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:	Thu, 19 Jan 2012 15:40:14 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Indan Zupancic <indan@....nu>
Cc:	Chris Evans <scarybeasts@...il.com>,
	Andi Kleen <andi@...stfloor.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,
	torvalds@...ux-foundation.org, segoon@...nwall.com,
	rostedt@...dmis.org, jmorris@...ei.org, 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]

Indan Zupancic wrote:
> On Wed, January 18, 2012 22:13, Chris Evans wrote:
> > On Wed, Jan 18, 2012 at 4:12 AM, Indan Zupancic <indan@....nu> wrote:
> >> On Wed, January 18, 2012 06:43, Chris Evans wrote:
> >>> 2) Tracee traps
> >>> 2b) Tracee could take a SIGKILL here
> >>> 3) Tracer looks at registers; bad syscall
> >>> 3b) Or tracee could take a SIGKILL here
> >>> 4) The only way to stop the bad syscall from executing is to rewrite
> >>> orig_eax (PTRACE_CONT + SIGKILL only kills the process after the
> >>> syscall has finished)
> >>
> >> Yes, we rewrite it to -1.
> >>
> >>> 5) Disaster: the tracee took a SIGKILL so any attempt to address it by
> >>> pid (such as PTRACE_SETREGS) fails.
> >>
> >> I assume that if a task can execute system calls and we get ptrace events
> >> for that, that we can do other ptrace operations too. Are you saying that
> >> the kernel has this ptrace gap between SIGKILL and task exit where ptrace
> >> doesn't work but the task continues executing system calls? That would be
> >> a huge bug, but it seems very unlikely too, as the task is stopped and
> >> shouldn't be able to disappear till it is continued by the tracer.
> >>
> >> I mean, really? That would be stupid.
> 
> Okay, I tested this scenario and you're right, we're screwed.

Ha!

Perhaps this could be fixed generically in
tracehook_report_syscall_entry(), for those architectures which bother
to call it and bother to disable the syscall if it says to.

-- Jamie
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