[PATCH] fprobe: Protect fprobe_return() with guard(rcu)()

[PATCH] fprobe: Protect fprobe_return() with guard(rcu)()

[Masami Hiramatsu (Google)]

Hi,

Here is a bugfix (possible UAF) for fprobe found by Sashiko[1].
[1] https://sashiko.dev/#/bug/linux-e46bcd68-4a56-4f19-a255-e3772980e5e3

I think this fix is a short-term fix to make it safer. Eventually
I would like to replace all guard(rcu)() from fprobe with
preempt_disable_notrace(), because currently it introduces unneeded
overhead to fprobe.

- Introduce new call_rcu_tasks_rude() for async call.
- Add special non-preempt mode flag to rhashtable, which
  uses call_rcu_tasks_rude() instead of call_rcu() 
- Switching to use synchronize_rcu_tasks_rude() for unregistering.
- Replace call_rcu() with call_rcu_tasks_rude() in BPF.

But this is heavy depends on Tasks RCU, so I would like to check
with the RCU maintainers whether this idea aligns with the concept
behind Tasks RCU updates.

Thanks,

---

Masami Hiramatsu (Google) (1):
      fprobe: Protect fprobe_return() with guard(rcu)()


 kernel/trace/fprobe.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

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

[PATCH] fprobe: Protect fprobe_return() with guard(rcu)()

[Masami Hiramatsu (Google)]

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

In fprobe_return(), the shadow-stack iteration and exit_handler
invocations were protected by preempt_disable_notrace().
However, unregister_fprobe() and unregister_fprobe_async() (used
by BPF kprobe-multi) rely on standard RCU grace periods (synchronize_rcu()
and call_rcu()) to wait until the fprobe is no longer in use before
freeing it.

In preemptible kernels (CONFIG_PREEMPT_RCU=y), standard RCU grace
periods do not wait for pure preempt_disable_notrace() critical
sections. Consequently, an unregistered fprobe may be freed while
a concurrent CPU executing fprobe_return() is still running
fp->exit_handler(), causing a use-after-free.

To resolve this, protect fprobe_return() with guard(rcu)() matching
fprobe_fgraph_entry(). This ensures both synchronous unregister_fprobe()
and asynchronous unregister_fprobe_async() safely wait for in-flight
exit_handlers to complete via standard RCU grace periods.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/bug/linux-e46bcd68-4a56-4f19-a255-e3772980e5e3
Fixes: 657b594b2084 ("fprobe: Fix unregister_fprobe() to wait for RCU grace period")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 kernel/trace/fprobe.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
index 9f2d98181779..c3e1580bc648 100644
--- a/kernel/trace/fprobe.c
+++ b/kernel/trace/fprobe.c
@@ -671,7 +671,7 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
 	size_words = SIZE_IN_LONG(size);
 	ret_ip = ftrace_regs_get_instruction_pointer(fregs);
 
-	preempt_disable_notrace();
+	guard(rcu)();
 
 	curr = 0;
 	while (size_words > curr) {
@@ -687,7 +687,6 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
 		}
 		curr += size;
 	}
-	preempt_enable_notrace();
 }
 NOKPROBE_SYMBOL(fprobe_return);
 

Re: [PATCH] fprobe: Protect fprobe_return() with guard(rcu)()

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] tracing: fprobe UAF and lockdep splat due to invalid guard(rcu)
--

commit 4b472c8849c5257c65478348507af6f9fee7c3af
Author: Masami Hiramatsu (Google) <mhiramat@kernel.org>

fprobe: Protect fprobe_return() with guard(rcu)()

In fprobe_return(), the shadow-stack iteration and exit_handler
invocations were protected by preempt_disable_notrace().
However, unregister_fprobe() and unregister_fprobe_async() (used
by BPF kprobe-multi) rely on standard RCU grace periods (synchronize_rcu()
and call_rcu()) to wait until the fprobe is no longer in use before
freeing it.

In preemptible kernels (CONFIG_PREEMPT_RCU=y), standard RCU grace
periods do not wait for pure preempt_disable_notrace() critical
sections. Consequently, an unregistered fprobe may be freed while
a concurrent CPU executing fprobe_return() is still running
fp->exit_handler(), causing a use-after-free.

