[PATCH net v2] virtio_net: fix wrong buf address calculation when using xdp

Xuan Zhuo
2022-04-24T11:18:52Z

On Sun, 24 Apr 2022 13:56:17 +0300, Nikolay Aleksandrov <razor at
blackwall.org> wrote:
> On 24/04/2022 13:42, Xuan Zhuo wrote:
> > On Sun, 24 Apr 2022 13:21:21 +0300, Nikolay Aleksandrov <razor at
blackwall.org> wrote:
> >> We received a report[1] of kernel crashes when Cilium is used in
XDP
> >> mode with virtio_net after updating to newer kernels. After
> >> investigating the reason it turned out that when using mergeable
bufs
> >> with an XDP program which adjusts xdp.data or xdp.data_meta
page_to_buf()
> >> calculates the build_skb address wrong because the offset can
become less
> >> than the headroom so it gets the address of the previous page (-X
bytes
> >> depending on how lower offset is):
> >>  page_to_skb: page addr ffff9eb2923e2000 buf ffff9eb2923e1ffc
offset 252 headroom 256
> >>
> >> This is a pr_err() I added in the beginning of page_to_skb which
clearly
> >> shows offset that is less than headroom by adding 4 bytes of
metadata
> >> via an xdp prog. The calculations done are:
> >>  receive_mergeable():
> >>  headroom = VIRTIO_XDP_HEADROOM; // VIRTIO_XDP_HEADROOM == 256
bytes
> >>  offset = xdp.data - page_address(xdp_page) -
> >>           vi->hdr_len - metasize;
> >>
> >>  page_to_skb():
> >>  p = page_address(page) + offset;
> >>  ...
> >>  buf = p - headroom;
> >>
> >> Now buf goes -4 bytes from the page's starting address as can
be seen
> >> above which is set as skb->head and skb->data by build_skb
later. Depending
> >> on what's done with the skb (when it's freed most often)
we get all kinds
> >> of corruptions and BUG_ON() triggers in mm[2]. We have to
recalculate
> >> the new headroom after the xdp program has run, similar to how
offset
> >> and len are recalculated. Headroom is directly related to
> >> data_hard_start, data and data_meta, so we use them to get the new
size.
> >> The result is correct (similar pr_err() in page_to_skb, one case
of
> >> xdp_page and one case of virtnet buf):
> >>  a) Case with 4 bytes of metadata
> >>  [  115.949641] page_to_skb: page addr ffff8b4dcfad2000 offset 252
headroom 252
> >>  [  121.084105] page_to_skb: page addr ffff8b4dcf018000 offset
20732 headroom 252
> >>  b) Case of pushing data +32 bytes
> >>  [  153.181401] page_to_skb: page addr ffff8b4dd0c4d000 offset 288
headroom 288
> >>  [  158.480421] page_to_skb: page addr ffff8b4dd00b0000 offset
24864 headroom 288
> >>  c) Case of pushing data -33 bytes
> >>  [  835.906830] page_to_skb: page addr ffff8b4dd3270000 offset 223
headroom 223
> >>  [  840.839910] page_to_skb: page addr ffff8b4dcdd68000 offset
12511 headroom 223
> >>
> >> An example reproducer xdp prog[3] is below.
> >>
> >> [1] https://github.com/cilium/cilium/issues/19453
> >>
> >> [2] Two of the many traces:
> >>  [   40.437400] BUG: Bad page state in process swapper/0
pfn:14940
> >>  [   40.916726] BUG: Bad page state in process systemd-resolve
pfn:053b7
> >>  [   41.300891] kernel BUG at include/linux/mm.h:720!
> >>  [   41.301801] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
> >>  [   41.302784] CPU: 1 PID: 1181 Comm: kubelet Kdump: loaded
Tainted: G    B   W         5.18.0-rc1+ #37
> >>  [   41.304458] Hardware name: QEMU Standard PC (Q35 + ICH9,
2009), BIOS 1.15.0-1.fc35 04/01/2014
> >>  [   41.306018] RIP: 0010:page_frag_free+0x79/0xe0
> >>  [   41.306836] Code: 00 00 75 ea 48 8b 07 a9 00 00 01 00 74 e0 48
8b 47 48 48 8d 50 ff a8 01 48 0f 45 fa eb d0 48 c7 c6 18 b8 30 a6 e8 d7 f8 fc ff
<0f> 0b 48 8d 78 ff eb bc 48 8b 07 a9 00 00 01 00 74 3a 66 90 0f b6
> >>  [   41.310235] RSP: 0018:ffffac05c2a6bc78 EFLAGS: 00010292
