[PATCH] fprobe: Release rethook after the ftrace_ops is unregistered

[PATCH] fprobe: Release rethook after the ftrace_ops is unregistered

[Jiri Olsa]

While running bpf selftests it's possible to get following fault:

  general protection fault, probably for non-canonical address \
  0x6b6b6b6b6b6b6b6b: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC NOPTI
  ...
  Call Trace:
   <TASK>
   fprobe_handler+0xc1/0x270
   ? __pfx_bpf_testmod_init+0x10/0x10
   ? __pfx_bpf_testmod_init+0x10/0x10
   ? bpf_fentry_test1+0x5/0x10
   ? bpf_fentry_test1+0x5/0x10
   ? bpf_testmod_init+0x22/0x80
   ? do_one_initcall+0x63/0x2e0
   ? rcu_is_watching+0xd/0x40
   ? kmalloc_trace+0xaf/0xc0
   ? do_init_module+0x60/0x250
   ? __do_sys_finit_module+0xac/0x120
   ? do_syscall_64+0x37/0x90
   ? entry_SYSCALL_64_after_hwframe+0x72/0xdc
   </TASK>

In unregister_fprobe function we can't release fp->rethook while it's
possible there are some of its users still running on another cpu.

Moving rethook_free call after fp->ops is unregistered with
unregister_ftrace_function call.

Fixes: 5b0ab78998e3 ("fprobe: Add exit_handler support")
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 kernel/trace/fprobe.c | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
index 18d36842faf5..0121e8c0d54e 100644
--- a/kernel/trace/fprobe.c
+++ b/kernel/trace/fprobe.c
@@ -364,19 +364,13 @@ int unregister_fprobe(struct fprobe *fp)
 		    fp->ops.saved_func != fprobe_kprobe_handler))
 		return -EINVAL;
 
-	/*
-	 * rethook_free() starts disabling the rethook, but the rethook handlers
-	 * may be running on other processors at this point. To make sure that all
-	 * current running handlers are finished, call unregister_ftrace_function()
-	 * after this.
-	 */
-	if (fp->rethook)
-		rethook_free(fp->rethook);
-
 	ret = unregister_ftrace_function(&fp->ops);
 	if (ret < 0)
 		return ret;
 
+	if (fp->rethook)
+		rethook_free(fp->rethook);
+
 	ftrace_free_filter(&fp->ops);
 
 	return ret;
-- 
2.40.1

Re: [PATCH] fprobe: Release rethook after the ftrace_ops is unregistered

[Steven Rostedt]

Masami,

Want to take this via your probes/urgent branch and send it off to Linus?

-- Steve


On Thu, 15 Jun 2023 13:52:36 +0200
Jiri Olsa <jolsa@kernel.org> wrote:

> While running bpf selftests it's possible to get following fault:
> 
>   general protection fault, probably for non-canonical address \
>   0x6b6b6b6b6b6b6b6b: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC NOPTI
>   ...
>   Call Trace:
>    <TASK>
>    fprobe_handler+0xc1/0x270
>    ? __pfx_bpf_testmod_init+0x10/0x10
>    ? __pfx_bpf_testmod_init+0x10/0x10
>    ? bpf_fentry_test1+0x5/0x10
>    ? bpf_fentry_test1+0x5/0x10
>    ? bpf_testmod_init+0x22/0x80
>    ? do_one_initcall+0x63/0x2e0
>    ? rcu_is_watching+0xd/0x40
>    ? kmalloc_trace+0xaf/0xc0
>    ? do_init_module+0x60/0x250
>    ? __do_sys_finit_module+0xac/0x120
>    ? do_syscall_64+0x37/0x90
>    ? entry_SYSCALL_64_after_hwframe+0x72/0xdc
>    </TASK>
> 
> In unregister_fprobe function we can't release fp->rethook while it's
> possible there are some of its users still running on another cpu.
> 
> Moving rethook_free call after fp->ops is unregistered with
> unregister_ftrace_function call.
> 
> Fixes: 5b0ab78998e3 ("fprobe: Add exit_handler support")
> Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  kernel/trace/fprobe.c | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index 18d36842faf5..0121e8c0d54e 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -364,19 +364,13 @@ int unregister_fprobe(struct fprobe *fp)
>  		    fp->ops.saved_func != fprobe_kprobe_handler))
>  		return -EINVAL;
>  
> -	/*
> -	 * rethook_free() starts disabling the rethook, but the rethook handlers
> -	 * may be running on other processors at this point. To make sure that all
> -	 * current running handlers are finished, call unregister_ftrace_function()
> -	 * after this.
> -	 */
> -	if (fp->rethook)
> -		rethook_free(fp->rethook);
> -
>  	ret = unregister_ftrace_function(&fp->ops);
>  	if (ret < 0)
>  		return ret;
>  
> +	if (fp->rethook)
> +		rethook_free(fp->rethook);
> +
>  	ftrace_free_filter(&fp->ops);
>  
>  	return ret;

