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Date: Thu, 1 Dec 2022 17:21:55 +0000
From: Mark Rutland <mark.rutland@....com>
To: Masami Hiramatsu <mhiramat@...nel.org>
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 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>
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>
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