Re: [PATCH v2 0/3] kprobes: Fix kretprobe issues

[Masami Hiramatsu]

On Tue, 8 Jan 2019 11:31:01 +0100
Andrea Righi <righi.andrea@xxxxxxxxx> wrote:

> On Tue, Jan 08, 2019 at 01:43:55PM +0900, Masami Hiramatsu wrote:
> > Hello,
> > 
> > This is v2 series of fixing kretprobe incorrect stacking order patches.
> > In this version, I fixed a lack of kprobes.h including and added new
> > patch for kretprobe trampoline recursion issue. (and add Cc:stable)
> > 
> > (1) kprobe incorrct stacking order problem
> > 
> > On recent talk with Andrea, I started more precise investigation on
> > the kernel panic with kretprobes on notrace functions, which Francis
> > had been reported last year ( https://lkml.org/lkml/2017/7/14/466 ).
> > 
> > See the investigation details in 
> > https://lkml.kernel.org/r/154686789378.15479.2886543882215785247.stgit@devbox
> > 
> > When we put a kretprobe on ftrace_ops_assist_func() and put another
> > kretprobe on probed-function, below happens
> > 
> > <caller>
> >  -><probed-function>
> >    ->fentry
> >     ->ftrace_ops_assist_func()
> >      ->int3
> >       ->kprobe_int3_handler()
> >       ...->pre_handler_kretprobe()
> >            push the return address (*fentry*) of ftrace_ops_assist_func() to
> >            top of the kretprobe list and replace it with kretprobe_trampoline.
> >       <-kprobe_int3_handler()
> >      <-(int3)
> >      ->kprobe_ftrace_handler()
> >       ...->pre_handler_kretprobe()
> >            push the return address (caller) of probed-function to top of the
> >            kretprobe list and replace it with kretprobe_trampoline.
> >      <-(kprobe_ftrace_handler())
> >     <-(ftrace_ops_assist_func())
> >     [kretprobe_trampoline]
> >      ->tampoline_handler()
> >        pop the return address (caller) from top of the kretprobe list
> >      <-(trampoline_handler())
> >     <caller>
> >     [run caller with incorrect stack information]
> >    <-(<caller>)
> >   !!KERNEL PANIC!!
> > 
> > Therefore, this kernel panic happens only when we put 2 k*ret*probes on
> > ftrace_ops_assist_func() and other functions. If we put kprobes, it
> > doesn't cause any issue, since it doesn't change the return address.
> > 
> > To fix (or just avoid) this issue, we can introduce a frame pointer
> > verification to skip wrong order entries. And I also would like to
> > blacklist those functions because those are part of ftrace-based 
> > kprobe handling routine.
> > 
> > (2) kretprobe trampoline recursion problem
> > 
> > This was found by Andrea in the previous thread
> > https://lkml.kernel.org/r/20190107183444.GA5966@xps-13
> > 
> > ----
> >  echo "r:event_1 __fdget" >> kprobe_events
> >  echo "r:event_2 _raw_spin_lock_irqsave" >> kprobe_events
> >  echo 1 > events/kprobes/enable
> >  [DEADLOCK]
> > ----
> > 
> > Because kretprobe trampoline_handler uses spinlock for protecting
> > hash table, if we probe the spinlock itself, it causes deadlock.
> > Thank you Andrea and Steve for discovering this root cause!!
> > 
> > This bug has been introduced with the asm-coded trampoline
> > code, since previously it used another kprobe for hooking
> > the function return placeholder (which only has a nop) and
> > trampoline handler was called from that kprobe.
> > 
> > To fix this bug, I introduced a dummy kprobe and set it in
> > current_kprobe as we did in old days.
> > 
> > Thank you,
> 
> It looks all good to me, with this patch set I couldn't break the
> kernel in any way.
> 
> Tested-by: Andrea Righi <righi.andrea@xxxxxxxxx>

Thank you, Andrea!

Ingo, could you pick this series?


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>