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:	Wed, 18 Jan 2012 01:48:09 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Indan Zupancic <indan@....nu>
Cc:	Andrew Lutomirski <luto@....edu>, 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]

Indan Zupancic 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?

PTRACE_PEEKTEXT won't securely tell you if it's int 0x80 if there's
another thread modifying the code, or changing the mappings, or it's
executing from a file or shared memory that someone's writing to.

> 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?

It's a surprise to me too.  And like you I'm using ptrace, to trace
what a process touches, not restrict it, but it's subject to the same problem.

This looks like it needs a kernel patch.

-- Jamie
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