Re: [PATCH] kprobes: Rollback post_handler on failed arm_kprobe()

On Wed, Jun 15, 2022 at 8:34 AM Masami Hiramatsu mhiramat@kernel.org wrote:

Hi Chuang,

On Tue, 14 Jun 2022 17:06:33 +0800 Chuang W nashuiliang@gmail.com wrote:

...

In a scenario where livepatch and aggrprobe coexist, if arm_kprobe() returns an error, ap.post_handler, while has been modified to p.post_handler, is not rolled back.

Would you mean 'coexist' on the same function?

Yes, It's the same function.

...

When ap.post_handler is not NULL (not rolled back), the caller (e.g. register_kprobe/enable_kprobe) of arm_kprobe_ftrace() will always fail.

It seems this explanation and the actual code does not match. Can you tell me what actually you observed?

Thank you,

I briefly describe the steps involved, a patch (kprobes: Rollback kprobe flags on failed arm_kprobe, https://lore.kernel.org/all/20220612213156.1323776351ee1be3cabc7fcc@kernel.o...) must be added, otherwise it will panic:

1) add a livepatch

$ insmod livepatch-XXX.ko

2) add a kprobe using tracefs API

$ echo 'p:mykprobe XXX' > /sys/kernel/debug/tracing/kprobe_events

At this time, XXX is a simple kprobe, kprobe->post_handler = NULL.

3) add a second kprobe using raw kprobe API (i.e. register_kprobe), the new kprobe->post_handler != NULL

$ insmod kprobe_XXX.ko $ insmod: ERROR: could not insert module kprobe_XXX.ko: Device or resource busy

This will fail (as expected). However, XXX is modified to an aggrprobe. agKprobe->post_handler = aggr_post_handler, it's not rolled back on failed arm_kprobe().

4) add a third kprobe using bpftrace/bcc tool

$ bpftrace -e 'kprobe:XXX {printf("%s", kstack());}' Attaching 1 probe... perf_event_open(/sys/kernel/debug/tracing/events/kprobes/p_XXX_0_1_bcc_440/id): Device or resource busy Error attaching probe: 'kprobe:blkcg_destroy_blkgs' $ bpftrace -e 'kprobe:XXX {printf("%s", kstack());}' Attaching 1 probe... perf_event_open(/sys/kernel/debug/tracing/events/kprobes/p_XXX_0_1_bcc_440/id): Device or resource busy Error attaching probe: 'kprobe:blkcg_destroy_blkgs'

This will always fail (not as expected).

...

Fixes: 12310e343755 ("kprobes: Propagate error from arm_kprobe_ftrace()") Signed-off-by: Chuang W nashuiliang@gmail.com Cc: stable@vger.kernel.org

---

kernel/kprobes.c | 5 +++++ 1 file changed, 5 insertions(+)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c index f214f8c088ed..0610b02a3a05 100644 --- a/kernel/kprobes.c +++ b/kernel/kprobes.c @@ -1300,6 +1300,7 @@ static int register_aggr_kprobe(struct kprobe *orig_p, struct kprobe *p) { int ret = 0; struct kprobe *ap = orig_p;

• kprobe_post_handler_t old_post_handler = NULL;
  
  cpus_read_lock();
  
  

@@ -1351,6 +1352,9 @@ static int register_aggr_kprobe(struct kprobe *orig_p, struct kprobe *p)

  /* Copy the insn slot of 'p' to 'ap'. */
  copy_kprobe(ap, p);

• /* save the old post_handler */
  

• old_post_handler = ap->post_handler;
  ret = add_new_kprobe(ap, p);
  
  

out: @@ -1365,6 +1369,7 @@ static int register_aggr_kprobe(struct kprobe *orig_p, struct kprobe *p) ret = arm_kprobe(ap); if (ret) { ap->flags |= KPROBE_FLAG_DISABLED;

•                         ap->post_handler = old_post_handler;
                          list_del_rcu(&p->list);
                          synchronize_rcu();
                  }
  

-- 2.34.1

-- Masami Hiramatsu (Google) mhiramat@kernel.org