Re: KASAN veth use after free in XDP_REDIRECT

[Shawn Bohrer <sbohrer@cloudflare.com>]

On Wed, Jan 25, 2023 at 11:07:32AM +0900, Toshiaki Makita wrote:
> On 2023/01/25 10:54, Toke Høiland-Jørgensen wrote:
> > Shawn Bohrer <sbohrer@cloudflare.com> 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.

I have a few more details here.  We have some machines running 5.15
kernels and some are running 6.1 kernels.  So far it appears this only
happens on 6.1.  We also have a couple of different network cards but
it appears that only the machines with Solarflare cards using the sfc
driver hit the KASAN BUG.

718a18a0c8a67f97781e40bdef7cdd055c430996 "veth: Rework
veth_xdp_rcv_skb in order to accept non-linear skb" reworked and added
the veth_convert_skb_to_xdp_buff() call you mentioned.  Importantly it
also added a call to pskb_expand_head() which will kmalloc() the
skb->head.  This looks like a path that could be causing the KASAN
BUG, but I have not yet confirmed that is the path we are hitting.
This change was also added in 5.18 so might explain why we don't see
it on 5.15.

--
Shawn