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

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, May 18, 2026 at 12:19:06PM -0400, Steven Rostedt wrote:
> 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?

hi,

right, the original email states 5.15 is affected, but I dont see
__modify_ftrace_direct in stable version v5.15.207 .. what kernel
version is the patch for?

> 
> > -	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?

+1 

jirka