Re: [LTP] [x86/entry] 2bbc68f837: ltp.ptrace08.fail

[Andy Lutomirski]

On Wed, Jun 17, 2020 at 6:17 AM Cyril Hrubis <chrubis@xxxxxxx> wrote:
>
> Hi!
> > > >> FYI, we noticed the following commit (built with gcc-9):
> > > >>
> > > >> commit: 2bbc68f8373c0631ebf137f376fbea00e8086be7 ("x86/entry: Convert Debug exception to IDTENTRY_DB")
> > > >> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> > > >
> > > > Is the head of linux.git exposing the same problem or is this an
> > > > intermittent failure, which only affects bisectability?
> > >
> > > It sure looks deterministic:
> > >
> > > ptrace08.c:62: BROK: Cannot find address of kernel symbol "do_debug"
> >
> > ROFL
>
> It's nice to have a good laugh, however I would really appreciate if any
> of you would help me to fix the test.
>
> The test in question is a regression test for:
>
> commit f67b15037a7a50c57f72e69a6d59941ad90a0f0f
> Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Date:   Mon Mar 26 15:39:07 2018 -1000
>
>     perf/hwbp: Simplify the perf-hwbp code, fix documentation
>
>     Annoyingly, modify_user_hw_breakpoint() unnecessarily complicates the
>     modification of a breakpoint - simplify it and remove the pointless
>     local variables.
>
> And as far as I can tell it uses ptrace() with PTRACE_POKEUSER in order to
> trigger it. But I'm kind of lost on how exactly we trigger the kernel
> crash.
>
> What is does is to write:
>
>         (void*)1 to u_debugreg[0]
>         (void*)1 to u_debugreg[7]
>         do_debug addr to u_debugreg[0]
>
> Looking at the kernel code the write to register 7 enables the breakpoints and
> what we attempt here is to change an invalid address to a valid one after we
> enabled the breakpoint but that's as far I can go.
>
> So does anyone has an idea how to trigger the bug without the do_debug function
> address? Would any valid kernel function address suffice?
>

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.

--Andy