linux-kernel - Re: [syzbot] [bpf?] general protection fault in bpf_get_local

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Message-Id: <20251231072919.1018961-1-buaajxlj@163.com>
Date: Wed, 31 Dec 2025 15:29:19 +0800
From: Liang Jie <buaajxlj@....com>
To: ameryhung@...il.com
Cc: andrii@...nel.org,
	ast@...nel.org,
	bpf@...r.kernel.org,
	buaajxlj@....com,
	daniel@...earbox.net,
	eddyz87@...il.com,
	haoluo@...gle.com,
	john.fastabend@...il.com,
	jolsa@...nel.org,
	kpsingh@...nel.org,
	liangjie@...iang.com,
	linux-kernel@...r.kernel.org,
	martin.lau@...ux.dev,
	sdf@...ichev.me,
	song@...nel.org,
	syzbot+4fe468a3f7fac86ea2c9@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com,
	yonghong.song@...ux.dev
Subject: Re: [syzbot] [bpf?] general protection fault in bpf_get_local_storage (2)

On Tue, 30 Dec 2025 22:18:00 -0800, Amery Hung <ameryhung@...il.com> wrote:
> >
> > #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 3f0e9c8cefa9
> >
> > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> > index 69988af44b37..2bc27ece5cc5 100644
> > --- a/kernel/bpf/cgroup.c
> > +++ b/kernel/bpf/cgroup.c
> > @@ -1768,6 +1768,9 @@ BPF_CALL_2(bpf_get_local_storage, struct bpf_map *, map, u64, flags)
> >         ctx = container_of(current->bpf_ctx, struct bpf_cg_run_ctx, run_ctx);
> >         storage = ctx->prog_item->cgroup_storage[stype];
> >
> > +       if (unlikely(!storage))
> > +               return (unsigned long)NULL;
> > +
> >         if (stype == BPF_CGROUP_STORAGE_SHARED)
> >                 ptr = &READ_ONCE(storage->buf)->data[0];
> >
> >
> 
> Hi Liang,
> 
> I don't think we can do this here due to backward compatibility. The
> return type of the helper is RET_PTR_TO_MAP_VALUE. Your proposed fix
> would require adding a PTR_MAYBE_NULL and existing BPF programs would
> no longer pass the verifier.
> 
> Did you look into why the storage pointer is NULL in the first place?
> 
> BTW, there is also another similar report and a work-in-progress fix
> [1]. Do you think this is a separate issue from that?

Hi Amery,

Thanks for pointing this out.

Sorry, I initially missed your earlier WIP fix. Looking at it now, this
does seem to be addressing the same class of issues where
bpf_get_local_storage() can observe a NULL storage pointer.

I'm interested in this area and will take a closer look to see whether
your patch also covers the syzbot report.

Thanks,
Liang