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

[Will Drewry <wad@chromium.org>]

On Tue, Jan 17, 2012 at 11:01 AM, Andrew Lutomirski <luto@mit.edu> wrote:
> On Tue, Jan 17, 2012 at 8:56 AM, Will Drewry <wad@chromium.org> wrote:
>> On Tue, Jan 17, 2012 at 10:45 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>>> On 01/16, Will Drewry wrote:
>>>>
>>>> On Mon, Jan 16, 2012 at 12:37 PM, Oleg Nesterov <oleg@redhat.com> wrote:
>>>> >
>>>> > Yes, thanks, I forgot about compat tasks again. But this is easy, just
>>>> > we need regs_64_to_32().
>>>>
>>>> Yup - we could make the assumption that is_compat_task is always
>>>> 32-bit and the pt_regs is always 64-bit, then copy_and_truncate with
>>>> regs_64_to_32.  Seems kinda wonky though :/
>>>
>>> much simpler/faster than what regset does to create the artificial
>>> user_regs_struct32.
>>
>> True, I could collapse pt_regs to looks like the exported ABI pt_regs.
>>  Then only compat processes would get the copy overhead.  That could
>> be tidy and not break ABI.  It would mean that I have to assume that
>> if unsigned long == 64-bit and is_compat_task(), then the task is
>> 32-bit.  Do you think if we ever add a crazy 128-bit "supercomputer"
>> arch that we will add a is_compat64_task() so that I could properly
>> collapse? :)
>>
>> I like this idea!
>
> FWIW, it's possible for a task to execute in 32-bit mode when
> !is_compat_task or in 64-bit mode when is_compat_task.  From earlier
> in the thread, I think you were planning to block the wrong-bitness
> syscall entries, but it's worth double-checking that you don't open up
> a hole when a compat task issues the 64-bit syscall instruction.

Yup - I had to (see below).

> (is_compat_task says whether the executable was marked as 32-bit.  The
> actual execution mode is determined by the cs register, which the user
> can control.  See the user_64bit_mode function in
> arch/asm/x86/ptrace.h.  But maybe it would make more sense to have a
> separate 32-bit and 64-bit BPF program and select which one to use
> based on the entry point.)

So that was my original design, but the problem was with how regviews
decides on the user_regs_struct.  It decides using TIF_IA32 while I
can only check the cross-arch is_compat_task() which checks TS_COMPAT
on x86.  If I'm just collapsing registers for compat calls (which I am
exploring the viability of right now), then I guess I could re-fork
the filtering to support compat versus non-compat.  The nastier bits
there were that I don't want to allow a compat call to be allowed
because a process only defined non-compat. I think that can be made
manage-able though.

I'll finish proving out the possibilities here.

Thanks!
will