linux-kernel - Re: [Linux-kernel-mentees] [PATCH] bpf: Fix NULL pointer dereference in __btf_resolve_helper

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Message-ID: <CAEf4BzZRGOsTiW3uFWd1aY6K5Yi+QBrTeC5FNOo6uVXviXuX4g@mail.gmail.com>
Date:   Mon, 13 Jul 2020 17:53:28 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Peilin Ye <yepeilin.cs@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        open list <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [Linux-kernel-mentees] [PATCH] bpf: Fix NULL pointer dereference
 in __btf_resolve_helper_id()

On Mon, Jul 13, 2020 at 5:43 PM Peilin Ye <yepeilin.cs@...il.com> wrote:
>
> Prevent __btf_resolve_helper_id() from dereferencing `btf_vmlinux`
> as NULL. This patch fixes the following syzbot bug:
>
>     https://syzkaller.appspot.com/bug?id=5edd146856fd513747c1992442732e5a0e9ba355
>
> Reported-by: syzbot+ee09bda7017345f1fbe6@...kaller.appspotmail.com
> Signed-off-by: Peilin Ye <yepeilin.cs@...il.com>
> ---
>  kernel/bpf/btf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 30721f2c2d10..3e981b183fa4 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -4088,7 +4088,7 @@ static int __btf_resolve_helper_id(struct bpf_verifier_log *log, void *fn,
>         const char *tname, *sym;
>         u32 btf_id, i;
>
> -       if (IS_ERR(btf_vmlinux)) {
> +       if (IS_ERR_OR_NULL(btf_vmlinux)) {
>                 bpf_log(log, "btf_vmlinux is malformed\n");

Can you please split IS_ERR and NULL cases and emit different messages
to log? If the kernel is not built with btf_vmlinux, saying that it's
"malformed" will confuse people. Thanks.

>                 return -EINVAL;
>         }
> --
> 2.25.1
>