> >>  [   41.311201] RAX: 000000000000003e RBX: 0000000000000000 RCX:
0000000000000000
> >>  [   41.312502] RDX: 0000000000000001 RSI: ffffffffa6423004 RDI:
00000000ffffffff
> >>  [   41.313794] RBP: ffff993c98823600 R08: 0000000000000000 R09:
00000000ffffdfff
> >>  [   41.315089] R10: ffffac05c2a6ba68 R11: ffffffffa698ca28 R12:
ffff993c98823600
> >>  [   41.316398] R13: ffff993c86311ebc R14: 0000000000000000 R15:
000000000000005c
> >>  [   41.317700] FS:  00007fe13fc56740(0000)
GS:ffff993cdd900000(0000) knlGS:0000000000000000
> >>  [   41.319150] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >>  [   41.320152] CR2: 000000c00008a000 CR3: 0000000014908000 CR4:
0000000000350ee0
> >>  [   41.321387] Call Trace:
> >>  [   41.321819]  <TASK>
> >>  [   41.322193]  skb_release_data+0x13f/0x1c0
> >>  [   41.322902]  __kfree_skb+0x20/0x30
> >>  [   41.343870]  tcp_recvmsg_locked+0x671/0x880
> >>  [   41.363764]  tcp_recvmsg+0x5e/0x1c0
> >>  [   41.384102]  inet_recvmsg+0x42/0x100
> >>  [   41.406783]  ? sock_recvmsg+0x1d/0x70
> >>  [   41.428201]  sock_read_iter+0x84/0xd0
> >>  [   41.445592]  ? 0xffffffffa3000000
> >>  [   41.462442]  new_sync_read+0x148/0x160
> >>  [   41.479314]  ? 0xffffffffa3000000
> >>  [   41.496937]  vfs_read+0x138/0x190
> >>  [   41.517198]  ksys_read+0x87/0xc0
> >>  [   41.535336]  do_syscall_64+0x3b/0x90
> >>  [   41.551637]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> >>  [   41.568050] RIP: 0033:0x48765b
> >>  [   41.583955] Code: e8 4a 35 fe ff eb 88 cc cc cc cc cc cc cc cc
e8 fb 7a fe ff 48 8b 7c 24 10 48 8b 74 24 18 48 8b 54 24 20 48 8b 44 24 08 0f 05
<48> 3d 01 f0 ff ff 76 20 48 c7 44 24 28 ff ff ff ff 48 c7 44 24 30
> >>  [   41.632818] RSP: 002b:000000c000a2f5b8 EFLAGS: 00000212
ORIG_RAX: 0000000000000000
> >>  [   41.664588] RAX: ffffffffffffffda RBX: 000000c000062000 RCX:
000000000048765b
> >>  [   41.681205] RDX: 0000000000005e54 RSI: 000000c000e66000 RDI:
0000000000000016
> >>  [   41.697164] RBP: 000000c000a2f608 R08: 0000000000000001 R09:
00000000000001b4
> >>  [   41.713034] R10: 00000000000000b6 R11: 0000000000000212 R12:
00000000000000e9
> >>  [   41.728755] R13: 0000000000000001 R14: 000000c000a92000 R15:
ffffffffffffffff
> >>  [   41.744254]  </TASK>
> >>  [   41.758585] Modules linked in: br_netfilter bridge veth
netconsole virtio_net
> >>
> >>  and
> >>
> >>  [   33.524802] BUG: Bad page state in process systemd-network
pfn:11e60
> >>  [   33.528617] page ffffe05dc0147b00 ffffe05dc04e7a00
ffff8ae9851ec000 (1) len 82 offset 252 metasize 4 hroom 0 hdr_len 12 data
ffff8ae9851ec10c data_meta ffff8ae9851ec108 data_end ffff8ae9851ec14e
> >>  [   33.529764] page:000000003792b5ba refcount:0 mapcount:-512
mapping:0000000000000000 index:0x0 pfn:0x11e60
> >>  [   33.532463] flags:
0xfffffc0000000(node=0|zone=1|lastcpupid=0x1fffff)
> >>  [   33.532468] raw: 000fffffc0000000 0000000000000000
dead000000000122 0000000000000000
> >>  [   33.532470] raw: 0000000000000000 0000000000000000
00000000fffffdff 0000000000000000
> >>  [   33.532471] page dumped because: nonzero mapcount
> >>  [   33.532472] Modules linked in: br_netfilter bridge veth
netconsole virtio_net
> >>  [   33.532479] CPU: 0 PID: 791 Comm: systemd-network Kdump:
loaded Not tainted 5.18.0-rc1+ #37
> >>  [   33.532482] Hardware name: QEMU Standard PC (Q35 + ICH9,
2009), BIOS 1.15.0-1.fc35 04/01/2014
> >>  [   33.532484] Call Trace:
> >>  [   33.532496]  <TASK>
> >>  [   33.532500]  dump_stack_lvl+0x45/0x5a
> >>  [   33.532506]  bad_page.cold+0x63/0x94
> >>  [   33.532510]  free_pcp_prepare+0x290/0x420
> >>  [   33.532515]  free_unref_page+0x1b/0x100
> >>  [   33.532518]  skb_release_data+0x13f/0x1c0
