CVE-2023-54112

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>] __allocskb+0x1ef/0x230 net/core/skbuff.c:634 [<ffffffff84606e59>] allocskb include/linux/skbuff.h:1289 [inline] [<ffffffff84606e59>] kcmsendmsg+0x269/0x1050 net/kcm/kcmsock.c:815 [<ffffffff83e479c6>] socksendmsgnosec net/socket.c:725 [inline] [<ffffffff83e479c6>] socksendmsg+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>] _syssendmsg+0xa6/0x120 net/socket.c:2577 [<ffffffff84ad7bb8>] dosyscallx64 arch/x86/entry/common.c:50 [inline] [<ffffffff84ad7bb8>] dosyscall64+0x38/0xb0 arch/x86/entry/common.c:80 [<ffffffff84c0008b>] entrySYSCALL64afterhwframe+0x63/0xcd

In kcmsendmsg(), kcmtxmsg(head)->lastskb is used as a cursor to append newly allocated skbs to 'head'. If some bytes are copied, an error occurred, and jumped to outerror label, 'lastskb' is left unmodified. A later kcmsendmsg() will use an obsoleted 'lastskb' 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'.