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 01:29:04 +0100
From:	"Indan Zupancic" <indan@....nu>
To:	"Oleg Nesterov" <oleg@...hat.com>
Cc:	"Chris Evans" <scarybeasts@...il.com>,
	"Andi Kleen" <andi@...stfloor.org>,
	"Jamie Lokier" <jamie@...reable.org>,
	"Andrew Lutomirski" <luto@....edu>,
	"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, January 18, 2012 18:00, Oleg Nesterov wrote:
> On 01/17, Chris Evans wrote:
>>
>> 1) Tracee is compromised; executes fork() which is syscall that isn't allowed
>> 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)
>> 5) Disaster: the tracee took a SIGKILL so any attempt to address it by
>> pid (such as PTRACE_SETREGS) fails.
>> 6) Syscall fork() executes; possible unsupervised process now running
>> since the tracer wasn't expecting the fork() to be allowed.
>
> As for fork() in particular, it can't succeed after SIGKILL.

That was sadly exactly the system call I used for testing my code...

> But I agree, probably it makes sense to change ptrace_stop() to check
> fatal_signal_pending() and do do_group_exit(SIGKILL) after it sleeps
> in TASK_TRACED. Or we can change tracehook_report_syscall_entry()
>
> 	-	return 0;
> 	+	return !fatal_signal_pending();
>
> (no, I do not literally mean the change above)
>
> Not only for security. The current behaviour sometime confuses the
> users. Debugger sends SIGKILL to the tracee and assumes it should
> die asap, but the tracee exits only after syscall.

I didn't expect the tracer to die asap when sending SIGKILL, but I
did for PTRACE_KILL.

Improving this behaviour is highly appreciated, thanks!

Greetings,

Indan


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