> >>  [   33.532524]  kfree_skb_reason+0x3e/0xc0
> >>  [   33.532527]  ip6_mc_input+0x23c/0x2b0
> >>  [   33.532531]  ip6_sublist_rcv_finish+0x83/0x90
> >>  [   33.532534]  ip6_sublist_rcv+0x22b/0x2b0
> >>
> >> [3] XDP program to reproduce(xdp_pass.c):
> >>  #include <linux/bpf.h>
> >>  #include <bpf/bpf_helpers.h>
> >>
> >>  SEC("xdp_pass")
> >>  int xdp_pkt_pass(struct xdp_md *ctx)
> >>  {
> >>           bpf_xdp_adjust_head(ctx, -(int)32);
> >>           return XDP_PASS;
> >>  }
> >>
> >>  char _license[] SEC("license") = "GPL";
> >>
> >>  compile: clang -O2 -g -Wall -target bpf -c xdp_pass.c -o
xdp_pass.o
> >>  load on virtio_net: ip link set enp1s0 xdpdrv obj xdp_pass.o sec
xdp_pass
> >>
> >> CC: stable at vger.kernel.org
> >> CC: Jason Wang <jasowang at redhat.com>
> >> CC: Xuan Zhuo <xuanzhuo at linux.alibaba.com>
> >> CC: Daniel Borkmann <daniel at iogearbox.net>
> >> CC: "Michael S. Tsirkin" <mst at redhat.com>
> >> CC: virtualization at lists.linux-foundation.org
> >> Fixes: 8fb7da9e9907 ("virtio_net: get build_skb() buf by data
ptr")
> >> Signed-off-by: Nikolay Aleksandrov <razor at blackwall.org>
> >> ---
> >> v2: Recalculate headroom based on data, data_hard_start and
data_meta
> >>
> >>  drivers/net/virtio_net.c | 8 +++++++-
> >>  1 file changed, 7 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >> index 87838cbe38cf..a12338de7ef1 100644
> >> --- a/drivers/net/virtio_net.c
> >> +++ b/drivers/net/virtio_net.c
> >> @@ -1005,6 +1005,12 @@ static struct sk_buff
*receive_mergeable(struct net_device *dev,
> >>  			 * xdp.data_meta were adjusted
> >>  			 */
> >>  			len = xdp.data_end - xdp.data + vi->hdr_len + metasize;
> >> +
> >> +			/* recalculate headroom if xdp.data or xdp.data_meta
> >> +			 * were adjusted
> >> +			 */
> >> +			headroom = xdp.data - xdp.data_hard_start - metasize;
> >
> >
> > This is incorrect.
> >
> >
> > 		data = page_address(xdp_page) + offset;
> > 		xdp_init_buff(&xdp, frame_sz - vi->hdr_len,
&rq->xdp_rxq);
> > 		xdp_prepare_buff(&xdp, data - VIRTIO_XDP_HEADROOM +
vi->hdr_len,
> > 				 VIRTIO_XDP_HEADROOM, len - vi->hdr_len, true);
> >
> > so: xdp.data_hard_start = page_address(xdp_page) + offset -
VIRTIO_XDP_HEADROOM + vi->hdr_len
> >
> > (page_address(xdp_page) + offset) points to virtio-net hdr.
> > (page_address(xdp_page) + offset - VIRTIO_XDP_HEADROOM) points to the
allocated buf.
> >
> > xdp.data_hard_start points to buf + vi->hdr_len
> >
> > Thanks.
> >
>
> xdp.data points to buf + vi->hdr_len + VIRTIO_XDP_HEADROOM, so we
calculate
> xdp.data - xdp.data_hard_start, i.e. buf + vi->hdr_len +
VIRTIO_XDP_HEADROOM - (buf + vi->hdr_len)
>
> You can see the headrooms from my tests above, they are correct and they
match exactly
> the values from the headroom calculations that you suggested earlier.


OK. You are right, xdp.data, xdp.data_hard_start have an offset of hdr_len. I
hope this can be explained in the comments, because the headroom we want to get
is virtio_hdr - buf. Although the value here are equal.

In addition, if you are going to post v2, I think you should post a new thread
separately instead of replying in the previous thread.

Thanks.


>
> >
> >> +
> >>  			/* We can only create skb based on xdp_page. */
> >>  			if (unlikely(xdp_page != page)) {
> >>  				rcu_read_unlock();
> >> @@ -1012,7 +1018,7 @@ static struct sk_buff
*receive_mergeable(struct net_device *dev,
> >>  				head_skb = page_to_skb(vi, rq, xdp_page, offset,
> >>  						       len, PAGE_SIZE, false,
> >>  						       metasize,
> >> -						       VIRTIO_XDP_HEADROOM);
> >> +						       headroom);
> >>  				return head_skb;
> >>  			}
> >>  			break;
> >> --
> >> 2.35.1
> >>
>