ip6mr: Fix skb_under_panic in ip6mr_cache_report()

In the Linux kernel, the following vulnerability has been resolved:

ip6mr: Fix skb_under_panic in ip6mr_cache_report()

skbuff: skb_under_panic: text:ffffffff88771f69 len:56 put:-4

head:ffff88805f86a800 data:ffff887f5f86a850 tail:0x88 end:0x2c0 dev:pim6reg

------------[ cut here ]------------

kernel BUG at net/core/skbuff.c:192!

invalid opcode: 0000 [#1] PREEMPT SMP KASAN

CPU: 2 PID: 22968 Comm: kworker/2:11 Not tainted 6.5.0-rc3-00044-g0a8db05b571a #236

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014

Workqueue: ipv6_addrconf addrconf_dad_work

RIP: 0010:skb_panic+0x152/0x1d0

Call Trace:

<TASK>

skb_push+0xc4/0xe0

ip6mr_cache_report+0xd69/0x19b0

reg_vif_xmit+0x406/0x690

dev_hard_start_xmit+0x17e/0x6e0

__dev_queue_xmit+0x2d6a/0x3d20

vlan_dev_hard_start_xmit+0x3ab/0x5c0

dev_hard_start_xmit+0x17e/0x6e0

__dev_queue_xmit+0x2d6a/0x3d20

neigh_connected_output+0x3ed/0x570

ip6_finish_output2+0x5b5/0x1950

ip6_finish_output+0x693/0x11c0

ip6_output+0x24b/0x880

NF_HOOK.constprop.0+0xfd/0x530

ndisc_send_skb+0x9db/0x1400

ndisc_send_rs+0x12a/0x6c0

addrconf_dad_completed+0x3c9/0xea0

addrconf_dad_work+0x849/0x1420

process_one_work+0xa22/0x16e0

worker_thread+0x679/0x10c0

ret_from_fork+0x28/0x60

ret_from_fork_asm+0x11/0x20

When setup a vlan device on dev pim6reg, DAD ns packet may sent on reg_vif_xmit().

reg_vif_xmit()

ip6mr_cache_report()

skb_push(skb, -skb_network_offset(pkt));//skb_network_offset(pkt) is 4

And skb_push declared as:

void *skb_push(struct sk_buff *skb, unsigned int len);

skb->data -= len;

//0xffff88805f86a84c - 0xfffffffc = 0xffff887f5f86a850

skb->data is set to 0xffff887f5f86a850, which is invalid mem addr, lead to skb_push() fails.