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

[Andrew Lutomirski]

On Tue, Jan 17, 2012 at 8:56 AM, Will Drewry <wad@xxxxxxxxxxxx> wrote:
> On Tue, Jan 17, 2012 at 10:45 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>> On 01/16, Will Drewry wrote:
>>>
>>> On Mon, Jan 16, 2012 at 12:37 PM, Oleg Nesterov <oleg@xxxxxxxxxx> 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.

(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.)

--Andy
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