On 23/04/2022 16:31, Xuan Zhuo wrote:
> On Sat, 23 Apr 2022 14:26:12 +0300, Nikolay Aleksandrov <razor@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx
>> CC: Jason Wang <jasowang@xxxxxxxxxx>
>> CC: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
>> CC: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
>> CC: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
>> CC: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
>> Fixes: 8fb7da9e9907 ("virtio_net: get build_skb() buf by data ptr")
>> Signed-off-by: Nikolay Aleksandrov <razor@xxxxxxxxxxxxx>
>> ---
>> 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.
