kcm: Fix memory leak in error path of kcm_sendmsg()

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

kcm: Fix memory leak in error path of kcm_sendmsg()

syzbot reported a memory leak like below:

BUG: memory leak

unreferenced object 0xffff88810b088c00 (size 240):

comm "syz-executor186", pid 5012, jiffies 4294943306 (age 13.680s)

hex dump (first 32 bytes):

00 89 08 0b 81 88 ff ff 00 00 00 00 00 00 00 00 ................

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

backtrace:

[<ffffffff83e5d5ff>] __alloc_skb+0x1ef/0x230 net/core/skbuff.c:634

[<ffffffff84606e59>] alloc_skb include/linux/skbuff.h:1289 [inline]

[<ffffffff84606e59>] kcm_sendmsg+0x269/0x1050 net/kcm/kcmsock.c:815

[<ffffffff83e479c6>] sock_sendmsg_nosec net/socket.c:725 [inline]

[<ffffffff83e479c6>] sock_sendmsg+0x56/0xb0 net/socket.c:748

[<ffffffff83e47f55>] ____sys_sendmsg+0x365/0x470 net/socket.c:2494

[<ffffffff83e4c389>] ___sys_sendmsg+0xc9/0x130 net/socket.c:2548

[<ffffffff83e4c536>] __sys_sendmsg+0xa6/0x120 net/socket.c:2577

[<ffffffff84ad7bb8>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]

[<ffffffff84ad7bb8>] do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80

[<ffffffff84c0008b>] entry_SYSCALL_64_after_hwframe+0x63/0xcd

In kcm_sendmsg(), kcm_tx_msg(head)->last_skb is used as a cursor to append

newly allocated skbs to 'head'. If some bytes are copied, an error occurred,

and jumped to out_error label, 'last_skb' is left unmodified. A later

kcm_sendmsg() will use an obsoleted 'last_skb' reference, corrupting the

'head' frag_list and causing the leak.

This patch fixes this issue by properly updating the last allocated skb in

'last_skb'.