the article, there are some projects looking
forward to this feature like QEMU, vsftpd or Chromium. Another (but
maybe not very convincing) point is that there are some other operating
systems that have such feature successfully implemented and used, like BSD.

> Having said that, to address the patch itself:
> 
>> 2. Performance. It's not that bad: I measured 5% more sys time for
>> process on root level, and 8% more sys time for processes on first
>> level. However, I think it may and should be improved but currently
>> I have no idea how to do it.
> 
> Do you mean for all processes, or just for processes that had deny
> bits set? If the former, then 8% is a non-starter, I think. But if you
> hook into the ptrace as I suggested above, you can probably get it to
> zero overhead for threads in allow-all cgroups, and I suspect people
> would scream much less about slowing down threads that are being
> specifically limited anyway.

The overhead is for all processes, not only the 'denying one'.
Implementing it in a way that you suggested (setting bits in all
descendants) is a very good idea. It enables us to have constant
overhead (same on all levels) - currently 5%.
All existing syscalls tracing solutions use ptrace hooking. It's clean
but the performance of ptrace is horrible. I ran the same benchmark as I
used for my code on ptrace based tracing and the overhead was immense.
It was 100 times (sic!) slower. I have another good paper on the
syscalls tracing that you may look through:
http://www.linux-kongress.org/2009/slides/system_call_tracing_overhead_joerg_zinke.pdf
All in all, I think that 5% overhead tracing is quite good trade-off
between no tracing and extremely slow tracing.
I have to say it again - I'm not satisfied with the current performance
and I'm investigating into making it lower but I wonder that there's a
way to go under, let's say - 3%.
However, if such overhead is still unacceptable maybe we should provide
a way to turn it off (besides not compiling it, of course)? My current
idea is to introduce a per-cgroup triggering bit (which will not be
hierarchical, of course). Then, the overhead for cgroups with tracing
turned off would be minimal (additional bit checking and short jumping
only). Or maybe there's a better way (for ex. a per-process triggering
bit)?

>> 3. Naming convention - it's not bad either but I don't like the names -
>> 'scs' abbreviation sound a little bit silly but full form (syscalls)
>> makes lines far too long.
> 
> I'd suggest syscall rather than scs.

Thanks, I prefer syscall too, but I'll have to cope with long lines.

>> +       switch (cftype->private) {
>> +       case SCS_CGROUP_ALLOW:
>> +               for (bit = 0; bit < NR_syscalls; ++bit) {
>> +                       if (__scs_cgroup_perm(scg, bit))
>> +                               seq_printf(seq_file, "%d\n", bit);
> 
> Since this is presumably meant to be only used programmatically (as
> syscall numbers can be different on different platforms, so you'll
> need programs to translate between names and numbers) I wouldn't
> bother with the \n - a plain space will be fine for the process
> reading it, and less destructive to screen real-estate if someone
> happens to cat it by hand.

Plane space is a good idea, I'll make it this way.
Relaying on the names is in my opinion very error prone, so I think it's
best to left it for sysadmins scripts working on unistd.h file, fetching
numbers based on the names of syscalls.
 
>> +       switch (cftype->private) {
>> +       case SCS_CGROUP_ALLOW:
>> +               if (__scs_cgroup_perm(parent_scg, number)) {
>> +                       write_seqlock(&scg->seqlock);
>> +                       set_bit(number, scg->syscalls_bitmap);
>> +                       write_sequnlock(&scg->seqlock);
>> +               } else {
>> +                       return -EPERM;
>> +               }
>> +               break;
>> +       case SCS_CGROUP_DENY:
>> +               write_seqlock(&scg->seqlock);
>> +               clear_bit(number, scg->syscalls_bitmap);
>> +               write_sequnlock(&scg->seqlock);
>> +               break;
> 
> Deny needs to clear the bit in all descendants as well. And there
> would need to be synchronization between checking the parent bit
> during an ALLOW write, and someone changing the parent bit to a DENY.

As I said above, it's a good idea.

Thanks a lot again,
Lukasz Sowa

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