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

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On Wed, 15 Jun 2022 11:09:16 +0800
chuang <nashuiliang@xxxxxxxxx> wrote:

> On Wed, Jun 15, 2022 at 8:34 AM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> >
> > Hi Chuang,
> >
> > On Tue, 14 Jun 2022 17:06:33 +0800
> > Chuang W <nashuiliang@xxxxxxxxx> 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@xxxxxxxxxx/T/)
> 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

Ah, OK. In this case, "p->post_handler != NULL" indicates this
kprobe will modify regs->ip. Thus the ftrace will conflict
with livepatch. In this case, if ap->post_handler is not
rolled back, the ap will never be enabled.

Can you update the patch description something like below?

-----
In a scenario where livepatch and a aggrprobe coexist on the same
function entry, and if the aggrprobe has a post_handler, the
arm_kprobe() always fail because both of livepatch and the aggrprobe
with post_handler will use FTRACE_OPS_FL_IPMODIFY. This flag is not
allowed to be used by the different ftrace user on the same function
entry.
Since the register_aggr_kprobe() doesn't roll back the post_handler
when the arm_kprobe() is failed, this aggrprobe will not be available
from now on even if all kprobes on the aggrprobe don't have the
post_handler.

Fix to roll back the post_handler of the aggrprobe for this case.
With this fix, if the kprobe which has the post_handler is removed
from the aggrprobe (since arm_kprobe() failed), it will be available
again. 
-----

This will explains the technical background, what will happen
with current code, how it is fixed and what is the corrected
behavior.

Thank you,

> 
> 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@xxxxxxxxx>
> > > Cc: <stable@xxxxxxxxxxxxxxx>
> > > ---
> > >  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@xxxxxxxxxx>


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>