Re: [PATCH] perf: Fix data race in perf_event_set_bpf_handler()

[Qing Wang <wangqing7171@gmail.com>]

On Tue, 27 Jan 2026 at 16:37, Qing Wang <wangqing7171@gmail.com> wrote:
> On Tue, 27 Jan 2026 at 10:36, Henry Zhang <henryzhangjcle@gmail.com> wrote:
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index a0fa488bce84..1f3ed9e87507 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -10349,7 +10349,7 @@ static inline int perf_event_set_bpf_handler(struct perf_event *event,
> >  		return -EPROTO;
> >  	}
> >  
> > -	event->prog = prog;
> > +	WRITE_ONCE(event->prog, prog);
> >  	event->bpf_cookie = bpf_cookie;
> >  	return 0;
> >  }
> > @@ -10407,7 +10407,9 @@ static int __perf_event_overflow(struct perf_event *event,
> >  	if (event->attr.aux_pause)
> >  		perf_event_aux_pause(event->aux_event, true);
> >  
> > -	if (event->prog && event->prog->type == BPF_PROG_TYPE_PERF_EVENT &&
> > +	struct bpf_prog *prog = READ_ONCE(event->prog);
> > +
> > +	if (prog && prog->type == BPF_PROG_TYPE_PERF_EVENT &&
> >  	    !bpf_overflow_handler(event, data, regs))
> >  		goto out;
> 
> Looking at this code, I guess there may be an serious issue: a potential
> use-after-free (UAF) risk when accessing event->prog in __perf_event_overflow.
> 
> CPU 0 (interrupt context)               CPU 1 (process context)
> read event->prog
>                                         perf_event_free_bpf_handler()
>                                             put(prog)
>                                                 free(prog)
> access memory pointed to by prog
> 
> This scenario need to be more analysis.
> 
> --
> Qing

This is my idea for solving the problem of data competition and potential UAF.

diff --git a/kernel/events/core.c b/kernel/events/core.c
index a0fa488bce84..3abf3689157d 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -10291,7 +10291,12 @@ static inline bool sample_is_allowed(struct perf_event *event, struct pt_regs *r
 }
 
 #ifdef CONFIG_BPF_SYSCALL
+/*
+ * Execute the attached BPF program. Caller must ensure prog is non-NULL
+ * and of type BPF_PROG_TYPE_PERF_EVENT under RCU protection.
+ */
 static int bpf_overflow_handler(struct perf_event *event,
+				struct bpf_prog *prog,
 				struct perf_sample_data *data,
 				struct pt_regs *regs)
 {
@@ -10299,22 +10304,17 @@ static int bpf_overflow_handler(struct perf_event *event,
 		.data = data,
 		.event = event,
 	};
-	struct bpf_prog *prog;
 	int ret = 0;
 
 	ctx.regs = perf_arch_bpf_user_pt_regs(regs);
 	if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1))
 		goto out;
-	rcu_read_lock();
-	prog = READ_ONCE(event->prog);
-	if (prog) {
-		perf_prepare_sample(data, event, regs);
-		ret = bpf_prog_run(prog, &ctx);
-	}
-	rcu_read_unlock();
+
+	perf_prepare_sample(data, event, regs);
+	ret = bpf_prog_run(prog, &ctx);
+
 out:
 	__this_cpu_dec(bpf_prog_active);
-
 	return ret;
 }
 
@@ -10349,7 +10349,7 @@ static inline int perf_event_set_bpf_handler(struct perf_event *event,
 		return -EPROTO;
 	}
 
-	event->prog = prog;
+	WRITE_ONCE(event->prog, prog);
 	event->bpf_cookie = bpf_cookie;
 	return 0;
 }
@@ -10361,13 +10361,14 @@ static inline void perf_event_free_bpf_handler(struct perf_event *event)
 	if (!prog)
 		return;
 
-	event->prog = NULL;
+	WRITE_ONCE(event->prog, NULL);
 	bpf_prog_put(prog);
 }
 #else
 static inline int bpf_overflow_handler(struct perf_event *event,
-				       struct perf_sample_data *data,
-				       struct pt_regs *regs)
+				struct bpf_prog *prog,
+				struct perf_sample_data *data,
+				struct pt_regs *regs)
 {
 	return 1;
 }
@@ -10407,9 +10408,19 @@ static int __perf_event_overflow(struct perf_event *event,
 	if (event->attr.aux_pause)
 		perf_event_aux_pause(event->aux_event, true);
 
-	if (event->prog && event->prog->type == BPF_PROG_TYPE_PERF_EVENT &&
-	    !bpf_overflow_handler(event, data, regs))
+	/*
+	 * For BPF-based overflow handling. If a BPF_PROG_TYPE_PERF_EVENT
+	 * program is attached, execute it and skip default overflow handling.
+	 */
+	rcu_read_lock();
+	struct bpf_prog *prog = rcu_dereference(event->prog);
+
+	if (prog && prog->type == BPF_PROG_TYPE_PERF_EVENT &&
+	    !bpf_overflow_handler(event, prog, data, regs)) {
+		rcu_read_unlock();
 		goto out;
+	}
+	rcu_read_unlock();
 
 	/*
 	 * XXX event_limit might not quite work as expected on inherited

What do you think about this solution? Looking forward to your review.
--
Qing