CVE-2023-53489

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

tcp/udp: Fix memleaks of sk and zerocopy skbs with TX timestamp.

syzkaller reported [0] memory leaks of an UDP socket and ZEROCOPY
skbs. We can reproduce the problem with these sequences:

sk = socket(AF_INET, SOCK_DGRAM, 0)
sk.setsockopt(SOL_SOCKET, SO_TIMESTAMPING, SOF_TIMESTAMPING_TX_SOFTWARE)
sk.setsockopt(SOL_SOCKET, SO_ZEROCOPY, 1)
sk.sendto(b”, MSG_ZEROCOPY, (‘127.0.0.1’, 53))
sk.close()

sendmsg() calls msg_zerocopy_alloc(), which allocates a skb, sets
skb->cb->ubuf.refcnt to 1, and calls sock_hold(). Here, struct
ubuf_info_msgzc indirectly holds a refcnt of the socket. When the
skb is sent, __skb_tstamp_tx() clones it and puts the clone into
the socket’s error queue with the TX timestamp.

When the original skb is received locally, skb_copy_ubufs() calls
skb_unclone(), and pskb_expand_head() increments skb->cb->ubuf.refcnt.
This additional count is decremented while freeing the skb, but struct
ubuf_info_msgzc still has a refcnt, so __msg_zerocopy_callback() is
not called.

The last refcnt is not released unless we retrieve the TX timestamped
skb by recvmsg(). Since we clear the error queue in inet_sock_destruct()
after the socket’s refcnt reaches 0, there is a circular dependency.
If we close() the socket holding such skbs, we never call sock_put()
and leak the count, sk, and skb.

TCP has the same problem, and commit e0c8bccd40fc (“net: stream:
purge sk_error_queue in sk_stream_kill_queues()”) tried to fix it
by calling skb_queue_purge() during close(). However, there is a
small chance that skb queued in a qdisc or device could be put
into the error queue after the skb_queue_purge() call.

In __skb_tstamp_tx(), the cloned skb should not have a reference
to the ubuf to remove the circular dependency, but skb_clone() does
not call skb_copy_ubufs() for zerocopy skb. So, we need to call
skb_orphan_frags_rx() for the cloned skb to call skb_copy_ubufs().

[0]:
BUG: memory leak
unreferenced object 0xffff88800c6d2d00 (size 1152):
comm “syz-executor392”, pid 264, jiffies 4294785440 (age 13.044s)
hex dump (first 32 bytes):
00 00 00 00 00 00 00 00 cd af e8 81 00 00 00 00 …………….
02 00 07 40 00 00 00 00 00 00 00 00 00 00 00 00 …@…………
backtrace:
[] sk_prot_alloc+0x64/0x2a0 net/core/sock.c:2024
[] sk_alloc+0x3b/0x800 net/core/sock.c:2083
[] inet_create net/ipv4/af_inet.c:319 [inline]
[] inet_create+0x31e/0xe40 net/ipv4/af_inet.c:245
[] __sock_create+0x2ab/0x550 net/socket.c:1515
[] sock_create net/socket.c:1566 [inline]
[] __sys_socket_create net/socket.c:1603 [inline]
[] __sys_socket_create net/socket.c:1588 [inline]
[] __sys_socket+0x138/0x250 net/socket.c:1636
[] __do_sys_socket net/socket.c:1649 [inline]
[] __se_sys_socket net/socket.c:1647 [inline]
[] __x64_sys_socket+0x73/0xb0 net/socket.c:1647
[] do_syscall_x64 arch/x86/entry/common.c:50 [inline]
[] do_syscall_64+0x38/0x90 arch/x86/entry/common.c:80
[] entry_SYSCALL_64_after_hwframe+0x63/0xcd

BUG: memory leak
unreferenced object 0xffff888017633a00 (size 240):
comm “syz-executor392”, pid 264, jiffies 4294785440 (age 13.044s)
hex dump (first 32 bytes):
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 …………….
00 00 00 00 00 00 00 00 00 2d 6d 0c 80 88 ff ff ………-m…..
backtrace:
[] __alloc_skb+0x229/0x320 net/core/skbuff.c:497
[] alloc_skb include/linux/skbuff.h:1265 [inline]
[] sock_omalloc+0xaa/0x190 net/core/sock.c:2596
[] msg_zerocopy_alloc net/core/skbuff.c:1294 [inline]
[]
—truncated—