netdev - Re: [PATCH bpf-next] net: Use skb->len to check the validity of the parameters in bpf_skb_load

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Date:   Wed, 16 Mar 2022 08:09:10 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     "liujian (CE)" <liujian56@...wei.com>
Cc:     John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii@...nel.org" <andrii@...nel.org>,
        "songliubraving@...com" <songliubraving@...com>,
        "yhs@...com" <yhs@...com>,
        "kpsingh@...nel.org" <kpsingh@...nel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next] net: Use skb->len to check the validity of the
 parameters in bpf_skb_load_bytes

On Wed, Mar 16, 2022 at 6:08 AM liujian (CE) <liujian56@...wei.com> wrote:
>
>
>
> > -----Original Message-----
> > From: John Fastabend [mailto:john.fastabend@...il.com]
> > Sent: Wednesday, March 16, 2022 12:00 PM
> > To: liujian (CE) <liujian56@...wei.com>; Martin KaFai Lau <kafai@...com>
> > Cc: ast@...nel.org; daniel@...earbox.net; andrii@...nel.org;
> > songliubraving@...com; yhs@...com; john.fastabend@...il.com;
> > kpsingh@...nel.org; davem@...emloft.net; kuba@...nel.org;
> > sdf@...gle.com; netdev@...r.kernel.org; bpf@...r.kernel.org
> > Subject: RE: [PATCH bpf-next] net: Use skb->len to check the validity of the
> > parameters in bpf_skb_load_bytes
> >
> > liujian (CE) wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: Martin KaFai Lau [mailto:kafai@...com]
> > > > Sent: Wednesday, March 16, 2022 3:58 AM
> > > > To: liujian (CE) <liujian56@...wei.com>
> > > > Cc: ast@...nel.org; daniel@...earbox.net; andrii@...nel.org;
> > > > songliubraving@...com; yhs@...com; john.fastabend@...il.com;
> > > > kpsingh@...nel.org; davem@...emloft.net; kuba@...nel.org;
> > > > sdf@...gle.com; netdev@...r.kernel.org; bpf@...r.kernel.org
> > > > Subject: Re: [PATCH bpf-next] net: Use skb->len to check the
> > > > validity of the parameters in bpf_skb_load_bytes
> > > >
> > > > On Tue, Mar 15, 2022 at 08:39:16PM +0800, Liu Jian wrote:
> > > > > The data length of skb frags + frag_list may be greater than
> > > > > 0xffff, so here use skb->len to check the validity of the parameters.
> > > > What is the use case that needs to look beyond 0xffff ?
> >
> > > I use sockmap with strparser, the stm->strp.offset (the begin of one
> > > application layer protocol message) maybe beyond 0xffff, but i need
> > > load the message head to do something.
> >
> > This would explain skb_load_bytes but not the other two right? Also if we
> Yes, I just see that these two functions have the same judgment.
> > are doing this why not just remove those two checks in
> > flow_dissector_load() I think skb_header_pointer() does duplicate checks.
> > Please check.
> Yes, skb_header_pointer() have checked as below, and I will send v2 to remove 0xffff check.
> ----skb_header_pointer
> -------- __skb_header_pointer
> ------------skb_copy_bits
> ---------------- if (offset > (int)skb->len - len)
> --------------------goto fault;
>
> Thank you~

Do we need to have at least "offset <= 0x7fffffff" check? IOW, do we
need to enforce the unsignedness of the offset? Or does
skb_header_pointer et all properly work with the negative offsets?