To resolve this, protect fprobe_return() with guard(rcu)() matching
fprobe_fgraph_entry(). This ensures both synchronous unregister_fprobe()
and asynchronous unregister_fprobe_async() safely wait for in-flight
exit_handlers to complete via standard RCU grace periods.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/bug/linux-e46bcd68-4a56-4f19-a255-e3772980e5e3
Fixes: 657b594b2084 ("fprobe: Fix unregister_fprobe() to wait for RCU grace period")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index 1e9b00997ff27..e34c3f7667848 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -656,7 +656,7 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
>  	size_words = SIZE_IN_LONG(size);
>  	ret_ip = ftrace_regs_get_instruction_pointer(fregs);
>  
> -	preempt_disable_notrace();
> +	guard(rcu)();

[Severity: High]
Since ftrace operations lacking the FTRACE_OPS_FL_RCU flag can execute in
idle contexts where RCU is not watching, does adding guard(rcu)() here
trigger an illegal RCU lock usage splat?

Additionally, since RCU doesn't track read-side critical sections that
start when RCU isn't watching, does this mean synchronize_rcu() during
unregistration will fail to wait for these handlers, leaving the fprobe
vulnerable to the same use-after-free this patch attempts to fix?

[ ... ]


Looking at fprobe_fgraph_entry() in the same file:

kernel/trace/fprobe.c:fprobe_fgraph_entry() {
    ...
	if (WARN_ON_ONCE(!fregs))
		return 0;

	guard(rcu)();
	head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params);
    ...
}

[Severity: High]
This isn't a bug introduced by this patch, but fprobe_fgraph_entry() also
uses guard(rcu)(). Could this pre-existing usage cause the same lockdep
splats in idle contexts?


Looking at how fgraph operations are initialized:

kernel/trace/fgraph.c:fgraph_init_ops() {
	dst_ops->flags = FTRACE_OPS_FL_PID | FTRACE_OPS_GRAPH_STUB;

#ifdef CONFIG_DYNAMIC_FTRACE
    ...
}

[Severity: High]
This isn't a bug introduced by this patch, but does the lack of
FTRACE_OPS_FL_RCU in these global fgraph operations expose all fgraph
users to execution when RCU is not watching? Should this flag be added
here to prevent tracing callbacks from being invoked in contexts where
RCU is offline?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/179055575009.241711.6358052647499787191.stgit@devnote2?part=1

Re: [PATCH] fprobe: Protect fprobe_return() with guard(rcu)()

[Masami Hiramatsu]

On Mon, 28 Sep 2026 00:51:14 +0000
sashiko-bot@kernel.org wrote:

> > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> > index 1e9b00997ff27..e34c3f7667848 100644
> > --- a/kernel/trace/fprobe.c
> > +++ b/kernel/trace/fprobe.c
> > @@ -656,7 +656,7 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
> >  	size_words = SIZE_IN_LONG(size);
> >  	ret_ip = ftrace_regs_get_instruction_pointer(fregs);
> >  
> > -	preempt_disable_notrace();
> > +	guard(rcu)();
> 
> [Severity: High]
> Since ftrace operations lacking the FTRACE_OPS_FL_RCU flag can execute in
> idle contexts where RCU is not watching, does adding guard(rcu)() here
> trigger an illegal RCU lock usage splat?

Hm, OK. Maybe we need to add something like checking rcu_watching, but
FTRACE_OPS_FL_RCU does not work correctly because fgraph_init_ops() only
configures subops (gops->ops). ftrace_startup_subops() does not propagete
that flag to graph_ops. (I guess that is yet another problem?)

To guarantee no illegal RCU usage or lockdep splats if an instrumented
function returns while RCU is offline, we can check 

 if (unlikely(!rcu_is_watching()))
	return;

before acquiring guard(rcu)().

> 
> Additionally, since RCU doesn't track read-side critical sections that
> start when RCU isn't watching, does this mean synchronize_rcu() during
> unregistration will fail to wait for these handlers, leaving the fprobe
> vulnerable to the same use-after-free this patch attempts to fix?

As I said above, if we add the rcu_is_watching() checks, we can avoid
this issue.


> 
> [ ... ]
> 
> 
> Looking at fprobe_fgraph_entry() in the same file:
> 
> kernel/trace/fprobe.c:fprobe_fgraph_entry() {
>     ...
> 	if (WARN_ON_ONCE(!fregs))
> 		return 0;
> 
> 	guard(rcu)();
> 	head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params);
>     ...
> }
> 
> [Severity: High]
> This isn't a bug introduced by this patch, but fprobe_fgraph_entry() also
> uses guard(rcu)(). Could this pre-existing usage cause the same lockdep
> splats in idle contexts?

