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

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Date:	Tue, 17 Jan 2012 09:45:25 -0800
From:	Andrew Lutomirski <luto@....edu>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	keescook@...omium.org, john.johansen@...onical.com,
	serge.hallyn@...onical.com, coreyb@...ux.vnet.ibm.com,
	pmoore@...hat.com, eparis@...hat.com, djm@...drot.org,
	torvalds@...ux-foundation.org, segoon@...nwall.com,
	rostedt@...dmis.org, jmorris@...ei.org, scarybeasts@...il.com,
	avi@...hat.com, penberg@...helsinki.fi, viro@...iv.linux.org.uk,
	mingo@...e.hu, akpm@...ux-foundation.org, khilman@...com,
	borislav.petkov@....com, amwang@...hat.com, ak@...ux.intel.com,
	eric.dumazet@...il.com, gregkh@...e.de, dhowells@...hat.com,
	daniel.lezcano@...e.fr, linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com,
	Roland McGrath <mcgrathr@...omium.org>,
	Andi Kleen <andi@...stfloor.org>, indan@....nu
Subject: Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

On Tue, Jan 17, 2012 at 9:05 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 01/17, Andrew Lutomirski wrote:
>>
>> (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.
>
> Confused... Afaics, TIF_IA32 says that the binary is 32-bit (this comes
> along with TS_COMPAT).
>
> TS_COMPAT says that, say, the task did "int 80" to enters the kernel.
> 64-bit or not, we should treat is as 32-bit in this case.

I think you're right, and checking which entry was used is better than
checking the cs register (since 64-bit code can use int80).  That's
what I get for insufficiently careful reading of the assembly.  (And
for going from memory from when I wrote the vsyscall emulation code --
that code is entered from a page fault, so the entry point used is
irrelevant.)

--Andy
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