Re: [PATCH] fprobe: Release rethook after the ftrace_ops is unregistered

[Jiri Olsa]

On Thu, Jun 15, 2023 at 08:59:20AM -0400, Steven Rostedt wrote:
> 
> Masami,
> 
> Want to take this via your probes/urgent branch and send it off to Linus?

hi,
did this one make it into some tree?

thanks,
jirka


> 
> -- Steve
> 
> 
> On Thu, 15 Jun 2023 13:52:36 +0200
> Jiri Olsa <jolsa@kernel.org> wrote:
> 
> > While running bpf selftests it's possible to get following fault:
> > 
> >   general protection fault, probably for non-canonical address \
> >   0x6b6b6b6b6b6b6b6b: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC NOPTI
> >   ...
> >   Call Trace:
> >    <TASK>
> >    fprobe_handler+0xc1/0x270
> >    ? __pfx_bpf_testmod_init+0x10/0x10
> >    ? __pfx_bpf_testmod_init+0x10/0x10
> >    ? bpf_fentry_test1+0x5/0x10
> >    ? bpf_fentry_test1+0x5/0x10
> >    ? bpf_testmod_init+0x22/0x80
> >    ? do_one_initcall+0x63/0x2e0
> >    ? rcu_is_watching+0xd/0x40
> >    ? kmalloc_trace+0xaf/0xc0
> >    ? do_init_module+0x60/0x250
> >    ? __do_sys_finit_module+0xac/0x120
> >    ? do_syscall_64+0x37/0x90
> >    ? entry_SYSCALL_64_after_hwframe+0x72/0xdc
> >    </TASK>
> > 
> > In unregister_fprobe function we can't release fp->rethook while it's
> > possible there are some of its users still running on another cpu.
> > 
> > Moving rethook_free call after fp->ops is unregistered with
> > unregister_ftrace_function call.
> > 
> > Fixes: 5b0ab78998e3 ("fprobe: Add exit_handler support")
> > Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  kernel/trace/fprobe.c | 12 +++---------
> >  1 file changed, 3 insertions(+), 9 deletions(-)
> > 
> > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> > index 18d36842faf5..0121e8c0d54e 100644
> > --- a/kernel/trace/fprobe.c
> > +++ b/kernel/trace/fprobe.c
> > @@ -364,19 +364,13 @@ int unregister_fprobe(struct fprobe *fp)
> >  		    fp->ops.saved_func != fprobe_kprobe_handler))
> >  		return -EINVAL;
> >  
> > -	/*
> > -	 * rethook_free() starts disabling the rethook, but the rethook handlers
> > -	 * may be running on other processors at this point. To make sure that all
> > -	 * current running handlers are finished, call unregister_ftrace_function()
> > -	 * after this.
> > -	 */
> > -	if (fp->rethook)
> > -		rethook_free(fp->rethook);
> > -
> >  	ret = unregister_ftrace_function(&fp->ops);
> >  	if (ret < 0)
> >  		return ret;
> >  
> > +	if (fp->rethook)
> > +		rethook_free(fp->rethook);
> > +
> >  	ftrace_free_filter(&fp->ops);
> >  
> >  	return ret;
> 

