Re: [PATCH] ftrace: fix race in __modify_ftrace_direct() between tmp_ops registration and direct_functions update

[Steven Rostedt <rostedt@goodmis.org>]

On Sun, 17 May 2026 14:01:53 +0300
Andrii Kuchmenko <capyenglishlite@gmail.com> wrote:

> In __modify_ftrace_direct(), register_ftrace_function_nolock() makes
> tmp_ops visible in ftrace_ops_list before entry->direct is updated
> under ftrace_lock. During this window any CPU entering the traced
> function calls call_direct_funcs(), reads the old address from
> direct_functions via RCU, and jumps to it via
> arch_ftrace_set_direct_caller(). If the caller freed or invalidated
> the old trampoline before calling modify_ftrace_direct(), this is a
> use-after-free in executable code context.
> 
> The race window:
> 
>   CPU 0 (__modify_ftrace_direct)       CPU 1 (executing traced func)
>   ──────────────────────────────       ──────────────────────────────
>   register_ftrace_function_nolock()
>     -> tmp_ops visible in ops_list  
>                                         call_direct_funcs()
>                                           ftrace_find_rec_direct() -> old_addr
>                                           arch_ftrace_set_direct_caller(old_addr)
>                                           jump to old_addr  <- UAF if freed

You do not state where old_addr is freed.

>   mutex_lock(&ftrace_lock)
>   entry->direct = addr   <- too late
>   mutex_unlock(&ftrace_lock)
> 
> Fix: update entry->direct under ftrace_lock BEFORE registering tmp_ops.
> Any CPU that observes tmp_ops in ftrace_ops_list after this point will
> already see the new address when it calls ftrace_find_rec_direct().
> Add smp_wmb() between the store and the registration to ensure the
> write is visible on weakly-ordered architectures before tmp_ops
> becomes observable via ftrace_ops_list.
> 
> On error from register_ftrace_function_nolock(), restore entry->direct
> to old_addr since tmp_ops never became visible to other CPUs.

The above statement is incorrect. The tmp_ops hash entries are also
*shared* with the ops that is being updated. That is, by changing the entry->direct, you 

> 
> This affects all callers of __modify_ftrace_direct(), including:
>   - modify_ftrace_direct() used by kernel modules and live patching
>   - modify_ftrace_direct_nolock() used by BPF trampolines
>     (kernel/bpf/trampoline.c) reachable with CAP_BPF + CAP_PERFMON
> 
> Fixes: 0567d6809440 ("ftrace: Add modify_ftrace_direct()")
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: stable@vger.kernel.org
> Signed-off-by: Andrii Kuchmenko <capyenglishlite@gmail.com>
> ---
>  kernel/trace/ftrace.c | 35 +++++++++++++++++++++++++----------
>  1 file changed, 25 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index a1b2c3d4e5f6..b7c8d9e0f1a2 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -5950,6 +5950,7 @@ static int __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
>  	struct ftrace_func_entry *entry;
>  	struct ftrace_ops tmp_ops;
> +	unsigned long old_addr;
>  	int err;
>  
>  	lockdep_assert_held(&direct_mutex);
> @@ -5960,22 +5961,36 @@ static int __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
>  	if (!entry)
>  		return -ENODEV;
>  
> -	/*
> -	 * tmp_ops is registered into ftrace_ops_list here, making it
> -	 * visible to all CPUs executing the traced function. However,
> -	 * entry->direct is not updated until after this call returns,
> -	 * leaving a window where CPUs read the stale (possibly freed)
> -	 * direct call address via ftrace_find_rec_direct().
> -	 */

Are you posting patches on top of your own patches that are not public?

> -	err = register_ftrace_function_nolock(&tmp_ops);
> -	if (err)
> -		return err;
> -
> +	/* Save old address in case we need to roll back on error. */
> +	old_addr = entry->direct;
> +
> +	/*
> +	 * Update entry->direct BEFORE registering tmp_ops into
> +	 * ftrace_ops_list. This closes the race window where a CPU
> +	 * executing the traced function could read the old (potentially
> +	 * freed) direct call address between tmp_ops becoming visible
> +	 * and entry->direct being updated.
> +	 *
> +	 * Any CPU that observes tmp_ops in ftrace_ops_list after the
> +	 * smp_wmb() below is guaranteed to see the new address when
> +	 * it calls ftrace_find_rec_direct().
> +	 */
>  	mutex_lock(&ftrace_lock);
>  	entry->direct = addr;
>  	mutex_unlock(&ftrace_lock);
>  
> +	/*
> +	 * Ensure entry->direct store is ordered before tmp_ops
> +	 * becomes visible via ftrace_ops_list on weakly-ordered archs.
> +	 */
> +	smp_wmb();

You do realize that register_ftrace_function_nolock() is itself a full
memory barrier? It's doing code modification which requires lots of
barriers to work.

Still, the only bug I see that is possible is that the caller may need to
do some synchronize RCU calls before freeing an old trampoline.

Can you show a path that doesn't do that?

-- Steve


> +
> +	err = register_ftrace_function_nolock(&tmp_ops);
> +	if (err) {
> +		/* tmp_ops never became visible; safe to restore old_addr. */
> +		mutex_lock(&ftrace_lock);
> +		entry->direct = old_addr;
> +		mutex_unlock(&ftrace_lock);
> +		return err;
> +	}
> +
>  	/*
>  	 * Now that tmp_ops is registered and entry->direct is updated,
>  	 * unregister the original ops and clean up.