skbuff: fix coalescing for page_pool fragment recycling

In the Linux kernel, the following vulnerability has been resolved:

skbuff: fix coalescing for page_pool fragment recycling

Fix a use-after-free when using page_pool with page fragments. We
encountered this problem during normal RX in the hns3 driver:

(1) Initially we have three descriptors in the RX queue. The first one
allocates PAGE1 through page_pool, and the other two allocate one
half of PAGE2 each. Page references look like this:

RX_BD1 _______ PAGE1
RX_BD2 _______ PAGE2
RX_BD3 _________/

(2) Handle RX on the first descriptor. Allocate SKB1, eventually added
to the receive queue by tcp_queue_rcv().

(3) Handle RX on the second descriptor. Allocate SKB2 and pass it to
netif_receive_skb():

netif_receive_skb(SKB2)
ip_rcv(SKB2)
SKB3 = skb_clone(SKB2)

SKB2 and SKB3 share a reference to PAGE2 through
skb_shinfo()->dataref. The other ref to PAGE2 is still held by
RX_BD3:

SKB2 ---+- PAGE2
SKB3 __/ /
RX_BD3 _________/

(3b) Now while handling TCP, coalesce SKB3 with SKB1:

tcp_v4_rcv(SKB3)
tcp_try_coalesce(to=SKB1, from=SKB3) // succeeds
kfree_skb_partial(SKB3)
skb_release_data(SKB3) // drops one dataref

SKB1 _____ PAGE1
\____
SKB2 _____ PAGE2
/
RX_BD3 _________/

In skb_try_coalesce(), __skb_frag_ref() takes a page reference to
PAGE2, where it should instead have increased the page_pool frag
reference, pp_frag_count. Without coalescing, when releasing both
SKB2 and SKB3, a single reference to PAGE2 would be dropped. Now
when releasing SKB1 and SKB2, two references to PAGE2 will be
dropped, resulting in underflow.

(3c) Drop SKB2:

af_packet_rcv(SKB2)
consume_skb(SKB2)
skb_release_data(SKB2) // drops second dataref
page_pool_return_skb_page(PAGE2) // drops one pp_frag_count

SKB1 _____ PAGE1
\____
PAGE2
/
RX_BD3 _________/

(4) Userspace calls recvmsg()
Copies SKB1 and releases it. Since SKB3 was coalesced with SKB1, we
release the SKB3 page as well:

tcp_eat_recv_skb(SKB1)
skb_release_data(SKB1)
page_pool_return_skb_page(PAGE1)
page_pool_return_skb_page(PAGE2) // drops second pp_frag_count

(5) PAGE2 is freed, but the third RX descriptor was still using it!
In our case this causes IOMMU faults, but it would silently corrupt
memory if the IOMMU was disabled.

Change the logic that checks whether pp_recycle SKBs can be coalesced.
We still reject differing pp_recycle between 'from' and 'to' SKBs, but
in order to avoid the situation described above, we also reject
coalescing when both 'from' and 'to' are pp_recycled and 'from' is
cloned.

The new logic allows coalescing a cloned pp_recycle SKB into a page
refcounted one, because in this case the release (4) will drop the right
reference, the one taken by skb_try_coalesce().