Re: [PATCH] ptrace: allow restriction of ptrace scope

Kees Cook
2010-06-16T23:40:44Z

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