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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Message-ID: <CA+55aFysH3h8sLHwNcaLnYMsgA28vmHgd3DSLPVw-=TzgfBrbg@mail.gmail.com>
Date:	Tue, 17 Jan 2012 21:23:51 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Indan Zupancic <indan@....nu>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Jamie Lokier <jamie@...reable.org>,
	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,
	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>
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 8:22 PM, Indan Zupancic <indan@....nu> wrote:
>
> Looking at EIP - 2 seems like a secure way to check how we entered the kernel.

Secure? No. Not at all.

It's actually very easy to fool it. Do something like this:

 - map the same physical page executably at one address, and writably
4kB above it (use shared memory, and map it twice).

 - in that page, do this:

      lea 1f,%edx
      movl $SYSCALL,%eax
      movl $-1,4096(%edx)
  1:
      int 0x80

and what happens is that the move that *overwrites* the int 0x80 will
not be noticed by the I$ coherency because it's at another address,
but by the time you read at $pc-2, you'll get -1, not "int 0x80"

                  Linus
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