[PATCH v2] fprobe: Ensure running fprobe_exit_handler() finished before calling rethook_free()

[PATCH v2] 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.

Here is the possible code flow that can call the exit handler after
unregister_fprobe().

------
 CPU1                              CPU2
 call unregister_fprobe(fp)
 ...
                                   __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() <- (*)
------

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

This fixes 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;
------


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>
---
 Changes in v2:
 - Update changelog to add a problematic code flow.
---
 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 v2] fprobe: Ensure running fprobe_exit_handler() finished before calling rethook_free()

[Steven Rostedt]

On Fri,  7 Jul 2023 23:03:19 +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.
> 
> Here is the possible code flow that can call the exit handler after
> unregister_fprobe().
> 
> ------
>  CPU1                              CPU2
>  call unregister_fprobe(fp)
>  ...
>                                    __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() <- (*)
> ------
> 
> (*) At this point, the exit handler is called after returning from
> unregister_fprobe().
> 
> This fixes 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;
> ------
> 
> 
> 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>

Looks good.

Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>


> ---
>  Changes in v2:
>  - Update changelog to add a problematic code flow.

Nit, for making forensic analysis easier in the future, I now always add a
link to the previous version. That is:

Changes since v1: https://lore.kernel.org/linux-trace-kernel/168796344232.46347.7947681068822514750.stgit@devnote2/
- Update changelog to add a problematic code flow.

-- Steve


> ---
>  include/linux/rethook.h |    1 +
>  kernel/trace/fprobe.c   |    3 +++
>  kernel/trace/rethook.c  |   13 +++++++++++++
>  3 files changed, 17 insertions(+)
> 

Re: [PATCH v2] fprobe: Ensure running fprobe_exit_handler() finished before calling rethook_free()

[Masami Hiramatsu]

On Mon, 10 Jul 2023 18:04:22 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Fri,  7 Jul 2023 23:03:19 +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.
> > 
> > Here is the possible code flow that can call the exit handler after
> > unregister_fprobe().
> > 
> > ------
> >  CPU1                              CPU2
> >  call unregister_fprobe(fp)
> >  ...
> >                                    __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() <- (*)
> > ------
> > 
> > (*) At this point, the exit handler is called after returning from
> > unregister_fprobe().
> > 
> > This fixes 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;
> > ------
> > 
> > 
> > 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>
> 
> Looks good.
> 
> Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Thank you :)

> 
> 
> > ---
> >  Changes in v2:
> >  - Update changelog to add a problematic code flow.
> 
> Nit, for making forensic analysis easier in the future, I now always add a
> link to the previous version. That is:
> 
> Changes since v1: https://lore.kernel.org/linux-trace-kernel/168796344232.46347.7947681068822514750.stgit@devnote2/
> - Update changelog to add a problematic code flow.

OK, I'll add it for an isolated patch too.

Thanks!

> 
> -- Steve
> 
> 
> > ---
> >  include/linux/rethook.h |    1 +
> >  kernel/trace/fprobe.c   |    3 +++
> >  kernel/trace/rethook.c  |   13 +++++++++++++
> >  3 files changed, 17 insertions(+)
> > 


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