linux-kernel - Re: [PATCH v2] bpf: fix NULL pointer dereference in print_reg

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Message-ID: <CAADnVQLjwm=ruZ-4tK1aMtFw-nL5ppwub_a-OpL09xkPQVh4dw@mail.gmail.com>
Date: Wed, 24 Sep 2025 18:28:17 +0100
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Brahmajit Das <listout@...tout.xyz>
Cc: syzbot+d36d5ae81e1b0a53ef58@...kaller.appspotmail.com, 
	Andrii Nakryiko <andrii@...nel.org>, Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Eduard <eddyz87@...il.com>, Hao Luo <haoluo@...gle.com>, 
	John Fastabend <john.fastabend@...il.com>, Jiri Olsa <jolsa@...nel.org>, 
	KP Singh <kpsingh@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Stanislav Fomichev <sdf@...ichev.me>, Song Liu <song@...nel.org>, 
	syzkaller-bugs <syzkaller-bugs@...glegroups.com>, Yonghong Song <yonghong.song@...ux.dev>
Subject: Re: [PATCH v2] bpf: fix NULL pointer dereference in print_reg_state()

On Wed, Sep 24, 2025 at 4:41 PM Brahmajit Das <listout@...tout.xyz> wrote:
>
> On 24.09.2025 09:32, Alexei Starovoitov wrote:
> > On Wed, Sep 24, 2025 at 1:43 AM Brahmajit Das <listout@...tout.xyz> wrote:
> > >
> > > Syzkaller reported a general protection fault due to a NULL pointer
> > > dereference in print_reg_state() when accessing reg->map_ptr without
> > > checking if it is NULL.
> > >
> ...snip...
> > > -       if (type_is_map_ptr(t)) {
> > > +       if (type_is_map_ptr(t) && reg->map_ptr) {
> >
> > You ignored earlier feedback.
> > Fix the root cause, not the symptom.
> >
> > pw-bot: cr
>
> I'm not sure if I'm headed the write direction but it seems like in
> check_alu_op, we are calling adjust_scalar_min_max_vals when we get an
> BPF_NEG as opcode. Which has a call to __mark_reg_known when opcode is
> BPF_NEG. And __mark_reg_known clears map_ptr with

Looks like we're getting somewhere.
It seems the verifier is not clearing reg->type.
adjust_scalar_min_max_vals() should be called on scalar types only.