linux-kernel - Re: [PATCH] ptrace: allow restriction of ptrace scope

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20100616233937.GQ24749@outflux.net>
Date:	Wed, 16 Jun 2010 16:39:37 -0700
From:	Kees Cook <kees.cook@...onical.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...otime.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Dave Young <hidave.darkstar@...il.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	David Howells <dhowells@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH] ptrace: allow restriction of ptrace scope

Hi,

On Wed, Jun 16, 2010 at 04:10:06PM -0700, Roland McGrath wrote:
> This constraint seems fairly insane to me, but I don't really care about
> people using sysctl to enable insane things if that's what floats your
> boat.  GDB's "attach", "strace -p", etc. are pretty normal (and highly
> useful) things for ordinary users debugging their own programs.

Right, but I don't think "ordinary users" debug their own programs.
Ordinary developers and sysadmin do, absolutely, and for them, this
sysctl would probably stay set to 0.

> I tend to think that this constraint offers more a delusion of security
> than much real protection.  But I'm too lazy to try to come up with a more
> contorted exploit that this wouldn't prevent, so I won't belabor the point.

Well, I don't want to present it as something it's not.  It's only
designed to block access to what is immediately in memory.  It certainly
won't stop an attacker from tricking a user into divulging credentials
directly or launching a process and then ptracing it, but it would put
a stop to an automated worm that did not need to go phishing.

> I think those who are actually paranoid would use something more meaningful
> via the LSM ptrace check, like SELinux with a policy that only permits
> known debugger applications to use ptrace.  Aside from SELinux, it could
> also be done with a new capability like CAP_PTRACE_MINE and filesystem
> capabilities on installed debugger application binaries, for example.

This has been the area I've run into the most.  I like the idea of a
semi-privileged capability like you suggest.  It would solve a number
of iffy spots, like KDE and Chrome that fork/exec a debugger from the
crashing process and attach back to it.  Those programs could be given
fscap for CAP_PTRACE_MINE, or something.  Though, honestly, just trying to
get rid of PTRACE seems like the better place to spend time.

> You've described this as allowing ptrace on "children", but really it's
> "unorphaned descendants", i.e. also children of children, etc.

Right, I should say "descendants", which is the correct intent.

> I don't think "task->pid > 0" is a sort of check that is used elsewhere in
> the kernel for this.  Perhaps "task == &init_task" would be better.

Is this correct for pid_ns?  I thought pid 1 (regardless of NS) would have
a NULL parent?

> It's not immediately obvious to me how this interacts with pid_ns, or
> should.  Probably it shouldn't pay attention to pid_ns, as it doesn't.
> But I think it merits an explicit statement of intent about that.

Okay, I can do that.

> I suspect you really want to test same_thread_group(walker, current).
> You don't actually mean to rule out a debugger that forks children with
> one thread and calls ptrace with another, do you?

Won't they ultimately have the same parent, though?

> Oh, and surely you meant real_parent.  Off hand I think that might only
> really add up to a different constraint if you had some ptrace attaches
> already live when you did set the sysctl flag.  But I have the vague
> sensation I'm overlooking some other arcane case.  And it just doesn't
> logically match the stated intent of the thing to depend on parent
> rather than real_parent.

Oh, yes.  That seems right.  I can fix that.

-Kees

-- 
Kees Cook
Ubuntu Security Team
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/