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

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Date:	Tue, 17 Jan 2012 17:01:41 -0800
From:	Andrew Lutomirski <luto@....edu>
To:	Indan Zupancic <indan@....nu>
Cc:	Oleg Nesterov <oleg@...hat.com>, 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>
Subject: Re: Compat 32-bit syscall entry from 64-bit task!? [was: Re:
 [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF]

On Tue, Jan 17, 2012 at 4:56 PM, Indan Zupancic <indan@....nu> wrote:
> On Tue, January 17, 2012 18:45, Andrew Lutomirski wrote:
>> 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.)
>
> Wait: If a tasks is set to 64 bit mode, but calls into the kernel via
> int 0x80 it's changed to 32 bit mode for that system call and back to
> 64 bit mode when the system call is finished!?
>
> Our ptrace jailer is checking cs to figure out if a task is a compat task
> or not, if the kernel can change that behind our back it means our jailer
> isn't secure for x86_64 with compat enabled. Or is cs changed before the
> ptrace stuff and ptrace sees the "right" cs value? If not, we have to add
> an expensive PTRACE_PEEKTEXT to check if it's an int 0x80 or not. Or is
> there another way?

I don't know what your ptrace jailer does.  But a task can switch
itself between 32-bit and 64-bit execution at will, and there's
nothing the kernel can do about it.  (That isn't quite true -- in
theory the kernel could fiddle with the GDT, but that would be
expensive and wouldn't work on Xen.)

That being said, is_compat_task is apparently a good indication of
whether the current *syscall* entry is a 64-bit syscall or a 32-bit
syscall.  Perhaps the function should be renamed to in_compat_syscall,
because that's what it does.

>
> I think this behaviour is so unexpected that it can only cause security
> problems in the long run. Is anyone counting on this? Where is this
> behaviour documented?

Nowhere, I think.

--Andy
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