CVE-2025-21875: Vulnerability in Linux Linux

Severity: highType: vulnerabilityCVE-2025-21875

In the Linux kernel, the following vulnerability has been resolved: mptcp: always handle address removal under msk socket lock Syzkaller reported a lockdep splat in the PM control path: WARNING: CPU: 0 PID: 6693 at ./include/net/sock.h:1711 sock_owned_by_me include/net/sock.h:1711 [inline] WARNING: CPU: 0 PID: 6693 at ./include/net/sock.h:1711 msk_owned_by_me net/mptcp/protocol.h:363 [inline] WARNING: CPU: 0 PID: 6693 at ./include/net/sock.h:1711 mptcp_pm_nl_addr_send_ack+0x57c/0x610 net/mptcp/pm_netlink.c:788 Modules linked in: CPU: 0 UID: 0 PID: 6693 Comm: syz.0.205 Not tainted 6.14.0-rc2-syzkaller-00303-gad1b832bf1cf #0 Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 12/27/2024 RIP: 0010:sock_owned_by_me include/net/sock.h:1711 [inline] RIP: 0010:msk_owned_by_me net/mptcp/protocol.h:363 [inline] RIP: 0010:mptcp_pm_nl_addr_send_ack+0x57c/0x610 net/mptcp/pm_netlink.c:788 Code: 5b 41 5c 41 5d 41 5e 41 5f 5d c3 cc cc cc cc e8 ca 7b d3 f5 eb b9 e8 c3 7b d3 f5 90 0f 0b 90 e9 dd fb ff ff e8 b5 7b d3 f5 90 <0f> 0b 90 e9 3e fb ff ff 44 89 f1 80 e1 07 38 c1 0f 8c eb fb ff ff RSP: 0000:ffffc900034f6f60 EFLAGS: 00010283 RAX: ffffffff8bee3c2b RBX: 0000000000000001 RCX: 0000000000080000 RDX: ffffc90004d42000 RSI: 000000000000a407 RDI: 000000000000a408 RBP: ffffc900034f7030 R08: ffffffff8bee37f6 R09: 0100000000000000 R10: dffffc0000000000 R11: ffffed100bcc62e4 R12: ffff88805e6316e0 R13: ffff88805e630c00 R14: dffffc0000000000 R15: ffff88805e630c00 FS: 00007f7e9a7e96c0(0000) GS:ffff8880b8600000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000001b2fd18ff8 CR3: 0000000032c24000 CR4: 00000000003526f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: <TASK> mptcp_pm_remove_addr+0x103/0x1d0 net/mptcp/pm.c:59 mptcp_pm_remove_anno_addr+0x1f4/0x2f0 net/mptcp/pm_netlink.c:1486 mptcp_nl_remove_subflow_and_signal_addr net/mptcp/pm_netlink.c:1518 [inline] mptcp_pm_nl_del_addr_doit+0x118d/0x1af0 net/mptcp/pm_netlink.c:1629 genl_family_rcv_msg_doit net/netlink/genetlink.c:1115 [inline] genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline] genl_rcv_msg+0xb1f/0xec0 net/netlink/genetlink.c:1210 netlink_rcv_skb+0x206/0x480 net/netlink/af_netlink.c:2543 genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219 netlink_unicast_kernel net/netlink/af_netlink.c:1322 [inline] netlink_unicast+0x7f6/0x990 net/netlink/af_netlink.c:1348 netlink_sendmsg+0x8de/0xcb0 net/netlink/af_netlink.c:1892 sock_sendmsg_nosec net/socket.c:718 [inline] __sock_sendmsg+0x221/0x270 net/socket.c:733 ____sys_sendmsg+0x53a/0x860 net/socket.c:2573 ___sys_sendmsg net/socket.c:2627 [inline] __sys_sendmsg+0x269/0x350 net/socket.c:2659 do_syscall_x64 arch/x86/entry/common.c:52 [inline] do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7f7e9998cde9 Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007f7e9a7e9038 EFLAGS: 00000246 ORIG_RAX: 000000000000002e RAX: ffffffffffffffda RBX: 00007f7e99ba5fa0 RCX: 00007f7e9998cde9 RDX: 000000002000c094 RSI: 0000400000000000 RDI: 0000000000000007 RBP: 00007f7e99a0e2a0 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 R13: 0000000000000000 R14: 00007f7e99ba5fa0 R15: 00007fff49231088 Indeed the PM can try to send a RM_ADDR over a msk without acquiring first the msk socket lock. The bugged code-path comes from an early optimization: when there are no subflows, the PM should (usually) not send RM_ADDR notifications. The above statement is incorrect, as without locks another process could concur ---truncated---

