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

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Message-ID: <7f4lg3hnkue3qcxc6ej6yqeix4cb2scwjlb4rzhrjb4idjnvqb@kpjtqar55jrq>
Date: Wed, 24 Sep 2025 14:39:27 +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.
> >
> > The existing code assumes reg->map_ptr is always valid before
> > dereferencing reg->map_ptr->name, reg->map_ptr->key_size, and
> > reg->map_ptr->value_size.
> >
> > Fix this by adding explicit NULL checks before accessing reg->map_ptr
> > and its members. This prevents crashes when reg->map_ptr is NULL,
> > improving the robustness of the BPF verifier's verbose logging.
> >
> > Reported-by: syzbot+d36d5ae81e1b0a53ef58@...kaller.appspotmail.com
> > Signed-off-by: Brahmajit Das <listout@...tout.xyz>
> > ---
> >  kernel/bpf/log.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/log.c b/kernel/bpf/log.c
> > index f50533169cc3..5ffb8d778b92 100644
> > --- a/kernel/bpf/log.c
> > +++ b/kernel/bpf/log.c
> > @@ -704,7 +704,7 @@ static void print_reg_state(struct bpf_verifier_env *env,
> >                 verbose_a("ref_obj_id=%d", reg->ref_obj_id);
> >         if (type_is_non_owning_ref(reg->type))
> >                 verbose_a("%s", "non_own_ref");
> > -       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

Alexei, I did not, the patches (v1 and v2) were sent in a very short
timeframe, when you gave me the feedback I had already sent the v2 so
your feedback applies to v2 as well :)

I'm working on fixing/understanding the issue. I went one function lower
from where print_reg_state is being called and added a few debugging
statements like this

--- a/kernel/bpf/log.c
+++ b/kernel/bpf/log.c
@@ -758,6 +758,12 @@ void print_verifier_state(struct bpf_verifier_env *env, const struct bpf_verifie
                        continue;
                if (!print_all && !reg_scratched(env, i))
                        continue;
+               pr_err("&state->regs[%d] = %p\n", i, (void *)&state->regs[i]);
+               pr_err("reg               = %p\n", (void *)reg);
+               pr_err("&reg->map_ptr      = %p\n", (void *)&reg->map_ptr);
+               pr_err("&state->regs[%d].map_ptr = %p\n", i, (void *)&state->regs[i].map_ptr);
+               pr_err("state->regs[%d].map_ptr is NULL %d\n", i, state->regs[i].map_ptr == NULL);
+               pr_err("regs->map_ptr is NULL %d\n", reg->map_ptr == NULL);
                verbose(env, " R%d", i);
                verbose(env, "=");
                print_reg_state(env, state, reg);

Both reg->map_ptr and state->regs[i].map_ptr reports map_ptr is NULL.
For now I'm bit stuck and trying to understand why that would be.
I got the reproducer from
https://syzkaller.appspot.com/text?tag=ReproC&x=1608c27c580000

-- 
Regards,
listout