Yeah, so we also need !rcu_is_watching() check. BTW for x86-64, it seems
those call path are removed from ftrace by objtool.

> 
> 
> Looking at how fgraph operations are initialized:
> 
> kernel/trace/fgraph.c:fgraph_init_ops() {
> 	dst_ops->flags = FTRACE_OPS_FL_PID | FTRACE_OPS_GRAPH_STUB;
> 
> #ifdef CONFIG_DYNAMIC_FTRACE
>     ...
> }
> 
> [Severity: High]
> This isn't a bug introduced by this patch, but does the lack of
> FTRACE_OPS_FL_RCU in these global fgraph operations expose all fgraph
> users to execution when RCU is not watching? Should this flag be added
> here to prevent tracing callbacks from being invoked in contexts where
> RCU is offline?

Yeah, but the fgraph_ops in fprobe becomes a subops, whose flags
are ignored. So fprobe needs to check the rcu_is_watching() by
itself.

Let me update it.

However, I think this is fundamentally unneeded check if we replace
guard(rcu)() with preempt-disabled.

Thank you,


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

Re: [PATCH] fprobe: Protect fprobe_return() with guard(rcu)()

[Steven Rostedt]

On Mon, 28 Sep 2026 21:22:51 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> 
> Hm, OK. Maybe we need to add something like checking rcu_watching, but
> FTRACE_OPS_FL_RCU does not work correctly because fgraph_init_ops() only
> configures subops (gops->ops). ftrace_startup_subops() does not propagete
> that flag to graph_ops. (I guess that is yet another problem?)
> 
> To guarantee no illegal RCU usage or lockdep splats if an instrumented
> function returns while RCU is offline, we can check 
> 
>  if (unlikely(!rcu_is_watching()))
> 	return;

If rcu isn't watching then that is a bug.

If you enable CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING then the
ftrace_test_recursion_trylock() tests for rcu_is_watching(). We
actually had that as default for some time to trigger any ftrace calls
that were outside of RCU watching and was fixed.

Perhaps we need to enable that on more test machines to see if anything
came back and allowed ftrace to trigger without it watching?

-- Steve

Re: [PATCH] fprobe: Protect fprobe_return() with guard(rcu)()

[Masami Hiramatsu]

On Tue, 29 Sep 2026 15:39:06 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Mon, 28 Sep 2026 21:22:51 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > 
> > Hm, OK. Maybe we need to add something like checking rcu_watching, but
> > FTRACE_OPS_FL_RCU does not work correctly because fgraph_init_ops() only
> > configures subops (gops->ops). ftrace_startup_subops() does not propagete
> > that flag to graph_ops. (I guess that is yet another problem?)
> > 
> > To guarantee no illegal RCU usage or lockdep splats if an instrumented
> > function returns while RCU is offline, we can check 
> > 
> >  if (unlikely(!rcu_is_watching()))
> > 	return;
> 
> If rcu isn't watching then that is a bug.

Yeah, I made another fix for this bug.

https://lore.kernel.org/all/179064115227.394389.16910234241400391996.stgit@devnote2/

> 
> If you enable CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING then the
> ftrace_test_recursion_trylock() tests for rcu_is_watching(). We
> actually had that as default for some time to trigger any ftrace calls
> that were outside of RCU watching and was fixed.
> 
> Perhaps we need to enable that on more test machines to see if anything
> came back and allowed ftrace to trigger without it watching?

What about adding that config to tools/testing/selftests/ftrace/config?

Thank you,

> 
> -- Steve


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

Re: [PATCH] fprobe: Protect fprobe_return() with guard(rcu)()

[Paul E. McKenney]

On Mon, Sep 28, 2026 at 09:35:59AM +0900, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> In fprobe_return(), the shadow-stack iteration and exit_handler
> invocations were protected by preempt_disable_notrace().
> However, unregister_fprobe() and unregister_fprobe_async() (used
> by BPF kprobe-multi) rely on standard RCU grace periods (synchronize_rcu()
> and call_rcu()) to wait until the fprobe is no longer in use before
> freeing it.
> 
> In preemptible kernels (CONFIG_PREEMPT_RCU=y), standard RCU grace
> periods do not wait for pure preempt_disable_notrace() critical
> sections. Consequently, an unregistered fprobe may be freed while
> a concurrent CPU executing fprobe_return() is still running
> fp->exit_handler(), causing a use-after-free.

