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

Masami Hiramatsu
2019-01-09T04:38:04Z

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

Thank you, Andrea!

Ingo, could you pick this series?

--
Masami Hiramatsu <mhiramat@kernel.org>