CVE-2023-53489 in Linux
Summary
by MITRE • 10/01/2025
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: [<0000000055636812>] sk_prot_alloc+0x64/0x2a0 net/core/sock.c:2024
[<0000000054d77b7a>] sk_alloc+0x3b/0x800 net/core/sock.c:2083
[<0000000066f3c7e0>] inet_create net/ipv4/af_inet.c:319 [inline]
[<0000000066f3c7e0>] inet_create+0x31e/0xe40 net/ipv4/af_inet.c:245
[<000000009b83af97>] __sock_create+0x2ab/0x550 net/socket.c:1515
[<00000000b9b11231>] sock_create net/socket.c:1566 [inline]
[<00000000b9b11231>] __sys_socket_create net/socket.c:1603 [inline]
[<00000000b9b11231>] __sys_socket_create net/socket.c:1588 [inline]
[<00000000b9b11231>] __sys_socket+0x138/0x250 net/socket.c:1636
[<000000004fb45142>] __do_sys_socket net/socket.c:1649 [inline]
[<000000004fb45142>] __se_sys_socket net/socket.c:1647 [inline]
[<000000004fb45142>] __x64_sys_socket+0x73/0xb0 net/socket.c:1647
[<0000000066999e0e>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]
[<0000000066999e0e>] do_syscall_64+0x38/0x90 arch/x86/entry/common.c:80
[<0000000017f238c1>] 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: [<000000002b1c4368>] __alloc_skb+0x229/0x320 net/core/skbuff.c:497
[<00000000143579a6>] alloc_skb include/linux/skbuff.h:1265 [inline]
[<00000000143579a6>] sock_omalloc+0xaa/0x190 net/core/sock.c:2596
[<00000000be626478>] msg_zerocopy_alloc net/core/skbuff.c:1294 [inline]
[<00000000be626478>]
---truncated---
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Analysis
by VulDB Data Team • 01/22/2026
The vulnerability described in CVE-2023-53489 represents a memory leak issue within the Linux kernel's networking subsystem, specifically affecting TCP and UDP socket implementations when utilizing zerocopy functionality alongside transmit timestamping. This flaw arises from a circular reference problem involving socket structures and scatter-gather buffers that prevents proper resource cleanup upon socket closure. The root cause lies in how the kernel handles reference counting for zerocopy skbs (socket buffer) when transmit timestamps are enabled, leading to unreleased memory allocations that persist until system reboot or manual intervention.
The technical mechanism behind this vulnerability involves the interaction between several kernel components including socket creation, zerocopy buffer allocation, and timestamping mechanisms. When a socket is created with both SO_TIMESTAMPING and SO_ZEROCOPY options enabled, the sendmsg() function triggers msg_zerocopy_alloc() which allocates an skb and sets up reference counting through ubuf_info_msgzc. This structure indirectly maintains a reference to the socket, creating a circular dependency. During transmission, __skb_tstamp_tx() clones the skb and places it in the socket's error queue, but the cloned skb retains references to the zerocopy buffers that were never properly released during normal socket destruction.
This issue manifests when a socket is closed while holding zerocopy skbs with pending timestamps. The circular dependency prevents the normal socket cleanup path from executing because inet_sock_destruct() clears the error queue after the socket's reference count reaches zero, but the error queue still contains references to skbs that cannot be freed due to the remaining reference in ubuf_info_msgzc. The problem affects both TCP and UDP protocols, with TCP having received a partial fix in commit e0c8bccd40fc, but this fix does not address all edge cases, particularly those involving qdisc or device queues where skbs may be queued after the initial purge operation.
The operational impact of this vulnerability includes sustained memory leaks that can accumulate over time, potentially leading to system instability, performance degradation, and resource exhaustion. Attackers could exploit this by repeatedly creating and closing sockets with zerocopy and timestamping enabled, causing progressive memory consumption. This vulnerability aligns with CWE-401 (Memory Leak) and can be categorized under ATT&CK technique T1499.1 (Endpoint Resource Exhaustion) due to the potential for sustained memory consumption. The issue is particularly concerning in environments where high-volume socket operations are common, such as network services, load balancers, or systems handling large numbers of concurrent connections.
Mitigation strategies include applying the kernel patch that addresses the circular reference by calling skb_orphan_frags_rx() on cloned skbs in __skb_tstamp_tx() to ensure proper buffer reference cleanup. System administrators should prioritize updating affected kernel versions and monitoring for memory usage patterns that might indicate this vulnerability's exploitation. Additionally, the recommended approach involves avoiding the simultaneous use of SO_ZEROCOPY and SO_TIMESTAMPING options when possible, or implementing application-level resource management to minimize exposure. The fix ensures that cloned skbs do not retain references to zerocopy buffers, breaking the circular dependency and allowing proper socket destruction and memory cleanup. Organizations should also implement regular kernel updates and vulnerability scanning to prevent exploitation of similar memory management issues in the networking stack.