Actually, standard RCU grace periods wait for preemption-disabled regions
of code regardless of kernel configuration.  So if the original code
below was broken, that indicates a bug in RCU.

So do you have a reproducer for this?

							Thanx, Paul

> To resolve this, protect fprobe_return() with guard(rcu)() matching
> fprobe_fgraph_entry(). This ensures both synchronous unregister_fprobe()
> and asynchronous unregister_fprobe_async() safely wait for in-flight
> exit_handlers to complete via standard RCU grace periods.
> 
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://sashiko.dev/#/bug/linux-e46bcd68-4a56-4f19-a255-e3772980e5e3
> Fixes: 657b594b2084 ("fprobe: Fix unregister_fprobe() to wait for RCU grace period")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>  kernel/trace/fprobe.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index 9f2d98181779..c3e1580bc648 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -671,7 +671,7 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
>  	size_words = SIZE_IN_LONG(size);
>  	ret_ip = ftrace_regs_get_instruction_pointer(fregs);
>  
> -	preempt_disable_notrace();
> +	guard(rcu)();
>  
>  	curr = 0;
>  	while (size_words > curr) {
> @@ -687,7 +687,6 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
>  		}
>  		curr += size;
>  	}
> -	preempt_enable_notrace();
>  }
>  NOKPROBE_SYMBOL(fprobe_return);
>  
> 

Re: [PATCH] fprobe: Protect fprobe_return() with guard(rcu)()

[Masami Hiramatsu]

On Mon, 28 Sep 2026 09:10:10 -0700
"Paul E. McKenney" <paulmck@kernel.org> wrote:

> On Mon, Sep 28, 2026 at 09:35:59AM +0900, Masami Hiramatsu (Google) wrote:
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > 
> > In fprobe_return(), the shadow-stack iteration and exit_handler
> > invocations were protected by preempt_disable_notrace().
> > However, unregister_fprobe() and unregister_fprobe_async() (used
> > by BPF kprobe-multi) rely on standard RCU grace periods (synchronize_rcu()
> > and call_rcu()) to wait until the fprobe is no longer in use before
> > freeing it.
> > 
> > In preemptible kernels (CONFIG_PREEMPT_RCU=y), standard RCU grace
> > periods do not wait for pure preempt_disable_notrace() critical
> > sections. Consequently, an unregistered fprobe may be freed while
> > a concurrent CPU executing fprobe_return() is still running
> > fp->exit_handler(), causing a use-after-free.
> 
> Actually, standard RCU grace periods wait for preemption-disabled regions
> of code regardless of kernel configuration.  So if the original code
> below was broken, that indicates a bug in RCU.

Thanks for pointing, this was my mistake. Sorry about that.
And I still think we need a fix to add rcu_is_watching() check.

Thank you,

> 
> So do you have a reproducer for this?
> 
> 							Thanx, Paul
> 
> > To resolve this, protect fprobe_return() with guard(rcu)() matching
> > fprobe_fgraph_entry(). This ensures both synchronous unregister_fprobe()
> > and asynchronous unregister_fprobe_async() safely wait for in-flight
> > exit_handlers to complete via standard RCU grace periods.
> > 
> > Reported-by: Sashiko <sashiko-bot@kernel.org>
> > Closes: https://sashiko.dev/#/bug/linux-e46bcd68-4a56-4f19-a255-e3772980e5e3
> > Fixes: 657b594b2084 ("fprobe: Fix unregister_fprobe() to wait for RCU grace period")
> > Cc: stable@vger.kernel.org
> > Assisted-by: LLM
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > ---
> >  kernel/trace/fprobe.c |    3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> > index 9f2d98181779..c3e1580bc648 100644
> > --- a/kernel/trace/fprobe.c
> > +++ b/kernel/trace/fprobe.c
> > @@ -671,7 +671,7 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
> >  	size_words = SIZE_IN_LONG(size);
> >  	ret_ip = ftrace_regs_get_instruction_pointer(fregs);
> >  
> > -	preempt_disable_notrace();
> > +	guard(rcu)();
> >  
> >  	curr = 0;
> >  	while (size_words > curr) {
> > @@ -687,7 +687,6 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
> >  		}
> >  		curr += size;
> >  	}
> > -	preempt_enable_notrace();
> >  }
> >  NOKPROBE_SYMBOL(fprobe_return);
> >  
> > 


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