packet: use ubuf_info completion for TX_RING packets

Overview

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

packet: use ubuf_info completion for TX_RING packets

tpacket_snd sends skbs with frags pointing into its ring slots. Slots are released when skb->destructor is called.

A call to skb_orphan calls skb->destructor before the skb is freed. This can cause the slot to be reused while still linked into the skb.

Switch to standard zerocopy completion (ubuf_info) so the slot is only released once all references to the payload are freed or copied. Restore skb->destructor to standard sock_wfree.

The ubuf_info completion callback can be called with a NULL skb, but only from net_zcopy_put and related API, used by zerocopy implementations that hold their own reference on the uarg, such as MSG_ZEROCOPY. This uarg is only ever completed from skb_zcopy_clear, so skb is always set.

To prevent userspace from aliasing in-flight state on shared ring slots, allocate tpacket_uarg per packet, rather than per slot. This adds a small allocation to the transmit path. Use standard kmalloc to allow backporting to stable kernels.

The uarg holds an sk_wmem_alloc reference, rather than an sk_refcnt reference. packet_free_tx_ring waits on sk_wmem_alloc before freeing the ring pages. Always allocate vec->deferred for tx_ring so page-backed rings also wait on sk_wmem_alloc when skb_copy_ubufs drops page refs before calling tpacket_ubuf_complete.

Drop the tx_ring.pg_vec test that tpacket_destruct_skb performed before accessing the slot. The sk_wmem_alloc reference now guarantees that the slot is valid. The test is also not sufficient by itself, as it reads pg_vec without pg_vec_lock, so it can race with packet_set_ring.

As a result a slot is released when its payload is copied, which can be before transmission (e.g., in skb_orphan_frags_rx). If copied before skb_tx_timestamp() is called, no slot timestamp is recorded, similar to when skb_orphan() was called early in the datapath before this patch.

Revert the now unused previous skb_zcopy_.._nouarg infra.

Depends on commit 992cc9f94ca9 ("net/packet: defer vmalloc TX_RING free until skbs finish").

Affected

• Linux >= 5cd8d46ea1562be80063f53c7c6a5f40224de623 < e67c455d79fbf709a94965f09a2be6bf4f4ba5fd
• Linux >= 5cd8d46ea1562be80063f53c7c6a5f40224de623 < eda9bcb721f15a93746a99eabda33d80b86404fd
• Linux >= 5cd8d46ea1562be80063f53c7c6a5f40224de623 < 2a0c9afefc0ecea2cc30ccfbbbc98182a0986191
• Linux >= 5cd8d46ea1562be80063f53c7c6a5f40224de623 < 9518405613863d0bf0700927a21367f5942cf058
• Linux 67f6fba765ef8cb19089d476e3c367212a2fbdfd
• Linux f2a67e68dbaeae7c88e5522873d7b908911f9f77
• Linux >= 4.14.86 < 4.15
• Linux >= 4.19.7 < 4.20
• Linux 4.20

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.