crash handlers and PTRACE in Ubuntu 10.10

[Serge E. Hallyn]

Quoting Kees Cook (kees at ubuntu.com):
> 5) "declared PTRACEing"
>    Recognize that the PTRACEing we want to allow is in very specific
>    declared situations (i.e. the PID of the debugger is always known by
>    the debugee, so have the debugee "declare" that it wishes to be
>    debugged).  This interface does not exist.  It would require the
>    creation of something like:
>         ptrace(PTRACE_TRACEDBY, debugger_pid, NULL, NULL);
>    This would be, at least for now, an Ubuntu-specific extension.  For
>    KDE applications, this is a trivial change, since the KDE crash
>    handler stays in a while loop, calling sleep() until the debugger
>    process quits.  Chromium would likely need to pass the debugger pid
>    back through the segfault handler in the sandbox -- I got lost reading
>    the syscall emulation-through-seccomp, so maybe it's as easy as KDE's.

While ptrace(PTRACE_TRACEDBY) is more elegant, if you were to use a
new prctl option then your yama lsm could just handle that.  Whereas
to have it handle PTRACE_TRACEDBY, I think you'd have to introduce a
new lsm hook.

-serge