Re: [PATCH] fprobe: Release rethook after the ftrace_ops is unregistered

[Masami Hiramatsu]

Hi Jiri,

On Thu, 15 Jun 2023 13:52:36 +0200
Jiri Olsa <jolsa@kernel.org> wrote:

> While running bpf selftests it's possible to get following fault:
> 
>   general protection fault, probably for non-canonical address \
>   0x6b6b6b6b6b6b6b6b: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC NOPTI
>   ...
>   Call Trace:
>    <TASK>
>    fprobe_handler+0xc1/0x270
>    ? __pfx_bpf_testmod_init+0x10/0x10
>    ? __pfx_bpf_testmod_init+0x10/0x10
>    ? bpf_fentry_test1+0x5/0x10
>    ? bpf_fentry_test1+0x5/0x10
>    ? bpf_testmod_init+0x22/0x80
>    ? do_one_initcall+0x63/0x2e0
>    ? rcu_is_watching+0xd/0x40
>    ? kmalloc_trace+0xaf/0xc0
>    ? do_init_module+0x60/0x250
>    ? __do_sys_finit_module+0xac/0x120
>    ? do_syscall_64+0x37/0x90
>    ? entry_SYSCALL_64_after_hwframe+0x72/0xdc
>    </TASK>
> 
> In unregister_fprobe function we can't release fp->rethook while it's
> possible there are some of its users still running on another cpu.

Ah, OK. rethook_free() invoked call_rcu(rethook_free_rcu) to free the
rethook, and it is possible rethook_free_rcu() is called before disabling
all fprobe, then `rethook_try_get(fp->rethook)` will access fp->rethook
which has been freed.

> 
> Moving rethook_free call after fp->ops is unregistered with
> unregister_ftrace_function call.
> 
> Fixes: 5b0ab78998e3 ("fprobe: Add exit_handler support")
> Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>

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

Thank you!


> ---
>  kernel/trace/fprobe.c | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index 18d36842faf5..0121e8c0d54e 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -364,19 +364,13 @@ int unregister_fprobe(struct fprobe *fp)
>  		    fp->ops.saved_func != fprobe_kprobe_handler))
>  		return -EINVAL;
>  
> -	/*
> -	 * rethook_free() starts disabling the rethook, but the rethook handlers
> -	 * may be running on other processors at this point. To make sure that all
> -	 * current running handlers are finished, call unregister_ftrace_function()
> -	 * after this.
> -	 */
> -	if (fp->rethook)
> -		rethook_free(fp->rethook);
> -
>  	ret = unregister_ftrace_function(&fp->ops);
>  	if (ret < 0)
>  		return ret;
>  
> +	if (fp->rethook)
> +		rethook_free(fp->rethook);
> +
>  	ftrace_free_filter(&fp->ops);
>  
>  	return ret;
> -- 
> 2.40.1
> 


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

Re: [PATCH] fprobe: Release rethook after the ftrace_ops is unregistered

[Masami Hiramatsu]

On Tue, 27 Jun 2023 23:33:06 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> > index 18d36842faf5..0121e8c0d54e 100644
> > --- a/kernel/trace/fprobe.c
> > +++ b/kernel/trace/fprobe.c
> > @@ -364,19 +364,13 @@ int unregister_fprobe(struct fprobe *fp)
> >  		    fp->ops.saved_func != fprobe_kprobe_handler))
> >  		return -EINVAL;
> >  
> > -	/*
> > -	 * rethook_free() starts disabling the rethook, but the rethook handlers
> > -	 * may be running on other processors at this point. To make sure that all
> > -	 * current running handlers are finished, call unregister_ftrace_function()
> > -	 * after this.
> > -	 */

Oh, wait, here is an important comment. If a rethook handler is still running
(because it hooks target function exit), returning from unregister_fprobe()
right after rethook_free() may cause another issue.

rethook_free() clears 'rh->handler', so after calling rethook_free(), we
can ensure no NEW rethook handler (means fprobe_exit_handler()) is called.
However, it doesn't mean there is no current running fprobe_exit_handler().
Thus if unregister_fprobe() caller releases the 'fp' right after returning
from unregister_fprobe(), current running fprobe_exit_handler() can access
'fp' (use-after-free).

