Date:   Tue, 8 Jan 2019 11:31:01 +0100
From:   Andrea Righi <righi.andrea@...il.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>, peterz@...radead.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>, stable@...r.kernel.org
Subject: Re: [PATCH v2 0/3] kprobes: Fix kretprobe issues

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@...il.com>

Thanks,
-Andrea

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