KASAN veth use after free in XDP_REDIRECT

[Shawn Bohrer <sbohrer@cloudflare.com>]

Hello,

We've seen the following KASAN report on our systems. When using
AF_XDP on a veth.

KASAN report:

BUG: KASAN: use-after-free in __xsk_rcv+0x18d/0x2c0
Read of size 78 at addr ffff888976250154 by task napi/iconduit-g/148640

CPU: 5 PID: 148640 Comm: napi/iconduit-g Kdump: loaded Tainted: G           O       6.1.4-cloudflare-kasan-2023.1.2 #1
Hardware name: Quanta Computer Inc. QuantaPlex T41S-2U/S2S-MB, BIOS S2S_3B10.03 06/21/2018
Call Trace:
 <TASK>
 dump_stack_lvl+0x34/0x48
 print_report+0x170/0x473
 ? __xsk_rcv+0x18d/0x2c0
 kasan_report+0xad/0x130
 ? __xsk_rcv+0x18d/0x2c0
 kasan_check_range+0x149/0x1a0
 memcpy+0x20/0x60
 __xsk_rcv+0x18d/0x2c0
 __xsk_map_redirect+0x1f3/0x490
 ? veth_xdp_rcv_skb+0x89c/0x1ba0 [veth]
 xdp_do_redirect+0x5ca/0xd60
 veth_xdp_rcv_skb+0x935/0x1ba0 [veth]
 ? __netif_receive_skb_list_core+0x671/0x920
 ? veth_xdp+0x670/0x670 [veth]
 veth_xdp_rcv+0x304/0xa20 [veth]
 ? do_xdp_generic+0x150/0x150
 ? veth_xdp_rcv_one+0xde0/0xde0 [veth]
 ? _raw_spin_lock_bh+0xe0/0xe0
 ? newidle_balance+0x887/0xe30
 ? __perf_event_task_sched_in+0xdb/0x800
 veth_poll+0x139/0x571 [veth]
 ? veth_xdp_rcv+0xa20/0xa20 [veth]
 ? _raw_spin_unlock+0x39/0x70
 ? finish_task_switch.isra.0+0x17e/0x7d0
 ? __switch_to+0x5cf/0x1070
 ? __schedule+0x95b/0x2640
 ? io_schedule_timeout+0x160/0x160
 __napi_poll+0xa1/0x440
 napi_threaded_poll+0x3d1/0x460
 ? __napi_poll+0x440/0x440
 ? __kthread_parkme+0xc6/0x1f0
 ? __napi_poll+0x440/0x440
 kthread+0x2a2/0x340
 ? kthread_complete_and_exit+0x20/0x20
 ret_from_fork+0x22/0x30
 </TASK>

Freed by task 148640:
 kasan_save_stack+0x23/0x50
 kasan_set_track+0x21/0x30
 kasan_save_free_info+0x2a/0x40
 ____kasan_slab_free+0x169/0x1d0
 slab_free_freelist_hook+0xd2/0x190
 __kmem_cache_free+0x1a1/0x2f0
 skb_release_data+0x449/0x600
 consume_skb+0x9f/0x1c0
 veth_xdp_rcv_skb+0x89c/0x1ba0 [veth]
 veth_xdp_rcv+0x304/0xa20 [veth]
 veth_poll+0x139/0x571 [veth]
 __napi_poll+0xa1/0x440
 napi_threaded_poll+0x3d1/0x460
 kthread+0x2a2/0x340
 ret_from_fork+0x22/0x30


The buggy address belongs to the object at ffff888976250000
 which belongs to the cache kmalloc-2k of size 2048
The buggy address is located 340 bytes inside of
 2048-byte region [ffff888976250000, ffff888976250800)

The buggy address belongs to the physical page:
page:00000000ae18262a refcount:2 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x976250
head:00000000ae18262a order:3 compound_mapcount:0 compound_pincount:0
flags: 0x2ffff800010200(slab|head|node=0|zone=2|lastcpupid=0x1ffff)
raw: 002ffff800010200 0000000000000000 dead000000000122 ffff88810004cf00
raw: 0000000000000000 0000000080080008 00000002ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff888976250000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888976250080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff888976250100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                                 ^
 ffff888976250180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888976250200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb


If I understand the code correctly it looks like a xdp_buf is
constructed pointing to the memory backed by a skb but consume_skb()
is called while the xdp_buf() is still in use.

```
	case XDP_REDIRECT:
		veth_xdp_get(&xdp);
		consume_skb(skb);
		xdp.rxq->mem = rq->xdp_mem;
		if (xdp_do_redirect(rq->dev, &xdp, xdp_prog)) {
			stats->rx_drops++;
			goto err_xdp;
		}
		stats->xdp_redirect++;
		rcu_read_unlock();
		goto xdp_xmit;
```

It is worth noting that I think XDP_TX has the exact same problem.

Again assuming I understand the problem one naive solution might be to
move the consum_skb() call after xdp_do_redirect().  I think this
might work for BPF_MAP_TYPE_XSKMAP, BPF_MAP_TYPE_DEVMAP, and
BPF_MAP_TYPE_DEVMAP_HASH since those all seem to copy the xdb_buf to
new memory.  The copy happens for XSKMAP in __xsk_rcv() and for the
DEVMAP cases happens in dev_map_enqueue_clone().

However, it would appear that for BPF_MAP_TYPE_CPUMAP that memory can
live much longer, possibly even after xdp_do_flush().  If I'm correct,
I'm not really sure where it would be safe to call consume_skb().

The XDP_TX case looks similarly complex but I have not spent as much
time looking at that one.

Thanks,
Shawn Bohrer