On 2023/01/25 10:54, Toke Høiland-Jørgensen wrote:
Shawn Bohrer <sbohrer@xxxxxxxxxxxxxx> writes:
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().
So the idea is that veth_xdp_get() does a
get_page(virt_to_page(xdp->data)), where xdp->data in this case points
to skb->head. This should keep the data page alive even if the skb
surrounding it is freed by the call to consume_skb().
However, because the skb->head in this case was allocated from a slab
allocator, taking a page refcount is not enough to prevent it from being
freed.
Not sure why skb->head is kmallocked here.
skb_head_is_locked() check in veth_convert_skb_to_xdp_buff() should ensure that
skb head is a page fragment.
Toshiaki Makita
