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Message-ID: <94172c53-2919-9eab-7933-91a78bdb87f0@blackwall.org>
Date: Sun, 24 Apr 2022 13:56:17 +0300
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: kuba@...nel.org, davem@...emloft.net, stable@...r.kernel.org,
Jason Wang <jasowang@...hat.com>,
Daniel Borkmann <daniel@...earbox.net>,
"Michael S. Tsirkin" <mst@...hat.com>,
virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH net v2] virtio_net: fix wrong buf address calculation when
using xdp
On 24/04/2022 13:42, Xuan Zhuo wrote:
> On Sun, 24 Apr 2022 13:21:21 +0300, Nikolay Aleksandrov <razor@...ckwall.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@...r.kernel.org
>> CC: Jason Wang <jasowang@...hat.com>
>> CC: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
>> CC: Daniel Borkmann <daniel@...earbox.net>
>> CC: "Michael S. Tsirkin" <mst@...hat.com>
>> CC: virtualization@...ts.linux-foundation.org
>> Fixes: 8fb7da9e9907 ("virtio_net: get build_skb() buf by data ptr")
>> Signed-off-by: Nikolay Aleksandrov <razor@...ckwall.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.
>
>> +
>> /* 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
>>
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