netlink: Fix kernel-infoleak-after-free in __skb_datagram_iter

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

netlink: Fix kernel-infoleak-after-free in __skb_datagram_iter

syzbot reported the following uninit-value access issue [1]:

netlink_to_full_skb() creates a new `skb` and puts the `skb->data`
passed as a 1st arg of netlink_to_full_skb() onto new `skb`. The data
size is specified as `len` and passed to skb_put_data(). This `len`
is based on `skb->end` that is not data offset but buffer offset. The
`skb->end` contains data and tailroom. Since the tailroom is not
initialized when the new `skb` created, KMSAN detects uninitialized
memory area when copying the data.

This patch resolved this issue by correct the len from `skb->end` to
`skb->len`, which is the actual data offset.

BUG: KMSAN: kernel-infoleak-after-free in instrument_copy_to_user include/linux/instrumented.h:114 [inline]
BUG: KMSAN: kernel-infoleak-after-free in copy_to_user_iter lib/iov_iter.c:24 [inline]
BUG: KMSAN: kernel-infoleak-after-free in iterate_ubuf include/linux/iov_iter.h:29 [inline]
BUG: KMSAN: kernel-infoleak-after-free in iterate_and_advance2 include/linux/iov_iter.h:245 [inline]
BUG: KMSAN: kernel-infoleak-after-free in iterate_and_advance include/linux/iov_iter.h:271 [inline]
BUG: KMSAN: kernel-infoleak-after-free in _copy_to_iter+0x364/0x2520 lib/iov_iter.c:186
instrument_copy_to_user include/linux/instrumented.h:114 [inline]
copy_to_user_iter lib/iov_iter.c:24 [inline]
iterate_ubuf include/linux/iov_iter.h:29 [inline]
iterate_and_advance2 include/linux/iov_iter.h:245 [inline]
iterate_and_advance include/linux/iov_iter.h:271 [inline]
_copy_to_iter+0x364/0x2520 lib/iov_iter.c:186
copy_to_iter include/linux/uio.h:197 [inline]
simple_copy_to_iter+0x68/0xa0 net/core/datagram.c:532
__skb_datagram_iter+0x123/0xdc0 net/core/datagram.c:420
skb_copy_datagram_iter+0x5c/0x200 net/core/datagram.c:546
skb_copy_datagram_msg include/linux/skbuff.h:3960 [inline]
packet_recvmsg+0xd9c/0x2000 net/packet/af_packet.c:3482
sock_recvmsg_nosec net/socket.c:1044 [inline]
sock_recvmsg net/socket.c:1066 [inline]
sock_read_iter+0x467/0x580 net/socket.c:1136
call_read_iter include/linux/fs.h:2014 [inline]
new_sync_read fs/read_write.c:389 [inline]
vfs_read+0x8f6/0xe00 fs/read_write.c:470
ksys_read+0x20f/0x4c0 fs/read_write.c:613
__do_sys_read fs/read_write.c:623 [inline]
__se_sys_read fs/read_write.c:621 [inline]
__x64_sys_read+0x93/0xd0 fs/read_write.c:621
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0x44/0x110 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x63/0x6b

Uninit was stored to memory at:
skb_put_data include/linux/skbuff.h:2622 [inline]
netlink_to_full_skb net/netlink/af_netlink.c:181 [inline]
__netlink_deliver_tap_skb net/netlink/af_netlink.c:298 [inline]
__netlink_deliver_tap+0x5be/0xc90 net/netlink/af_netlink.c:325
netlink_deliver_tap net/netlink/af_netlink.c:338 [inline]
netlink_deliver_tap_kernel net/netlink/af_netlink.c:347 [inline]
netlink_unicast_kernel net/netlink/af_netlink.c:1341 [inline]
netlink_unicast+0x10f1/0x1250 net/netlink/af_netlink.c:1368
netlink_sendmsg+0x1238/0x13d0 net/netlink/af_netlink.c:1910
sock_sendmsg_nosec net/socket.c:730 [inline]
__sock_sendmsg net/socket.c:745 [inline]
____sys_sendmsg+0x9c2/0xd60 net/socket.c:2584
___sys_sendmsg+0x28d/0x3c0 net/socket.c:2638
__sys_sendmsg net/socket.c:2667 [inline]
__do_sys_sendmsg net/socket.c:2676 [inline]
__se_sys_sendmsg net/socket.c:2674 [inline]
__x64_sys_sendmsg+0x307/0x490 net/socket.c:2674
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0x44/0x110 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x63/0x6b

Uninit was created at:
free_pages_prepare mm/page_alloc.c:1087 [inline]
free_unref_page_prepare+0xb0/0xa40 mm/page_alloc.c:2347
free_unref_page_list+0xeb/0x1100 mm/page_alloc.c:2533
release_pages+0x23d3/0x2410 mm/swap.c:1042
free_pages_and_swap_cache+0xd9/0xf0 mm/swap_state.c:316
tlb_batch_pages
---truncated---