kernel-hardening - Re: [PATCH v8 3/8] seccomp: add system call filtering using BPF

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Date: Thu, 16 Feb 2012 17:00:19 -0600
From: Will Drewry <wad@...omium.org>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Markus Gutschke <markus@...omium.org>, linux-kernel@...r.kernel.org, 
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org, 
	kernel-hardening@...ts.openwall.com, netdev@...r.kernel.org, x86@...nel.org, 
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	tglx@...utronix.de, luto@....edu, eparis@...hat.com, 
	serge.hallyn@...onical.com, djm@...drot.org, scarybeasts@...il.com, 
	indan@....nu, pmoore@...hat.com, akpm@...ux-foundation.org, corbet@....net, 
	eric.dumazet@...il.com, keescook@...omium.org
Subject: Re: [PATCH v8 3/8] seccomp: add system call filtering using BPF

On Thu, Feb 16, 2012 at 4:06 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 02/16/2012 01:51 PM, Will Drewry wrote:
>>>
>>> Put the bloody bit in there and let the pattern program make that decision.
>>
>> Easy enough to add a bit for the mode: 32-bit or 64-bit.  It seemed
>> like a waste of cycles for every 32-bit program or every 64-bit
>> program to check to see that its calling convention hadn't changed,
>> but it does take away a valid decision the pattern program should be
>> making.
>>
>> I'll add a flag for 32bit/64bit while cleaning up seccomp_data. I
>> think that will properly encapsulate the is_compat_task() behavior in
>> a way that is stable for compat and non-compat tasks to use.  If
>> there's a more obvious way, I'm all ears.
>>
>
> is_compat_task() is not going to be the right thing for x86 going
> forward, as we're introducing the x32 ABI (which uses the normal x86-64
> entry point, but with different eax numbers, and bit 30 set.)
>
> The actual state is the TS_COMPAT flag in the thread_info structure,
> which currently matches is_compat_task(), but perhaps we should add a
> new helper function syscall_namespace() or something like that...

Without the addition of x32, it is still the intersection of
is_compat_task()/TS_COMPAT and CONFIG_64BIT for all arches to
determine if the call is 32-bit or 64-bit, but this will add another
wrinkle.  Would it make sense to assume that system call namespaces
may be ever expanding and offer up an unsigned integer value?

struct seccomp_data {
  int nr;
  u32 namespace;
  u64 instruction_pointer;
  u64 args[6];
}

Then syscall_namespace(current, regs) returns
* 0 - SYSCALL_NS_32 (for existing 32 and config_compat)
* 1 - SYSCALL_NS_64 (for existing 64 bit)
* 2 - SYSCALL_NS_X32 (everything after 2 is arch specific)
* ..

This patch series is pegged to x86 right now, so it's not a big deal
to add a simple syscall_namespace to asm/syscall.h.  Of course, the
code is always the easy part.  Even easier would be to only assign 0
and 1 in the seccomp_data for 32-bit or 64-bit, then leave the rest of
the u32 untouched until x32 stabilizes and the TS_COMPAT interactions
are sorted.

The other option, of course, is to hide it from the users and peg to
is_compat_task and later to however x32 is exposed, but that might
just be me trying to avoid adding more dependencies to this patch
series :)

> Either that or we can just use another bit in the syscall number field...

That would simplify the case here. The seccomp_data bit would say the
call is 64-bit and then the syscall number with the extra bit would
say that it is x32 and wouldn't collide with the existing 64-bit
numbering, and the filter program author wouldn't make a filter
program that allows a call that it shouldn't.

Another option could be to expose the task_user_regset_view() of
e_machine and e_osabi for the current/active calling convention
assuming x32 gets a new marker there.  (There is a small amount
pain[1] there since x86 uses TIF_IA32 and not TS_COMPAT for regset's
e_machine value and not the current calling convention.  It's not
clear to me if TS_COMPAT would be set during a core
dump/fill_note_info, and not many people use ptrace's GET/SETREGSET,
but I'm not super confident unraveling that mystery myself. Perhaps,
current_user_regset_view() )

For now, I'll just drop in a u32 for the calling convention.

Thanks!
will

1 - http://lxr.linux.no/linux+v3.2.6/arch/x86/kernel/ptrace.c#L1310
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