Re: [PATCH] Yama: turn process ancestry check into function

[Kees Cook <kees.cook@canonical.com>]

On Wed, Jul 14, 2010 at 09:19:09AM +0900, Tetsuo Handa wrote:
> Kees Cook wrote:
> > +static int task_is_descendant(struct task_struct *parent,
> > +			      struct task_struct *child)
> > +{
> > +	int rc = 0;
> > +	struct task_struct *walker = child;
> > +
> > +	if (!parent || !child)
> > +		return 0;
> 
> parent (== current) is !NULL and
> child (in original code) is !NULL.
> You can remove this check unless you are planning to call
> this function from other places.

I'd like the flexibility to call it with NULLs.  But yes, at present, it
never will be NULL.

> > +	if (mode == PTRACE_MODE_ATTACH &&
> > +	    ptrace_scope &&
> > +	    !task_is_descendant(current, child) &&
> > +	    !capable(CAP_SYS_PTRACE))
> > +		rc = -EPERM;
> 
> I don't know how heavy capable(CAP_SYS_PTRACE) is.
> But checking !capable(CAP_SYS_PTRACE) before
> !task_is_descendant(current, child) might be lighter.

That's the order I had before, but in looking at some of the other code, it
seemed like moving it to the end made more logical sense.  Since checking
PTRACE attach isn't a common or time-sensitive operation, I figured trying
to tune it wasn't critical.

-Kees

-- 
Kees Cook
Ubuntu Security Team