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().