On 12/15/20 12:07 AM, Willem de Bruijn wrote:

On Mon, Dec 14, 2020 at 3:56 PM Jakub Kicinski [off-list ref] wrote:

quoted

On Sun, 13 Dec 2020 20:59:54 -0500 Willem de Bruijn wrote:

quoted

On Sun, Dec 13, 2020 at 2:37 PM Vasily Averin [off-list ref] wrote:

quoted

quoted

quoted

On 12/11/20 6:37 PM, Vasily Averin wrote:

quoted

It seems for me the similar problem can happen in __skb_trim_rcsum().
Also I doubt that that skb_checksum_start_offset(skb) checks in
__skb_postpull_rcsum() and skb_csum_unnecessary() are correct,
becasue they do not guarantee that skb have correct CHECKSUM_PARTIAL.
Could somebody confirm it?

I've rechecked the code and I think now that other places are not affected,
i.e. skb_push_rcsum() only should be patched.

Thanks for investigating this. So tun was able to insert a packet with
csum_start + csum_off + 2 beyond the packet after trimming, using
virtio_net_hdr.csum_...

Any packet with an offset beyond the end of the packet is bogus
really. No need to try to accept it by downgrading to CHECKSUM_NONE.

Do you mean it's better to force pskb_trim_rcsum() to return -EINVAL instead?

I would prefer to have more strict input validation in
tun/virtio/packet (virtio_net_hdr_to_skb), rather than new checks in
the hot path. But that is a larger change and not feasible
unconditionally due to performance impact and likely some false
positive drops. So out of scope here.

Could you please elaborate? Is it the case that syzbot constructed some
extremely convoluted frame to trigger this?

Somewhat convoluted, yes. A packet with a checksum offset beyond the
end of the ip packet.

skb_partial_csum_set (called from virtio_net_hdr_to_skb) verifies that
the offsets are within the linear buffer passed from userspace, but
without protocol parsing we don't know at that time that the offset is
beyond the end of the packet.

quoted

Otherwise the validation
at the source would work as well, no?

The problem with validation is two fold: it may add noticeable cost to
the hot path and it may have false positives: packets that the flow
dissector cannot fully dissect, but which are harmless and were
previously accepted.

I do want to add such strict source validation based on flow
dissection, but as an opt-in (sysctl) feature.

quoted

Does it actually trigger upstream? The linked syzbot report is for 4.14
but from the commit description it sounds like the problem should repro
rather reliably.

quoted

From the description, I would assume so, too. Haven't tested.

Original syzkaller reproducer fails on upstream because it prepares
invalid iptable ruleset, new kernels have more strict validation of iptable rules. 
I've commented this call im originsl reproducer and set the CHECKSUM rule manually,
then run of corrected reproducer triggered BUG_ON in skb_checksum_help().
I've crashed upstream 5.10-rc7 kernel by this way and then validated patched kernel.
originally we got such problem on RHEL7-based kernel, so I think the problem 
affects all stable and actual distribution kernels. 

Thank you,	
    Vasily Averin