CVE-2022-49093: 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().