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 13:13:02 -0800
From:	Chris Evans <scarybeasts@...il.com>
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,
	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]

On Wed, Jan 18, 2012 at 4:12 AM, Indan Zupancic <indan@....nu> wrote:
> On Wed, January 18, 2012 06:43, Chris Evans wrote:
>>> As far as I know, we fixed all races except symlink races caused by malicious
>>> code outside the jail.
>>
>> Are you sure? I've remembered possibly the worst one I encountered,
>> since my previous e-mail to Jamie:
>>
>> 1) Tracee is compromised; executes fork() which is syscall that isn't allowed
>
> How do you mean compromised? Tracees aren't trusted by definition. And fork is
> allowed in our jail, we're ptracing all tasks within the jail.

Right, the tracee isn't trusted because you're worried it _might_ get
compromised.
If it _does_ get compromised, you don't want it playing various tricks
to break our of the ptrace() sandbox.

>
>> 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.
>
> If true we have to work around it by disallowing SIGKILL and just sending
> them ourselves within the jail. Meh.
>
>> 6) Syscall fork() executes; possible unsupervised process now running
>> since the tracer wasn't expecting the fork() to be allowed.
>
> We use PTRACE_O_TRACEFORK (or replace it with clone and set CLONE_PTRACE
> for 2.4 kernels. Yes, I check for CLONE_UNTRACED in clone calls.)
>
>>
>> All this ptrace() security headache is why vsftpd is waiting for
>> Will's seccomp enhancements to hit the kernel. Then they will be used
>> pronto.
>
> How will you avoid file path races with BPF?

There is typically no need for file-path based access control in an FTP server.
Take for example anonymous FTP, which will typically be inside a
chroot() to /var/ftp. Inside that filesystem tree -- if you can open()
it, you can have it.


Cheers
Chris

>
> Greetings,
>
> Indan
>
>
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