Re: [PATCH tip/master] [BUGFIX] kprobes/x86: Fix to clear TF bit in fault-on-single-stepping

[Steven Rostedt]

On Mon, 13 Jun 2016 19:13:45 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:


> >   # cd /sys/kernel/debug/tracing
> >   # echo p copy_user_enhanced_fast_string+5 > kprobe_events
> >   # echo 1 > events/kprobes/enable
> > 
> > And you'll see a kernel panic on do_debug(), since the debug
> > trap is not handled by kprobes.
> > 
> > To fix this problem, we just need to clear the TF bit when
> > resetting running kprobe.
> >   
> 
> This should definitely be marked for stable, and I bisected it all the
> way down to this commit: f4cb1cc18f364d "x86-64, copy_user: Remove zero
> byte check before copy user buffer."
> 
> I reverted that commit and sure enough, this bug goes away. I'm not
> saying the revert should be done. I'm just doing an FYI, and showing how
> changes that appear to be a nice clean up can have subtle effects. I'm
> not even sure how that change caused this to be a problem with kprobes.
> 

Nevermind, reverting that commit only moved the location of the
"rep movsb" that you were placing the kprobe on. When I do:

  echo "p copy_user_enhanced_fast_string+9" > kprobe_events

I get the same result.

That means we need to make that stable tag even earlier.

-- Steve