Date:   Fri, 2 Dec 2022 09:42:58 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Mark Brown <broonie@...nel.org>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Sandeepa Prabhu <sandeepa.s.prabhu@...il.com>
Subject: Re: [PATCH 3/3] arm64: kprobes: Return DBG_HOOK_ERROR if kprobes
 can not handle a BRK

On Thu, 1 Dec 2022 17:21:55 +0000
Mark Rutland <mark.rutland@....com> wrote:

> On Fri, Dec 02, 2022 at 01:07:13AM +0900, Masami Hiramatsu wrote:
> > On Thu, 1 Dec 2022 15:08:52 +0000
> > Mark Rutland <mark.rutland@....com> wrote:
> > 
> > > On Thu, Dec 01, 2022 at 11:39:21PM +0900, Masami Hiramatsu (Google) wrote:
> > > > From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > > > 
> > > > 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.
> > 
> > Interesting idea... and it also need many changes in kprobe itself.
> > 
> > > 
> > > In case we do try to change that in future, it would be good to have a comment
> > > somewhere to that effect.
> > 
> > Hmm, it would fundamentally change the assumptions that kprobes relies on,
> > and would require a lot of thought again. (e.g. current running kprobe is
> > stored in per-cpu variable, it should be per-task. etc.)
> 
> Ah; I had not considered that.
> 
> Feel free to ignore the above; with the comments as below:
> 
> Acked-by: Mark Rutland <mark.rutland@....com>

OK, Thanks!

> 
> Thanks,
> Mark.
> 
> > 
> > > 
> > > 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@...nel.org>
> > > > ---
> > > >  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.
> > > 		 */
> > 
> > OK.
> > 
> > > 
> > > > +		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.
> > 
> > OK, I'll update both in v2.
> > 
> > Thank you!
> > 
> > > 
> > > 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).
> > > > 
> > 
> > 
> > -- 
> > Masami Hiramatsu (Google) <mhiramat@...nel.org>


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>

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