Thus we need to add below code with this patch;
	/*
	 * The rethook handlers may be running on other processors at this point.
	 * To make sure that all current running handlers are finished, disable
	 * rethook by clearing handler and call unregister_ftrace_function()
	 * to ensure all running rethook handlers exit. And call rethook_free().
	 */
	if (fp->rethook)
		WRITE_ONCE(fp->rethook->handler, NULL);

> > -	if (fp->rethook)
> > -		rethook_free(fp->rethook);
> > -
> >  	ret = unregister_ftrace_function(&fp->ops);
> >  	if (ret < 0)
> >  		return ret;
> >  
> > +	if (fp->rethook)
> > +		rethook_free(fp->rethook);
> > +
> >  	ftrace_free_filter(&fp->ops);
> >  
> >  	return ret;

Thank you,

> > -- 
> > 2.40.1
> > 
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@kernel.org>


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

[PATCH] fprobe: Ensure running fprobe_exit_handler() finished before calling rethook_free()

[Masami Hiramatsu (Google)]

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

Ensure running fprobe_exit_handler() has finished before
calling rethook_free() in the unregister_fprobe() so that caller can free
the fprobe right after unregister_fprobe().

unregister_fprobe() ensured that all running fprobe_entry/exit_handler()
have finished by calling unregister_ftrace_function() which synchronizes
RCU. But commit 5f81018753df ("fprobe: Release rethook after the ftrace_ops
is unregistered") changed to call rethook_free() after
unregister_ftrace_function(). So call rethook_stop() to make rethook
disabled before unregister_ftrace_function() and ensure it again.

Fixes: 5f81018753df ("fprobe: Release rethook after the ftrace_ops is unregistered")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 include/linux/rethook.h |    1 +
 kernel/trace/fprobe.c   |    3 +++
 kernel/trace/rethook.c  |   13 +++++++++++++
 3 files changed, 17 insertions(+)

diff --git a/include/linux/rethook.h b/include/linux/rethook.h
index c8ac1e5afcd1..bdbe6717f45a 100644
--- a/include/linux/rethook.h
+++ b/include/linux/rethook.h
@@ -59,6 +59,7 @@ struct rethook_node {
 };
 
 struct rethook *rethook_alloc(void *data, rethook_handler_t handler);
+void rethook_stop(struct rethook *rh);
 void rethook_free(struct rethook *rh);
 void rethook_add_node(struct rethook *rh, struct rethook_node *node);
 struct rethook_node *rethook_try_get(struct rethook *rh);
diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
index 0121e8c0d54e..75517667b54f 100644
--- a/kernel/trace/fprobe.c
+++ b/kernel/trace/fprobe.c
@@ -364,6 +364,9 @@ int unregister_fprobe(struct fprobe *fp)
 		    fp->ops.saved_func != fprobe_kprobe_handler))
 		return -EINVAL;
 
+	if (fp->rethook)
+		rethook_stop(fp->rethook);
+
 	ret = unregister_ftrace_function(&fp->ops);
 	if (ret < 0)
 		return ret;
diff --git a/kernel/trace/rethook.c b/kernel/trace/rethook.c
index 60f6cb2b486b..468006cce7ca 100644
--- a/kernel/trace/rethook.c
+++ b/kernel/trace/rethook.c
@@ -53,6 +53,19 @@ static void rethook_free_rcu(struct rcu_head *head)
 		kfree(rh);
 }
 
+/**
+ * rethook_stop() - Stop using a rethook.
+ * @rh: the struct rethook to stop.
+ *
+ * Stop using a rethook to prepare for freeing it. If you want to wait for
+ * all running rethook handler before calling rethook_free(), you need to
+ * call this first and wait RCU, and call rethook_free().
+ */
+void rethook_stop(struct rethook *rh)
+{
+	WRITE_ONCE(rh->handler, NULL);
+}
+
 /**
  * rethook_free() - Free struct rethook.
  * @rh: the struct rethook to be freed.

