Re: [PATCH] perf/bpf: Fix data race in __perf_event_overflow

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, Aug 12, 2026 at 12:08:58AM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] Lockless TOCTOU race in perf_event_alloc() causes NULL pointer dereference or Use-After-Free during BPF program inheritance.
> - [Medium] Incomplete fix for data races and missing memory barriers in BPF attach/detach paths.
> - [Medium] Time-of-Check to Time-of-Use (TOCTOU) race drops valid perf events during concurrent BPF detach.
> --
> 
> --- Patch [1]: [PATCH] perf/bpf: Fix data race in __perf_event_overflow ---
> commit f63a2be5ceb79a2a8bb0263ba6723f7bf4ae763d
> Author: Deepanshu Kartikey <kartikey406@gmail.com>
> 
> perf/bpf: Fix data race in __perf_event_overflow
> 
> This patch addresses a data race where __perf_event_overflow() concurrently
> reads event->prog without lock protection while perf_event_detach_bpf_prog()
> writes it to NULL. It fixes the race by introducing READ_ONCE() and
> WRITE_ONCE() in the respective read and write paths.
> 
> Reported-by: syzbot+651d2774bd1d8395595f@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=651d2774bd1d8395595f
> Fixes: f11f10bfa1ca ("perf/bpf: Call BPF handler directly, not through overflow machinery")
> Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
> 
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> 
> [ ... ]
> 
> > @@ -10766,7 +10767,8 @@ 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 &&
> > +	prog = READ_ONCE(event->prog);
> > +	if (prog && prog->type == BPF_PROG_TYPE_PERF_EVENT &&
> >  	    !bpf_overflow_handler(event, data, regs))
> >  		goto out;
> 
> [Severity: Medium]
> This isn't a bug introduced by this patch, but does the independent second
> read in bpf_overflow_handler() create a time-of-check to time-of-use race?
> 
> If a concurrent detach sets event->prog to NULL between the READ_ONCE() here
> in __perf_event_overflow() and the second READ_ONCE() inside
> bpf_overflow_handler():
> 
> bpf_overflow_handler() {
>     prog = READ_ONCE(event->prog);
>     if (prog) {
>         ...
>     }
>     return 0;
> }
> 
> Wouldn't bpf_overflow_handler() evaluate prog as NULL and return 0? The
> caller __perf_event_overflow() evaluates !0 as true and executes goto out,
> which unconditionally drops the perf sample.
> 
> Should the already-validated prog pointer be passed directly to
> bpf_overflow_handler()?

seems legit

> 
> 
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -2018,7 +2018,7 @@ void perf_event_detach_bpf_prog(struct perf_event *event)
> >  
> >  put:
> >  	prog = event->prog;
> > -	event->prog = NULL;
> > +	WRITE_ONCE(event->prog, NULL);
> >  
> >  unlock:
> 
> [Severity: Medium]
> This is a pre-existing issue, but does this incomplete fix leave other data
> races in the attach and detach paths?
> 
> Other functions like perf_event_set_bpf_handler(),
> perf_event_free_bpf_handler(), and perf_event_attach_bpf_prog() still
> perform plain writes to event->prog without WRITE_ONCE().

was wondering the same, the perf_event_detach_bpf_prog is called for
tracing events (tracepoint/kprobe/uprobe) the perf event release goes
through perf_event_free_bpf_handler, check perf_event_free_bpf_prog

jirka

> 
> Additionally, does this need release and acquire semantics? If the writes to
> event->prog and event->bpf_cookie in the attach paths are reordered by the
> CPU, could the NMI handler read a valid event->prog but a stale or
> uninitialized event->bpf_cookie?
> 
> [Severity: High]
> This is also a pre-existing issue, but does the lockless access in
> perf_event_alloc() create a use-after-free or NULL pointer dereference
> during BPF program inheritance?
> 
> During fork, perf_event_alloc() inherits the BPF program:
> 
> perf_event_alloc() {
>     ...
>     if (parent_event->prog) {
>         struct bpf_prog *prog = parent_event->prog;
>         bpf_prog_inc(prog);
>         ...
>     }
> }
> 
> Since this accesses parent_event->prog locklessly, can a concurrent
> bpf_perf_link_release() clear the pointer? If the compiler reloads the
> pointer after the check, bpf_prog_inc() would dereference NULL.
> 
> Even if it is optimized to a single read, perf_event_alloc() is not inside
> an RCU read-side critical section. Could a concurrent detach finish its RCU
> grace period and free the BPF program, causing bpf_prog_inc() to write to
> freed memory?
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260811235331.10044-1-kartikey406@gmail.com?part=1
>