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Subject: Re: [Linux-kernel-mentees] [PATCH v3] bpf: Fix NULL pointer
 dereference in __btf_resolve_helper_id()
To:     Peilin Ye <yepeilin.cs@gmail.com>,
        Andrii Nakryiko <andriin@fb.com>,
        Alexei Starovoitov <ast@kernel.org>
Cc:     Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
        Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        syzkaller-bugs@googlegroups.com,
        linux-kernel-mentees@lists.linuxfoundation.org,
        linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com
References: <CAADnVQ+jUPGJapkvKW=AfXESD6Vz2iuONvJm8eJm5Yd+u9mJ+w@mail.gmail.com>
 <20200714180904.277512-1-yepeilin.cs@gmail.com>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <3b97c5bf-9f07-0353-ea4d-f90574fbcdc0@iogearbox.net>
Date:   Wed, 15 Jul 2020 23:07:17 +0200
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On 7/14/20 8:09 PM, Peilin Ye 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=f823224ada908fa5c207902a5a62065e53ca0fcc
> 
> Reported-by: syzbot+ee09bda7017345f1fbe6@syzkaller.appspotmail.com
> Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>

Looks good, applied, thanks! As far as I can tell all the other occurrences are
gated behind btf_parse_vmlinux() where we also init struct_opts, etc.

So for bpf-next this would then end up looking like ...

int btf_resolve_helper_id(struct bpf_verifier_log *log,
                           const struct bpf_func_proto *fn, int arg)
{
         int id;

         if (fn->arg_type[arg] != ARG_PTR_TO_BTF_ID)
                 return -EINVAL;
         id = fn->btf_id[arg];
         if (!id || !btf_vmlinux || id > btf_vmlinux->nr_types)
                 return -EINVAL;
         return id;
}

> ---
> Sorry, I got the link wrong. Thank you for pointing that out.
> 
> Change in v3:
>      - Fix incorrect syzbot dashboard link.
> 
> Change in v2:
>      - Split NULL and IS_ERR cases.
> 
>   kernel/bpf/btf.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 30721f2c2d10..092116a311f4 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -4088,6 +4088,11 @@ static int __btf_resolve_helper_id(struct bpf_verifier_log *log, void *fn,
>   	const char *tname, *sym;
>   	u32 btf_id, i;
>   
> +	if (!btf_vmlinux) {
> +		bpf_log(log, "btf_vmlinux doesn't exist\n");
> +		return -EINVAL;
> +	}
> +
>   	if (IS_ERR(btf_vmlinux)) {
>   		bpf_log(log, "btf_vmlinux is malformed\n");
>   		return -EINVAL;
>