Re: [PATCH] fprobe: Ensure running fprobe_exit_handler() finished before calling rethook_free()

[Masami Hiramatsu]

On Wed, 28 Jun 2023 23:44:02 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Ensure running fprobe_exit_handler() has finished before
> calling rethook_free() in the unregister_fprobe() so that caller can free
> the fprobe right after unregister_fprobe().
> 
> unregister_fprobe() ensured that all running fprobe_entry/exit_handler()
> have finished by calling unregister_ftrace_function() which synchronizes
> RCU. But commit 5f81018753df ("fprobe: Release rethook after the ftrace_ops
> is unregistered") changed to call rethook_free() after
> unregister_ftrace_function(). So call rethook_stop() to make rethook
> disabled before unregister_ftrace_function() and ensure it again.
>

Steve, can you review this? without this fix, Jiri's patch may cause another
timing issue.

Thanks, 
 
> Fixes: 5f81018753df ("fprobe: Release rethook after the ftrace_ops is unregistered")
> Cc: stable@vger.kernel.org
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>  include/linux/rethook.h |    1 +
>  kernel/trace/fprobe.c   |    3 +++
>  kernel/trace/rethook.c  |   13 +++++++++++++
>  3 files changed, 17 insertions(+)
> 
> diff --git a/include/linux/rethook.h b/include/linux/rethook.h
> index c8ac1e5afcd1..bdbe6717f45a 100644
> --- a/include/linux/rethook.h
> +++ b/include/linux/rethook.h
> @@ -59,6 +59,7 @@ struct rethook_node {
>  };
>  
>  struct rethook *rethook_alloc(void *data, rethook_handler_t handler);
> +void rethook_stop(struct rethook *rh);
>  void rethook_free(struct rethook *rh);
>  void rethook_add_node(struct rethook *rh, struct rethook_node *node);
>  struct rethook_node *rethook_try_get(struct rethook *rh);
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index 0121e8c0d54e..75517667b54f 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -364,6 +364,9 @@ int unregister_fprobe(struct fprobe *fp)
>  		    fp->ops.saved_func != fprobe_kprobe_handler))
>  		return -EINVAL;
>  
> +	if (fp->rethook)
> +		rethook_stop(fp->rethook);
> +
>  	ret = unregister_ftrace_function(&fp->ops);
>  	if (ret < 0)
>  		return ret;
> diff --git a/kernel/trace/rethook.c b/kernel/trace/rethook.c
> index 60f6cb2b486b..468006cce7ca 100644
> --- a/kernel/trace/rethook.c
> +++ b/kernel/trace/rethook.c
> @@ -53,6 +53,19 @@ static void rethook_free_rcu(struct rcu_head *head)
>  		kfree(rh);
>  }
>  
> +/**
> + * rethook_stop() - Stop using a rethook.
> + * @rh: the struct rethook to stop.
> + *
> + * Stop using a rethook to prepare for freeing it. If you want to wait for
> + * all running rethook handler before calling rethook_free(), you need to
> + * call this first and wait RCU, and call rethook_free().
> + */
> +void rethook_stop(struct rethook *rh)
> +{
> +	WRITE_ONCE(rh->handler, NULL);
> +}
> +
>  /**
>   * rethook_free() - Free struct rethook.
>   * @rh: the struct rethook to be freed.
> 


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

Re: [PATCH] fprobe: Ensure running fprobe_exit_handler() finished before calling rethook_free()

