Re: 答复: [PATCH net] net: Add check for csum_start in skb_partial_csum_set()

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

luwei (O) wrote:
> 
> 在 2023/4/15 9:47 PM, Willem de Bruijn 写道:
> > 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.
> 
>     OK, Thanks for your reply

We still should address the unnecessary warning triggerable by syzbot.

If I'm correct that any offset programmable through virtio_net_hdr ends
up in the skb linear section and skb_checksum_help will compute it fine,
then the WARN_ON_ONCE just needs an explicit cast to signed.

I have only skimmed, so not 100% sure yet. But that's the short take.
 
> >   
> >>> 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)
> > .
> 
> -- 
> Best Regards,
> Lu Wei
>