netdev - Re: 答复: [PATCH net] net: Add check for csum_start in skb_partial_csum

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Date:   Sat, 15 Apr 2023 09:47:59 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "luwei (O)" <luwei32@...wei.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "asml.silence@...il.com" <asml.silence@...il.com>,
        "imagedong@...cent.com" <imagedong@...cent.com>,
        "brouer@...hat.com" <brouer@...hat.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "jbenc@...hat.com" <jbenc@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: 答复: [PATCH net] net: Add check for csum_start in skb_partial_csum_set()

luwei (O) wrote:
> 
> 在 2023/4/15 1:20 AM, Willem de Bruijn 写道:
> > 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 = 0x11;
> >>>    *(uint16_t*)0x200000c2 = htobe16(0);
> >>>    *(uint32_t*)0x200000c4 = r[3];
> >>>    *(uint16_t*)0x200000c8 = 1;
> >>>    *(uint8_t*)0x200000ca = 0;
> >>>    *(uint8_t*)0x200000cb = 6;
> >>>    memset((void*)0x200000cc, 170, 5);
> >>>    *(uint8_t*)0x200000d1 = 0;
> >>>    memset((void*)0x200000d2, 0, 2);
> >>>    syscall(__NR_sendto, r[1], 0x20000200ul, 0xe45ful, 0ul, 0x200000c0ul, 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_packet,
> >> 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 have
> >> a good solution so far. Maybe others have a good suggestiong based on
> >> this added context.
> > Specific to skb_checksum_help, it appears that skb_checksum will
> > work with negative offset just fine.
> 
>        In this case maybe not, since it checksums from within the mac 
> header, and the mac header
> 
>         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
> >   >= skb_headlen(skb)