[Steven Rostedt]

On Wed, 28 Jun 2023 23:44:02 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Ensure running fprobe_exit_handler() has finished before
> calling rethook_free() in the unregister_fprobe() so that caller can free
> the fprobe right after unregister_fprobe().
> 
> unregister_fprobe() ensured that all running fprobe_entry/exit_handler()
> have finished by calling unregister_ftrace_function() which synchronizes
> RCU. But commit 5f81018753df ("fprobe: Release rethook after the
> ftrace_ops is unregistered") changed to call rethook_free() after
> unregister_ftrace_function(). So call rethook_stop() to make rethook
> disabled before unregister_ftrace_function() and ensure it again.

I'm confused. I still don't understand why it is bad to call
unregister_ftrace_function() *before* rethook_free().

Can you show the race condition you are trying to avoid?

-- Steve



> 
> Fixes: 5f81018753df ("fprobe: Release rethook after the ftrace_ops is
> unregistered") Cc: stable@vger.kernel.org
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>  include/linux/rethook.h |    1 +
>  kernel/trace/fprobe.c   |    3 +++
>  kernel/trace/rethook.c  |   13 +++++++++++++
>  3 files changed, 17 insertions(+)
> 
> diff --git a/include/linux/rethook.h b/include/linux/rethook.h
> index c8ac1e5afcd1..bdbe6717f45a 100644
> --- a/include/linux/rethook.h
> +++ b/include/linux/rethook.h
> @@ -59,6 +59,7 @@ struct rethook_node {
>  };
>  
>  struct rethook *rethook_alloc(void *data, rethook_handler_t handler);
> +void rethook_stop(struct rethook *rh);
>  void rethook_free(struct rethook *rh);
>  void rethook_add_node(struct rethook *rh, struct rethook_node *node);
>  struct rethook_node *rethook_try_get(struct rethook *rh);
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index 0121e8c0d54e..75517667b54f 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -364,6 +364,9 @@ int unregister_fprobe(struct fprobe *fp)
>  		    fp->ops.saved_func != fprobe_kprobe_handler))
>  		return -EINVAL;
>  
> +	if (fp->rethook)
> +		rethook_stop(fp->rethook);
> +
>  	ret = unregister_ftrace_function(&fp->ops);
>  	if (ret < 0)
>  		return ret;
> diff --git a/kernel/trace/rethook.c b/kernel/trace/rethook.c
> index 60f6cb2b486b..468006cce7ca 100644
> --- a/kernel/trace/rethook.c
> +++ b/kernel/trace/rethook.c
> @@ -53,6 +53,19 @@ static void rethook_free_rcu(struct rcu_head *head)
>  		kfree(rh);
>  }
>  
> +/**
> + * rethook_stop() - Stop using a rethook.
> + * @rh: the struct rethook to stop.
> + *
> + * Stop using a rethook to prepare for freeing it. If you want to wait
> for
> + * all running rethook handler before calling rethook_free(), you need to
> + * call this first and wait RCU, and call rethook_free().
> + */
> +void rethook_stop(struct rethook *rh)
> +{
> +	WRITE_ONCE(rh->handler, NULL);
> +}
> +
>  /**
>   * rethook_free() - Free struct rethook.
>   * @rh: the struct rethook to be freed.

Re: [PATCH] fprobe: Ensure running fprobe_exit_handler() finished before calling rethook_free()

[Masami Hiramatsu]

On Wed, 5 Jul 2023 21:26:57 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Wed, 28 Jun 2023 23:44:02 +0900
> "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
> 
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > 
> > Ensure running fprobe_exit_handler() has finished before
> > calling rethook_free() in the unregister_fprobe() so that caller can free
> > the fprobe right after unregister_fprobe().
> > 
> > unregister_fprobe() ensured that all running fprobe_entry/exit_handler()
> > have finished by calling unregister_ftrace_function() which synchronizes
> > RCU. But commit 5f81018753df ("fprobe: Release rethook after the
> > ftrace_ops is unregistered") changed to call rethook_free() after
> > unregister_ftrace_function(). So call rethook_stop() to make rethook
> > disabled before unregister_ftrace_function() and ensure it again.
> 
> I'm confused. I still don't understand why it is bad to call
> unregister_ftrace_function() *before* rethook_free().
> 
> Can you show the race condition you are trying to avoid?

