Re: [PATCH 3/3] arm64: kprobes: Return DBG_HOOK_ERROR if kprobes can not handle a BRK

[Mark Rutland]

On Thu, Dec 01, 2022 at 11:39:21PM +0900, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> 
> Return DBG_HOOK_ERROR if kprobes can not handle a BRK because it
> fails to find a kprobe corresponding to the address.
> 
> Since arm64 kprobes uses stop_machine based text patching for removing
> BRK, it ensures all running kprobe_break_handler() is done at that point.
> And after removing the BRK, it removes the kprobe from its hash list.
> Thus, if the kprobe_break_handler() fails to find kprobe from hash list,
> there is a bug.

IIUC this relies on BRK handling not being preemptible, which is something
we've repeatedly considered changing along with a bunch of other debug
exception handling.

In case we do try to change that in future, it would be good to have a comment
somewhere to that effect.

I think there are other ways we could synchronise against that (e.g. using RCU
tasks rude) if we ever do that, and this patch looks good to me.

> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> ---
>  arch/arm64/kernel/probes/kprobes.c |   79 +++++++++++++++++-------------------
>  1 file changed, 37 insertions(+), 42 deletions(-)
> 
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index d2ae37f89774..ea56b22d4da8 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -298,7 +298,8 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
>  	return 0;
>  }
>  
> -static void __kprobes kprobe_handler(struct pt_regs *regs)
> +static int __kprobes
> +kprobe_breakpoint_handler(struct pt_regs *regs, unsigned long esr)
>  {
>  	struct kprobe *p, *cur_kprobe;
>  	struct kprobe_ctlblk *kcb;
> @@ -308,39 +309,45 @@ static void __kprobes kprobe_handler(struct pt_regs *regs)
>  	cur_kprobe = kprobe_running();
>  
>  	p = get_kprobe((kprobe_opcode_t *) addr);
> +	if (WARN_ON_ONCE(!p)) {
> +		/*
> +		 * Something went wrong. This must be put by kprobe, but we
> +		 * could not find corresponding kprobes. Let the kernel handle
> +		 * this error case.
> +		 */

Could we make this:

		/*
		 * Something went wrong. This BRK used an immediate reserved
		 * for kprobes, but we couldn't find any corresponding probe.
		 */

> +		return DBG_HOOK_ERROR;
> +	}
>  
> -	if (p) {
> -		if (cur_kprobe) {
> -			if (reenter_kprobe(p, regs, kcb))
> -				return;
> -		} else {
> -			/* Probe hit */
> -			set_current_kprobe(p);
> -			kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> -
> -			/*
> -			 * If we have no pre-handler or it returned 0, we
> -			 * continue with normal processing.  If we have a
> -			 * pre-handler and it returned non-zero, it will
> -			 * modify the execution path and no need to single
> -			 * stepping. Let's just reset current kprobe and exit.
> -			 */
> -			if (!p->pre_handler || !p->pre_handler(p, regs)) {
> -				setup_singlestep(p, regs, kcb, 0);
> -			} else
> -				reset_current_kprobe();
> -		}
> +	if (cur_kprobe) {
> +		/* Hit a kprobe inside another kprobe */
> +		if (!reenter_kprobe(p, regs, kcb))
> +			return DBG_HOOK_ERROR;
> +	} else {
> +		/* Probe hit */
> +		set_current_kprobe(p);
> +		kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> +
> +		/*
> +		 * If we have no pre-handler or it returned 0, we
> +		 * continue with normal processing.  If we have a
> +		 * pre-handler and it returned non-zero, it will
> +		 * modify the execution path and no need to single
> +		 * stepping. Let's just reset current kprobe and exit.
> +		 */

Minor wording nit: could we replace:

	no need to single stepping.

With:
	
	not need to single-step.

Thanks,
Mark.

> +		if (!p->pre_handler || !p->pre_handler(p, regs))
> +			setup_singlestep(p, regs, kcb, 0);
> +		else
> +			reset_current_kprobe();
>  	}
> -	/*
> -	 * The breakpoint instruction was removed right
> -	 * after we hit it.  Another cpu has removed
> -	 * either a probepoint or a debugger breakpoint
> -	 * at this address.  In either case, no further
> -	 * handling of this interrupt is appropriate.
> -	 * Return back to original instruction, and continue.
> -	 */
> +
> +	return DBG_HOOK_HANDLED;
>  }
>  
> +static struct break_hook kprobes_break_hook = {
> +	.imm = KPROBES_BRK_IMM,
> +	.fn = kprobe_breakpoint_handler,
> +};
> +
>  static int __kprobes
>  kprobe_breakpoint_ss_handler(struct pt_regs *regs, unsigned long esr)
>  {
> @@ -365,18 +372,6 @@ static struct break_hook kprobes_break_ss_hook = {
>  	.fn = kprobe_breakpoint_ss_handler,
>  };
>  
> -static int __kprobes
> -kprobe_breakpoint_handler(struct pt_regs *regs, unsigned long esr)
> -{
> -	kprobe_handler(regs);
> -	return DBG_HOOK_HANDLED;
> -}
> -
> -static struct break_hook kprobes_break_hook = {
> -	.imm = KPROBES_BRK_IMM,
> -	.fn = kprobe_breakpoint_handler,
> -};
> -
>  /*
>   * Provide a blacklist of symbols identifying ranges which cannot be kprobed.
>   * This blacklist is exposed to userspace via debugfs (kprobes/blacklist).
>