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

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Message-ID: <417a8038817ce69e438a59e6916dc04372a47593.camel@linux.dev>
Date: Wed, 01 Oct 2025 13:08:02 +0800
From: KaFai Wan <kafai.wan@...ux.dev>
To: Brahmajit Das <listout@...tout.xyz>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, 
 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 Tue, 2025-09-30 at 23:51 +0530, Brahmajit Das wrote:
> On 26.09.2025 18:36, KaFai Wan wrote:
> > On Fri, 2025-09-26 at 06:34 +0530, Brahmajit Das wrote:
> > > On 25.09.2025 23:31, KaFai Wan wrote:
> > > > On Wed, 2025-09-24 at 23:58 +0530, Brahmajit Das wrote:
> > > > > On 25.09.2025 01:38, KaFai Wan wrote:
> > > > > > On Wed, 2025-09-24 at 21:10 +0530, Brahmajit Das 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...
> > 
> > You should add a Fixes label in the commit log and add selftest for it
> > in V3. 
> > Fixes label is Fixes: aced132599b3 ("bpf: Add range tracking for
> > BPF_NEG")
> > For selftest you may check the test in verifier_value_illegal_alu.c and
> > other files.  
> > 
> > The code in your next post would change the behavior of BPF_NEG and 
> > BPF_END, you can run the selftest to check that.
> > 
> 
> KaFai, I'm quite new to kernel development. I'm been trying to write a
> selftest for this unfortunately been having a hard time. I would really
> appreciate some help. For now I tried to create on from the initial test
> you used to verify this bug i.e. r0 -= r0.
> 
> I have tried testing my changes via sending a pull request on the
> kernel-patches/bpf repository, but seems like it's failing.
> My pull request: https://github.com/kernel-patches/bpf/pull/9900
> 

The attachment is the patch for selftest, you can apply it.

The patch#1 in your PR, Add the Oops call trace or the Closes label in commit
log could be better. 

Closes: https://lore.kernel.org/all/68d26227.a70a0220.1b52b.02a4.GAE@google.com/
-- 
Thanks,
KaFai

View attachment "0001-selftests-bpf-Add-test-for-BPF_NEG-alu-on-CONST_PTR_.patch" of type "text/x-patch" (1652 bytes)