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

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Message-ID: <qj5y7pjdx2f5alp7sfx2gepfylkk2bytiyeoiapyp3dpzwloyk@aljz7o77tt3m>
Date: Wed, 24 Sep 2025 21:10:56 +0530
From: Brahmajit Das <listout@...tout.xyz>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
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 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

	/* Clear off and union(map_ptr, range) */
	memset(((u8 *)reg) + sizeof(reg->type), 0,
	       offsetof(struct bpf_reg_state, var_off) - sizeof(reg->type));

-- 
Regards,
listout