Re: kprobes, livepatch and FTRACE_OPS_FL_IPMODIFY

[Masami Hiramatsu <mhiramat@kernel.org>]

On Wed, 24 Jul 2019 11:19:42 -0400
Joe Lawrence <joe.lawrence@redhat.com> wrote:

> Hi Masami,
> 
> I wanted to revisit FTRACE_OPS_FL_IPMODIFY blocking of kprobes and
> livepatch, at least in cases where kprobe pre_handlers don't modify
> regs->ip.

OK, now I think we can pass a flag to kprobe_register() to modify regs->ip
or not. Then we can introduce 2 different ftrace_ops for IPMODIFY
or just requires REGS.

> (We've discussed this previously at part of a kpatch github issue #47:
> https://github.com/dynup/kpatch/issues/47)
> 
> The particular use case I was wondering about was perf probing a
> particular function, then attempting to livepatch that same function:
> 
>   % uname -r
>   5.3.0-rc1+
> 
>   % dmesg -C
>   % perf probe --add cmdline_proc_show
>   Added new event:
>     probe:cmdline_proc_show (on cmdline_proc_show)
> 
>   You can now use it in all perf tools, such as:
> 
>           perf record -e probe:cmdline_proc_show -aR sleep 1
> 
>   % perf record -e probe:cmdline_proc_show -aR sleep 30 &
>   [1] 1007
>   % insmod samples/livepatch/livepatch-sample.ko
>   insmod: ERROR: could not insert module samples/livepatch/livepatch-sample.ko: Device or resource busy
>   % dmesg
>   [  440.913962] livepatch_sample: tainting kernel with TAINT_LIVEPATCH
>   [  440.917123] livepatch_sample: module verification failed: signature and/or required key missing - tainting kernel
>   [  440.942493] livepatch: enabling patch 'livepatch_sample'
>   [  440.943445] livepatch: failed to register ftrace handler for function 'cmdline_proc_show' (-16)
>   [  440.944576] livepatch: failed to patch object 'vmlinux'
>   [  440.945270] livepatch: failed to enable patch 'livepatch_sample'
>   [  440.946085] livepatch: 'livepatch_sample': unpatching complete
> 
> This same behavior holds in reverse, if we want to probe a livepatched
> function:
> 
>   % insmod samples/livepatch/livepatch-sample.ko
>   % perf probe --add cmdline_proc_show
>   Added new event:
>     probe:cmdline_proc_show (on cmdline_proc_show)
> 
>   You can now use it in all perf tools, such as:
> 
>           perf record -e probe:cmdline_proc_show -aR sleep 1
> 
>   % perf record -e probe:cmdline_proc_show -aR sleep 30
>   Error:
>   The sys_perf_event_open() syscall returned with 16 (Device or resource busy) for event (probe:cmdline_proc_show).
>   /bin/dmesg | grep -i perf may provide additional information.
> 
> 
> Now, if I read kernel/trace/trace_kprobe.c :: kprobe_dispatcher()
> correctly, it's only going to return !0 (indicating a modified regs->ip)
> when kprobe_perf_func() returns !0, i.e. regs->ip changes over a call to
> trace_call_bpf().
> 
> Aside: should kprobe_ftrace_handler() check that FTRACE_OPS_FL_IPMODIFY
> is set when a pre_handler returns !0?

NO, that flag has been shared among all ftrace-based kprobes, and checked
when registering. So what we need is to introduce a new kprobe flag which
states that this kprobe doesn't modify regs->ip. And kprobe prepare 2 ftrace_ops
1 is for IPMODIFY and 1 is for !IPMODIFY.


> 
> In kpatch #47, Josh suggested:
> 
> - If a kprobe handler needs to modify IP, user sets KPROBE_FLAG_IPMODIFY
>   flag to register_kprobe, and then kprobes sets FTRACE_OPS_FL_IPMODIFY
>   when registering with ftrace for that probe.
> 
> - If KPROBE_FLAG_IPMODIFY is not used, kprobe_ftrace_handler() can
>   detect when a kprobe handler changes regs->ip and restore it to its
>   original value (regs->ip = ip).
> 
> Is this something that could still be supported?  In cases like perf
> probe, could we get away with not setting FTRACE_OPS_FL_IPMODIFY?  The
> current way that we're applying that flag, kprobes and livepatch are
> mutually exclusive (for the same function).

It is not supported yet. But I can make it. wait a bit :)

Thank you,

> 
> Regards,
> 
> -- Joe


-- 
Masami Hiramatsu <mhiramat@kernel.org>