Affected: Linux

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

arp: Prevent overflow in arp_req_get().

syzkaller reported an overflown write in arp_req_get(). [0]

When ioctl(SIOCGARP) is issued, arp_req_get() looks up an neighbour entry and copies neigh->ha to struct arpreq.arp_ha.sa_data.

The arp_ha here is struct sockaddr, not struct sockaddr_storage, so the sa_data buffer is just 14 bytes.

In the splat below, 2 bytes are overflown to the next int field, arp_flags. We initialise the field just after the memcpy(), so it's not a problem.

However, when dev->addr_len is greater than 22 (e.g. MAX_ADDR_LEN), arp_netmask is overwritten, which could be set as htonl(0xFFFFFFFFUL) in arp_ioctl() before calling arp_req_get().

To avoid the overflow, let's limit the max length of memcpy().

Note that commit b5f0de6df6dc ("net: dev: Convert sa_data to flexible array in struct sockaddr") just silenced syzkaller.

[0]: memcpy: detected field-spanning write (size 16) of single field "r->arp_ha.sa_data" at net/ipv4/arp.c:1128 (size 14) WARNING: CPU: 0 PID: 144638 at net/ipv4/arp.c:1128 arp_req_get+0x411/0x4a0 net/ipv4/arp.c:1128 Modules linked in: CPU: 0 PID: 144638 Comm: syz-executor.4 Not tainted 6.1.74 #31 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.0-debian-1.16.0-5 04/01/2014 RIP: 0010:arp_req_get+0x411/0x4a0 net/ipv4/arp.c:1128 Code: fd ff ff e8 41 42 de fb b9 0e 00 00 00 4c 89 fe 48 c7 c2 20 6d ab 87 48 c7 c7 80 6d ab 87 c6 05 25 af 72 04 01 e8 5f 8d…