Yes. This is ensuring all handlers exit when returning from
unregister_fprobe() so that the caller can release the data which will be
accessed from the handlers. The entry handler is safe because
unregister_ftrace_function() waits for the ftrace handlers. But that is
not enough for the exit handler.

With only Jiri's patch, following flow can happen;

------
 CPU1                              CPU2
 call unregister_fprobe()
 ...
                                   __fprobe_handler()
                                   rethook_hook() on probed function
 unregister_ftrace_function()
                                   return from probed function
                                   rethook hooks
                                   find rh->handler == fprobe_exit_handler
                                   call fprobe_exit_handler()
 rethook_free():
   set rh->handler = NULL;
 return from unreigster_fprobe;
                                   call fp->exit_handler() <- (*)

(*) In this point, the exit handler is called after returning from 
unregister_fprobe().
------

So, this patch changes it as following;
------
 CPU1                              CPU2
 call unregister_fprobe()
 ...
 rethook_stop():
   set rh->handler = NULL;
                                   __fprobe_handler()
                                   rethook_hook() on probed function
 unregister_ftrace_function()
                                   return from probed function
                                   rethook hooks
                                   find rh->handler == NULL
                                   return from rethook
 rethook_free()
 return from unreigster_fprobe;
------

I can also just put a synchronize_sched_rcu() right after rethook_free()
to wait for all running fprobe_exit_handler() too.

Thank you,

> 
> -- Steve
> 
> 
> 
> > 
> > Fixes: 5f81018753df ("fprobe: Release rethook after the ftrace_ops is
> > unregistered") Cc: stable@vger.kernel.org
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > ---
> >  include/linux/rethook.h |    1 +
> >  kernel/trace/fprobe.c   |    3 +++
> >  kernel/trace/rethook.c  |   13 +++++++++++++
> >  3 files changed, 17 insertions(+)
> > 
> > diff --git a/include/linux/rethook.h b/include/linux/rethook.h
> > index c8ac1e5afcd1..bdbe6717f45a 100644
> > --- a/include/linux/rethook.h
> > +++ b/include/linux/rethook.h
> > @@ -59,6 +59,7 @@ struct rethook_node {
> >  };
> >  
> >  struct rethook *rethook_alloc(void *data, rethook_handler_t handler);
> > +void rethook_stop(struct rethook *rh);
> >  void rethook_free(struct rethook *rh);
> >  void rethook_add_node(struct rethook *rh, struct rethook_node *node);
> >  struct rethook_node *rethook_try_get(struct rethook *rh);
> > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> > index 0121e8c0d54e..75517667b54f 100644
> > --- a/kernel/trace/fprobe.c
> > +++ b/kernel/trace/fprobe.c
> > @@ -364,6 +364,9 @@ int unregister_fprobe(struct fprobe *fp)
> >  		    fp->ops.saved_func != fprobe_kprobe_handler))
> >  		return -EINVAL;
> >  
> > +	if (fp->rethook)
> > +		rethook_stop(fp->rethook);
> > +
> >  	ret = unregister_ftrace_function(&fp->ops);
> >  	if (ret < 0)
> >  		return ret;
> > diff --git a/kernel/trace/rethook.c b/kernel/trace/rethook.c
> > index 60f6cb2b486b..468006cce7ca 100644
> > --- a/kernel/trace/rethook.c
> > +++ b/kernel/trace/rethook.c
> > @@ -53,6 +53,19 @@ static void rethook_free_rcu(struct rcu_head *head)
> >  		kfree(rh);
> >  }
> >  
> > +/**
> > + * rethook_stop() - Stop using a rethook.
> > + * @rh: the struct rethook to stop.
> > + *
> > + * Stop using a rethook to prepare for freeing it. If you want to wait
> > for
> > + * all running rethook handler before calling rethook_free(), you need to
> > + * call this first and wait RCU, and call rethook_free().
> > + */
> > +void rethook_stop(struct rethook *rh)
> > +{
> > +	WRITE_ONCE(rh->handler, NULL);
> > +}
> > +
> >  /**
> >   * rethook_free() - Free struct rethook.
> >   * @rh: the struct rethook to be freed.
> 


