Linux Kernel Out-of-Bounds Write Vulnerability

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

netfilter: bridge: make ebt_snat ARP rewrite writable

The ebtables SNAT target keeps the Ethernet source address rewrite

behind skb_ensure_writable(skb, 0). This is intentional: at the bridge

ebtables hooks the Ethernet header is addressed through

skb_mac_header()/eth_hdr(), while skb->data points at the Ethernet

payload. Asking skb_ensure_writable() for ETH_HLEN bytes would check

the payload, not the Ethernet header, and would reintroduce the small

packet regression fixed by commit 63137bc5882a.

However, the optional ARP sender hardware address rewrite is different.

It writes through skb_store_bits() at an offset relative to skb->data:

skb_store_bits(skb, sizeof(struct arphdr), info->mac, ETH_ALEN)

skb_header_pointer() only safely reads the ARP header; it does not make

the later sender hardware address range writable. If that range is

still held in a nonlinear skb fragment backed by a splice-imported file

page, skb_store_bits() maps the frag page and copies the new MAC address

directly into it.

Ensure the ARP SHA range is writable before reading the ARP header and

before calling skb_store_bits().