Merge branch 'fix-header-length-on-skb-merging'

Merge branch 'fix-header-length-on-skb-merging'

Arseniy Krasnov says:

====================
fix header length on skb merging

this patchset fixes appending newly arrived skbuff to the last skbuff of
the socket's queue during rx path. Problem fires when we are trying to
append data to skbuff which was already processed in dequeue callback
at least once. Dequeue callback calls function 'skb_pull()' which changes
'skb->len'. In current implementation 'skb->len' is used to update length
in header of last skbuff after new data was copied to it. This is bug,
because value in header is used to calculate 'rx_bytes'/'fwd_cnt' and
thus must be constant during skbuff lifetime. Here is example, we have
two skbuffs: skb0 with length 10 and skb1 with length 4.

1) skb0 arrives, hdr->len == skb->len == 10, rx_bytes == 10
2) Read 3 bytes from skb0, skb->len == 7, hdr->len == 10, rx_bytes == 10
3) skb1 arrives, hdr->len == skb->len == 4, rx_bytes == 14
4) Append skb1 to skb0, skb0 now has skb->len == 11, hdr->len == 11.
   But value of 11 in header is invalid.
5) Read whole skb0, update rx_bytes by 11 from skb0's header.
6) At this moment rx_bytes == 3, but socket's queue is empty.

This bug starts to fire since:

commit
077706165717 ("virtio/vsock: don't use skbuff state to account credit")

In fact, it presents before, but didn't triggered due to a little bit
buggy implementation of credit calculation logic. So i'll use Fixes tag
for it.

I really forgot about this branch in rx path when implemented patch
077706165717.

This patchset contains 3 patches:
1) Fix itself.
2) Patch with WARN_ONCE() to catch such problems in future.
3) Patch with test which triggers skb appending logic. It looks like
   simple test with several 'send()' and 'recv()', but it checks, that
   skbuff appending works ok.
====================

Link: https://lore.kernel.org/r/0683cc6e-5130-484c-1105-ef2eb792d355@sberdevices.ru
Signed-off-by: Paolo Abeni <pabeni@redhat.com>