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Date:   Wed, 9 Jan 2019 11:08:16 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Ingo Molnar <mingo@redhat.com>, peterz@infradead.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Andrea Righi <righi.andrea@gmail.com>, stable@vger.kernel.org
Subject: Re: [PATCH v2 3/3] x86/kprobes: Fix to avoid kretprobe recursion
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        <154692272225.1133.799265848498671353.stgit@devbox>
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On Tue,  8 Jan 2019 13:45:22 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> Fix to avoid kretprobe recursion loop by setting a dummy
> kprobes to current_kprobe per-cpu variable.
> 
> 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.
> 
> This revives the old lost kprobe again.
> 
> With this fix, we don't see deadlock anymore.
> 
> # echo "r:event_1 __fdget" >> kprobe_events
> # echo "r:event_2 _raw_spin_lock_irqsave" >> kprobe_events
> # echo 1 > events/kprobes/enable
> 
> And you can see that all inner-called kretprobe are skipped.
> 
> # cat kprobe_profile
>   event_1                                  235               0
>   event_2                                19375           19612
> 
> The 1st column is recorded count and the 2nd is missed count.
> Above shows (event_1 rec) + (event_2 rec) ~= (event_2 missed)

I don't quite understand the above. Is the miss count because we missed
event_2 events for both event_1 and event_2?

 trace raw_spin_lock()
    handler calls raw_spin_lock()
          trace raw_spin_lock() [ skip ]

I'm also guessing that the 2 extra (19612 - (235 + 19375) = 2) are
possibly due to the displaying being racy?

> 
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> Reported-by: Andrea Righi <righi.andrea@gmail.com>
> Fixes: c9becf58d935 ("[PATCH] kretprobe: kretprobe-booster")
> Cc: stable@vger.kernel.org
> ---
>  arch/x86/kernel/kprobes/core.c |   22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 69b6400d1ce2..f4b954ff5b89 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -749,11 +749,16 @@ asm(
>  NOKPROBE_SYMBOL(kretprobe_trampoline);
>  STACK_FRAME_NON_STANDARD(kretprobe_trampoline);
>  
> +static struct kprobe kretprobe_kprobe = {
> +	.addr = (void *)kretprobe_trampoline,
> +};
> +
>  /*
>   * Called from kretprobe_trampoline
>   */
>  static __used void *trampoline_handler(struct pt_regs *regs)
>  {
> +	struct kprobe_ctlblk *kcb;
>  	struct kretprobe_instance *ri = NULL;
>  	struct hlist_head *head, empty_rp;
>  	struct hlist_node *tmp;
> @@ -763,6 +768,17 @@ static __used void *trampoline_handler(struct pt_regs *regs)
>  	void *frame_pointer;
>  	bool skipped = false;
>  
> +	preempt_disable();
> +
> +	/*
> +	 * Set a dummy kprobe for avoiding kretprobe recursion.
> +	 * Since kretprobe never run in kprobe handler, kprobe must not
> +	 * be running at this point.
> +	 */
> +	kcb = get_kprobe_ctlblk();
> +	__this_cpu_write(current_kprobe, &kretprobe_kprobe);

If an interrupt comes in here, is this still safe, if the interrupt
handler has a kretprobe too?

-- Steve

> +	kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> +
>  	INIT_HLIST_HEAD(&empty_rp);
>  	kretprobe_hash_lock(current, &head, &flags);
>  	/* fixup registers */
> @@ -838,10 +854,9 @@ static __used void *trampoline_handler(struct pt_regs *regs)
>  		orig_ret_address = (unsigned long)ri->ret_addr;
>  		if (ri->rp && ri->rp->handler) {
>  			__this_cpu_write(current_kprobe, &ri->rp->kp);
> -			get_kprobe_ctlblk()->kprobe_status = KPROBE_HIT_ACTIVE;
>  			ri->ret_addr = correct_ret_addr;
>  			ri->rp->handler(ri, regs);
> -			__this_cpu_write(current_kprobe, NULL);
> +			__this_cpu_write(current_kprobe, &kretprobe_kprobe);
>  		}
>  
>  		recycle_rp_inst(ri, &empty_rp);
> @@ -857,6 +872,9 @@ static __used void *trampoline_handler(struct pt_regs *regs)
>  
>  	kretprobe_hash_unlock(current, &flags);
>  
> +	__this_cpu_write(current_kprobe, NULL);
> +	preempt_enable();
> +
>  	hlist_for_each_entry_safe(ri, tmp, &empty_rp, hlist) {
>  		hlist_del(&ri->hlist);
>  		kfree(ri);