Re: [PATCH 2/2] Yama: add PTRACE exception tracking

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Kees Cook (kees.cook@canonical.com):
> Hi Serge,
> 
> On Tue, Jun 29, 2010 at 10:56:09PM -0500, Serge E. Hallyn wrote:
> > Quoting Kees Cook (kees.cook@canonical.com):
> > > Some application suites have external crash handlers that depend on
> > > being able to use PTRACE to generate crash reports (KDE, Chromium, etc).
> > > Since the inferior process generally knows the PID of the debugger,
> > > it can use PR_SET_PTRACER to allow a specific PID and its descendants
> > > to perform the PTRACE instead of only a direct ancestor.
> > > 
> > > Signed-off-by: Kees Cook <kees.cook@canonical.com>
> > > ---
> > 
> > Hi Kees - very nice, overall.  One little note though:
> 
> Thanks for looking it over!
> 
> > >  	rc = cap_ptrace_access_check(child, mode);
> > 
> > This means that if capable(CAP_SYS_PTRACE) we'll always shortcut
> > here, so
> > 
> > > +	if (mode == PTRACE_MODE_ATTACH &&
> > > +	    ptrace_scope &&
> > > +	    !capable(CAP_SYS_PTRACE) &&
> > > +	    !task_is_descendant(current, child) &&
> > > +	    !ptracer_exception_found(current, child))
> > > +		rc = -EPERM;
> > 
> > You don't need the CAP_SYS_PTRACE check here AFAICS.
> 
> I don't think that's true -- the capable(CAP_SYS_PTRACE) tests
> are always done in the negative since we only ever abort with error

Haha, you're right, I looked at that wrong  :)

-serge