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
Offset and headroom are equal because offset points to the start of reserved bytes for the virtio_net header which are at buf start + headroom, while data points at buf start + vnet hdr size + headroom so when data or data_meta are adjusted by the xdp prog both the headroom size and the offset change equally. We can use data_hard_start to compute the new headroom after the xdp prog (linearized / page start case, the virtnet buf case is similar just with bigger base offset): xdp.data_hard_start = page_address + vnet_hdr xdp.data = page_address + vnet_hdr + headroom new headroom after xdp prog = xdp.data - xdp.data_hard_start - metasize
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@vger.kernel.org CC: Jason Wang jasowang@redhat.com CC: Xuan Zhuo xuanzhuo@linux.alibaba.com CC: Daniel Borkmann daniel@iogearbox.net CC: "Michael S. Tsirkin" mst@redhat.com CC: virtualization@lists.linux-foundation.org Fixes: 8fb7da9e9907 ("virtio_net: get build_skb() buf by data ptr") Signed-off-by: Nikolay Aleksandrov razor@blackwall.org --- v3: Add a comment explaining why offset and headroom are equal, no code changes v2: Recalculate headroom based on data, data_hard_start and data_meta
drivers/net/virtio_net.c | 20 +++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-)
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index 87838cbe38cf..cbba9d2e8f32 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c @@ -1005,6 +1005,24 @@ 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, note that offset should always point + * to the start of the reserved bytes for virtio_net + * header which are followed by xdp.data, that means + * that offset is equal to the headroom (when buf is + * starting at the beginning of the page, otherwise + * there is a base offset inside the page) but it's used + * with a different starting point (buf start) than + * xdp.data (buf start + vnet hdr size). If xdp.data or + * data_meta were adjusted by the xdp prog then the + * headroom size has changed and so has the offset, we + * can use data_hard_start, which points at buf start + + * vnet hdr size, to calculate the new headroom and use + * it later to compute buf start in page_to_skb() + */ + headroom = xdp.data - xdp.data_hard_start - metasize; + /* We can only create skb based on xdp_page. */ if (unlikely(xdp_page != page)) { rcu_read_unlock(); @@ -1012,7 +1030,7 @@ 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;