Re: Compat 32-bit syscall entry from 64-bit task!? [was: Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF]

[Chris Evans <scarybeasts@gmail.com>]

On Wed, Jan 18, 2012 at 4:14 PM, Indan Zupancic <indan@nul.nu> wrote:
> On Wed, January 18, 2012 22:13, Chris Evans wrote:
>> On Wed, Jan 18, 2012 at 4:12 AM, Indan Zupancic <indan@nul.nu> wrote:
>>> On Wed, January 18, 2012 06:43, Chris Evans wrote:
>>>> 2) Tracee traps
>>>> 2b) Tracee could take a SIGKILL here
>>>> 3) Tracer looks at registers; bad syscall
>>>> 3b) Or tracee could take a SIGKILL here
>>>> 4) The only way to stop the bad syscall from executing is to rewrite
>>>> orig_eax (PTRACE_CONT + SIGKILL only kills the process after the
>>>> syscall has finished)
>>>
>>> Yes, we rewrite it to -1.
>>>
>>>> 5) Disaster: the tracee took a SIGKILL so any attempt to address it by
>>>> pid (such as PTRACE_SETREGS) fails.
>>>
>>> I assume that if a task can execute system calls and we get ptrace events
>>> for that, that we can do other ptrace operations too. Are you saying that
>>> the kernel has this ptrace gap between SIGKILL and task exit where ptrace
>>> doesn't work but the task continues executing system calls? That would be
>>> a huge bug, but it seems very unlikely too, as the task is stopped and
>>> shouldn't be able to disappear till it is continued by the tracer.
>>>
>>> I mean, really? That would be stupid.
>
> Okay, I tested this scenario and you're right, we're screwed.
>
> What the hell guys?

Steady on :) ptrace() has never been sold as a technology upon which
its safe to build security solutions.

> What about other PID checks in the kernel, are they still
> safe if the process looks dead but is still active? Or is it a ptrace-only
> problem?
>
>>> If true we have to work around it by disallowing SIGKILL and just sending
>>> them ourselves within the jail. Meh.
>
> I guess this helps a bit. It doesn't prevent external signals, but prisoners
> don't have control over that.

Well.... a prisoner may be able to play other tricks:
- Allocate lots of memory... kernel may start spraying around SIGKILLs
- Sending SIGKILL via prctl()
- Sending SIGKILL via fcntl()
- Sending SIGKILL via clone()

>
> Is this SIGKILL specific or is it true for all task ending signals?

Can't remember - try it?

>
>>> How will you avoid file path races with BPF?
>>
>> There is typically no need for file-path based access control in an FTP server.
>> Take for example anonymous FTP, which will typically be inside a
>> chroot() to /var/ftp. Inside that filesystem tree -- if you can open()
>> it, you can have it.
>
> Ah, you count on having root access. We don't.
>
> Do you know any more crazy security destroying holes?

Try spraying SIGCONT and / or SIGSTOP at tracees. It may be possible
to confuse the tracer about whether a SIGTRAP event is syscall entry
or exit.
Try doing an execve() that fails. May cause similar state confusion in
the tracer.


Cheers
Chris

>
> Thanks,
>
> Indan
>
>