CVE-2022-49093
Published: 26 February 2025
Description
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().
Security Summary
CVE-2022-49093 is a use-after-free vulnerability in the Linux kernel's skbuff subsystem, specifically during coalescing of page_pool fragments for recycling. The issue arises when handling receive (RX) packets, such as in the hns3 driver, where cloned SKBs sharing page_pool fragments lead to incorrect reference counting. During TCP coalescing of a cloned SKB into another, the page_pool fragment reference (pp_frag_count) is not properly incremented, causing an underflow when pages are released. This affects Linux kernel versions using page_pool for fragment allocation in networking paths, rated at CVSS 7.8 (AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H) and mapped to CWE-416.
A local attacker with low privileges can exploit this during normal network receive processing. The vulnerability triggers when an SKB cloned during IP receive (e.g., SKB3 from SKB2) is coalesced into a prior SKB (SKB1), dropping references prematurely. Subsequent release of the coalesced SKB and the original SKB2 frees a page still referenced by an RX descriptor (e.g., RX_BD3), leading to use-after-free. This manifests as IOMMU faults or silent memory corruption if IOMMU is disabled, potentially enabling arbitrary code execution, data leaks, or denial of service.
Kernel stable patches resolve the issue by modifying coalescing logic in skb_try_coalesce() to reject cases where both source and target SKBs use page_pool recycling and the source is cloned, while still allowing coalescing of cloned page_pool SKBs into page-refcounted ones. Relevant commits include 1effe8ca4e34c34cdd9318436a4232dcb582ebf4, 72bb856d16e883437023ff2ff77d0c498018728a, ba965e8605aee5387cecaa28fcf7ee9f61779a49, and c4fa19615806a9a7e518c295b39175aa47a685ac, available in kernel.org stable trees.
Details
- CWE(s)