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

Nikolay Aleksandrov
Sat, 23 Apr 2022 14:30:11 -0000

On 23/04/2022 17:16, Nikolay Aleksandrov wrote:
> On 23/04/2022 16:31, Xuan Zhuo wrote:
>> On Sat, 23 Apr 2022 14:26:12 +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]. The story of the
faulty
>>> commit is interesting because the patch was sent and applied twice
(it
>>> seems the first one got lost during merge back in 5.13 window). The
>>> first version of the patch that was applied as:
>>>  commit 7bf64460e3b2 ("virtio-net: get build_skb() buf by data
ptr")
>>> was actually correct because it calculated the page starting
address
>>> without relying on offset or headroom, but then the second version
that
>>> was applied as:
>>>  commit 8fb7da9e9907 ("virtio_net: get build_skb() buf by data
ptr")
>>> was wrong and added the above calculation.
>>> An example 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>
>>> ---
>>>  drivers/net/virtio_net.c | 8 ++++++--
>>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index 87838cbe38cf..0687dd88e97f 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -434,9 +434,13 @@ static struct sk_buff *page_to_skb(struct
virtnet_info *vi,
>>>  	 * Buffers with headroom use PAGE_SIZE as alloc size, see
>>>  	 * add_recvbuf_mergeable() + get_mergeable_buf_len()
>>>  	 */
>>> -	truesize = headroom ? PAGE_SIZE : truesize;
>>> +	if (headroom) {
>>> +		truesize = PAGE_SIZE;
>>> +		buf = (char *)((unsigned long)p & PAGE_MASK);
>>
>> The reason for not doing this is that buf and p may not be on the same
page, and
>> buf is probably not page-aligned.
>>
>> The implementation of virtio-net merge is add_recvbuf_mergeable(),
which
>> allocates a large block of memory at one time, and allocates from it
each time.
>> Although in xdp mode, each allocation is page_size, it does not
guarantee that
>> each allocation is page-aligned .
>>
>> The problem here is that the value of headroom is wrong, the package is
>> structured like this:
>>
>> from device    | headroom          | virtio-net hdr | data |
>> after xdp      | headroom  |  virtio-net hdr | meta | data |
>
> You're free to push data back (not necessarily through meta).
> You don't have virtio-net hdr for the xdp case (hdr_valid is false
there).
>
>>
>> The page_address(page) + offset we pass to page_to_skb() points to the
>> virtio-net hdr.
>>
>> So I think it might be better to change it this way.
>>
>> Thanks.
>>
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 87838cbe38cf..086ae835ec86 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -1012,7 +1012,7 @@ 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);
>> +
VIRTIO_XDP_HEADROOM - metazie);
>>                                 return head_skb;
>>                         }
>>                         break;
>
> That patch doesn't fix it, as I said with xdp you can move both data
and data_meta.
> So just doing that would take care of the meta, but won't take care of
moving data.
>

Also it doesn't take care of the case where page_to_skb() is called with the
original page
i.e. when we already have headroom, so we hit the next/standard page_to_skb()
call (xdp_page == page).

The above change guarantees that buf and p will be in the same page and the
skb_reserve() call will
make skb->data point to p - buf, i.e. to the beginning of the valid data in
that page.
Unfortunately the new headroom will not be correct if it is a frag, it will be
longer.