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

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

[Chuang W]

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.

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.

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

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

[Masami Hiramatsu]

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?

> 
> 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,

> 
> 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>

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

[chuang]

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.org/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

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>

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

[Masami Hiramatsu]

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

> 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.org/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@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>


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