netdev - Re: [PATCH v2 bpf-next 4/4] bpf: Replace bpf memory allocator with kmalloc

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Message-ID: <CAADnVQ+FC5dscjW0MQbG2qYP7KSQ2Ld6LCt5uK8+M2xreyeU7w@mail.gmail.com>
Date: Mon, 17 Nov 2025 15:36:08 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Amery Hung <ameryhung@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>
Cc: bpf <bpf@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, 
	Andrii Nakryiko <andrii@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Martin KaFai Lau <martin.lau@...nel.org>, Kumar Kartikeya Dwivedi <memxor@...il.com>, KP Singh <kpsingh@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, Song Liu <song@...nel.org>, 
	Kernel Team <kernel-team@...a.com>
Subject: Re: [PATCH v2 bpf-next 4/4] bpf: Replace bpf memory allocator with
 kmalloc_nolock() in local storage

On Mon, Nov 17, 2025 at 12:37 PM Amery Hung <ameryhung@...il.com> wrote:
>
> On Fri, Nov 14, 2025 at 6:01 PM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Fri, Nov 14, 2025 at 12:13 PM Amery Hung <ameryhung@...il.com> wrote:
> > >
> > >
> > > -       if (smap->bpf_ma) {
> > > +       if (smap->use_kmalloc_nolock) {
> > >                 rcu_barrier_tasks_trace();
> > > -               if (!rcu_trace_implies_rcu_gp())
> > > -                       rcu_barrier();
> > > -               bpf_mem_alloc_destroy(&smap->selem_ma);
> > > -               bpf_mem_alloc_destroy(&smap->storage_ma);
> > > +               rcu_barrier();
> >
> > Why unconditional rcu_barrier() ?
> > It's implied in rcu_barrier_tasks_trace().
>
> Hmm, I am not sure.
>
> > What am I missing?
>
> I hit a UAF in v1 in bpf_selem_free_rcu() when running selftests and
> making rcu_barrier() unconditional addressed it. I think the bug was
> due to map_free() not waiting for bpf_selem_free_rcu() (an RCU
> callback) to finish.
>
> Looking at rcu_barrier() and rcu_barrier_tasks_trace(), they pass
> different rtp to rcu_barrier_tasks_generic() so I think both are
> needed to make sure in-flight RCU and RCU tasks trace callbacks are
> done.
>
> Not an expert in RCU so I might be wrong and it was something else.

Paul,

Please help us here.
Does rcu_barrier_tasks_trace() imply rcu_barrier() ?