AI Analysis

Technical Summary

CVE-2025-21875 is a vulnerability identified in the Linux kernel's Multipath TCP (MPTCP) implementation, specifically within the path manager (PM) component that handles address removal notifications. The flaw arises because the PM attempts to send RM_ADDR (remove address) notifications over a multipath socket (msk) without properly acquiring the msk socket lock. This improper locking leads to a race condition and lock dependency (lockdep) splat, as reported by the Syzkaller fuzzing tool. The vulnerability manifests as a concurrency issue where multiple processes can simultaneously manipulate socket state without synchronization, potentially causing kernel warnings, deadlocks, or crashes. The technical details indicate that the bug stems from an early optimization intended to avoid sending RM_ADDR notifications when no subflows exist, but this assumption is incorrect and leads to unsynchronized access. The vulnerability affects Linux kernel versions around 6.14.0-rc2 and related commits identified by the given hashes. Although no known exploits are reported in the wild, the flaw could be triggered by local or privileged userspace processes interacting with MPTCP sockets, potentially leading to denial of service (DoS) through kernel panic or system instability. The vulnerability does not appear to allow privilege escalation or remote code execution directly but undermines kernel stability and reliability in systems using MPTCP.

Potential Impact

For European organizations, the impact of CVE-2025-21875 primarily concerns systems running Linux kernels with MPTCP enabled, which are increasingly used in data centers, cloud infrastructure, and network appliances to improve network resilience and throughput. Disruption of MPTCP functionality could degrade network performance or cause kernel crashes, leading to service outages. Critical infrastructure providers, cloud service operators, and enterprises relying on Linux-based networking stacks may experience availability issues. Given the kernel-level nature of the flaw, affected systems could require reboots or kernel patches, impacting operational continuity. Although exploitation requires local access and interaction with MPTCP sockets, insider threats or compromised hosts could leverage this vulnerability to cause denial of service. The vulnerability's impact on confidentiality and integrity is minimal, but availability risks are significant in environments where MPTCP is actively used for load balancing or failover. European organizations with high reliance on Linux servers, especially those in telecommunications, cloud services, and financial sectors, should prioritize patching to maintain network stability and service uptime.

Mitigation Recommendations

Mitigation requires applying the official Linux kernel patches that ensure the msk socket lock is properly acquired before sending RM_ADDR notifications in the MPTCP path manager. Organizations should: 1) Identify all Linux systems running kernel versions affected by this vulnerability, particularly those with MPTCP enabled. 2) Deploy updated kernel versions or backported patches from trusted Linux distributors as soon as they become available. 3) If immediate patching is not possible, consider disabling MPTCP functionality temporarily to prevent triggering the vulnerable code path. 4) Monitor kernel logs for lockdep warnings or unusual socket-related errors that may indicate attempted exploitation or instability. 5) Restrict local access to systems with MPTCP enabled to trusted users only, reducing the risk of malicious triggering. 6) Incorporate this vulnerability into vulnerability management and incident response workflows to ensure timely detection and remediation. 7) Engage with Linux vendor support channels to obtain patches and guidance tailored to specific distributions and kernel versions in use.