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Date:   Sat, 15 Apr 2023 09:47:59 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To:     "luwei (O)" <luwei32@huawei.com>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        Eric Dumazet <edumazet@google.com>
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        "keescook@chromium.org" <keescook@chromium.org>,
        "jbenc@redhat.com" <jbenc@redhat.com>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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Subject: =?UTF-8?Q?Re:_=E7=AD=94=E5=A4=8D:_[PATCH_net]_net:_Add_check_for?=
 =?UTF-8?Q?_csum=5Fstart_in_skb=5Fpartial=5Fcsum=5Fset=28=29?=
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luwei (O) wrote:
> =

> =E5=9C=A8 2023/4/15 1:20 AM, Willem de Bruijn =E5=86=99=E9=81=93:
> > Willem de Bruijn wrote:
> >> luwei (O) wrote:
> >>> yes, here is the vnet_hdr:
> >>>
> >>>      flags: 3
> >>>      gso_type: 3
> >>>      hdr_len: 23
> >>>      gso_size: 58452
> >>>      csum_start: 5
> >>>      csum_offset: 16
> >>>
> >>> and the packet:
> >>>
> >>> | vnet_hdr | mac header | network header | data ... |
> >>>
> >>>    memcpy((void*)0x20000200,
> >>>           "\x03\x03\x02\x00\x54\xe4\x05\x00\x10\x00\x80\x00\x00\x53=
\xcc\x9c\x2b"
> >>>           "\x19\x3b\x00\x00\x00\x89\x4f\x08\x03\x83\x81\x04",
> >>>           29);
> >>>    *(uint16_t*)0x200000c0 =3D 0x11;
> >>>    *(uint16_t*)0x200000c2 =3D htobe16(0);
> >>>    *(uint32_t*)0x200000c4 =3D r[3];
> >>>    *(uint16_t*)0x200000c8 =3D 1;
> >>>    *(uint8_t*)0x200000ca =3D 0;
> >>>    *(uint8_t*)0x200000cb =3D 6;
> >>>    memset((void*)0x200000cc, 170, 5);
> >>>    *(uint8_t*)0x200000d1 =3D 0;
> >>>    memset((void*)0x200000d2, 0, 2);
> >>>    syscall(__NR_sendto, r[1], 0x20000200ul, 0xe45ful, 0ul, 0x200000=
c0ul, 0x14ul);
> >> Thanks. So this can happen whenever a packet is injected into the tx=

> >> path with a virtio_net_hdr.
> >>
> >> Even if we add bounds checking for the link layer header in pf_packe=
t,
> >> it can still point to the network header.
> >>
> >> If packets are looped to the tx path, skb_pull is common if a packet=

> >> traverses tunnel devices. But csum_start does not directly matter in=

> >> the rx path (CHECKSUM_PARTIAL is just seen as CHECKSUM_UNNECESSARY).=

> >> Until it is forwarded again to the tx path.
> >>
> >> So the question is which code calls skb_checksum_start_offset on the=

> >> tx path. Clearly, skb_checksum_help. Also a lot of drivers. Which
> >> may cast the signed int return value to an unsigned. Even an u8 in
> >> the first driver I spotted (alx).
> >>
> >> skb_postpull_rcsum anticipates a negative return value, as do other
> >> core functions. So it clearly allowed in certain cases. We cannot
> >> just bound it.
> >>
> >> Summary after a long story: an initial investigation, but I don't ha=
ve
> >> a good solution so far. Maybe others have a good suggestiong based o=
n
> >> this added context.
> > Specific to skb_checksum_help, it appears that skb_checksum will
> > work with negative offset just fine.
> =

>  =C2=A0=C2=A0=C2=A0 =C2=A0 In this case maybe not, since it checksums f=
rom within the mac =

> header, and the mac header
> =

>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 will be stripped when the rx path=
 checks the checksum.

The header is pulled, but still present. Obviously something bogus gets
written if the virtio_net_hdr configures csum offload with a bogus
offset. But as long as the offset is zero or positive from skb->head,
the checksum helper works as intended.
 =

> >
> > Perhaps the only issue is that the WARN_ON_ONCE compares signed to
> > unsigned, and thus incorrectly interprets a negative offset as
> >   >=3D skb_headlen(skb)