net: xps: reject an out of range traffic class

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

net: xps: reject an out of range traffic class

Only the entries below dev->num_tc are valid in dev->tc_to_txq[], and
dev->prio_tc_map[] may only name classes below it. netdev_set_num_tc()
lowers dev->num_tc without touching either array.

netdev_txq_to_tc() walks all TC_MAX_QUEUE slots and
netdev_get_prio_tc_map() returns the entry as it stands, so a leftover
entry is handed out as a traffic class >= dev->num_tc. Taking that
class from netdev_txq_to_tc(), __netif_set_xps_queue() rejects only a
negative one and indexes an XPS map sized for dev->num_tc classes:

tci = j * num_tc + tc;
RCU_INIT_POINTER(new_dev_maps->attr_map[tci], map);

attr_map[] holds nr_ids * num_tc entries and j runs over the ids named
in the mask, so a class that is not below num_tc pushes tci past the end
of the map for the last ids and the store overruns it.

Any caller that lowers num_tc leaves such entries behind, and
mqprio_destroy() tears down with netdev_set_num_tc(dev, 0) rather than
netdev_reset_tc(). After mqprio with 8 classes then 1, tc_to_txq[1..7]
still describe txq 1..7. The splat is from an XPS write to txq 2 on a
veth with 8 rx queues: attr_map[] has 8 * 1 entries, tci = j + 2, and
j == 6 stores one past the end of the 88-byte map:

BUG: KASAN: slab-out-of-bounds in __netif_set_xps_queue (net/core/dev.c:2954)
Write of size 8 at addr ffff88813016bc58 by task xps_oob/634
__netif_set_xps_queue (net/core/dev.c:2954)
xps_rxqs_store (net/core/net-sysfs.c:1880)
netdev_queue_attr_store (net/core/net-sysfs.c:1390)
Allocated by task 634:
__kmalloc_noprof (mm/slub.c:5439)
__netif_set_xps_queue (net/core/dev.c:2937)
The buggy address is located 0 bytes to the right of
allocated 88-byte region [ffff88813016bc00, ffff88813016bc58)

Reject a class the map has no room for.