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().

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/1effe8ca4e34c34cdd9318436a4232dcb582ebf4
• https://git.kernel.org/stable/c/72bb856d16e883437023ff2ff77d0c498018728a
• https://git.kernel.org/stable/c/ba965e8605aee5387cecaa28fcf7ee9f61779a49
• https://git.kernel.org/stable/c/c4fa19615806a9a7e518c295b39175aa47a685ac