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

Re: [PATCH] fprobe: Ensure running fprobe_exit_handler() finished before calling rethook_free()

[Steven Rostedt]

On Thu, 6 Jul 2023 14:10:12 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> With only Jiri's patch, following flow can happen;
> 
> ------
>  CPU1                              CPU2
>  call unregister_fprobe()
>  ...
>                                    __fprobe_handler()
>                                    rethook_hook() on probed function
>  unregister_ftrace_function()
>                                    return from probed function
>                                    rethook hooks
>                                    find rh->handler == fprobe_exit_handler
>                                    call fprobe_exit_handler()
>  rethook_free():
>    set rh->handler = NULL;
>  return from unreigster_fprobe;
>                                    call fp->exit_handler() <- (*)
> 
> (*) In this point, the exit handler is called after returning from 
> unregister_fprobe().
> ------
> 
> So, this patch changes it as following;
> ------
>  CPU1                              CPU2
>  call unregister_fprobe()
>  ...
>  rethook_stop():
>    set rh->handler = NULL;
>                                    __fprobe_handler()
>                                    rethook_hook() on probed function
>  unregister_ftrace_function()
>                                    return from probed function
>                                    rethook hooks
>                                    find rh->handler == NULL
>                                    return from rethook
>  rethook_free()
>  return from unreigster_fprobe;
> ------
> 
> I can also just put a synchronize_sched_rcu() right after rethook_free()
> to wait for all running fprobe_exit_handler() too.
> 

This makes more sense. Can you please add the above to the change log.

Thanks,

-- Steve

Re: [PATCH] fprobe: Ensure running fprobe_exit_handler() finished before calling rethook_free()

[Masami Hiramatsu]

On Thu, 6 Jul 2023 09:56:24 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Thu, 6 Jul 2023 14:10:12 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > With only Jiri's patch, following flow can happen;
> > 
> > ------
> >  CPU1                              CPU2
> >  call unregister_fprobe()
> >  ...
> >                                    __fprobe_handler()
> >                                    rethook_hook() on probed function
> >  unregister_ftrace_function()
> >                                    return from probed function
> >                                    rethook hooks
> >                                    find rh->handler == fprobe_exit_handler
> >                                    call fprobe_exit_handler()
> >  rethook_free():
> >    set rh->handler = NULL;
> >  return from unreigster_fprobe;
> >                                    call fp->exit_handler() <- (*)
> > 
> > (*) In this point, the exit handler is called after returning from 
> > unregister_fprobe().
> > ------
> > 
> > So, this patch changes it as following;
> > ------
> >  CPU1                              CPU2
> >  call unregister_fprobe()
> >  ...
> >  rethook_stop():
> >    set rh->handler = NULL;
> >                                    __fprobe_handler()
> >                                    rethook_hook() on probed function
> >  unregister_ftrace_function()
> >                                    return from probed function
> >                                    rethook hooks
> >                                    find rh->handler == NULL
> >                                    return from rethook
> >  rethook_free()
> >  return from unreigster_fprobe;
> > ------
> > 
> > I can also just put a synchronize_sched_rcu() right after rethook_free()
> > to wait for all running fprobe_exit_handler() too.
> > 
> 
> This makes more sense. Can you please add the above to the change log.

OK, let me update it.

Thanks!

> 
> Thanks,
> 
> -- Steve


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