On Fri, Aug 14, 2020 at 7:58 AM Cyril Hrubis <chrubis(a)suse.cz> wrote:
> > do_debug is a bit of a red herring here. ptrace should not be able to
> > put a breakpoint on a kernel address, period. I would just pick a
> > fixed address that's in the kernel text range or even just in the
> > pre-KASLR text range and make sure it gets rejected. Maybe try a few
> > different addresses for good measure.
> I've looked at the code and it seems like this would be a bit more
> complicated since the breakpoint is set by an accident in a race and the
> call still fails. Which is why the test triggers the breakpoint and
> causes infinite loop in the kernel...
> I guess that we could instead read back the address with
> PTRACE_PEEKUSER, so something as:
> break_addr = ptrace(PTRACE_PEEKUSER, child_pid,
> (void *)offsetof(struct user, u_debugreg),
> if (break_addr == kernel_addr)
> tst_res(TFAIL, "ptrace() set break on a kernel address");
So this works actually nicely, even better than the original code.
Any hints on how to select a fixed address in the kernel range as you
pointed out in one of the previous emails? I guess that this would end
up as a per-architecture mess of ifdefs if we wanted to hardcode it.
It's fundamentally architecture dependent. Sane architectures like
s390x don't even have this concept.