CVE-2025-21642: Vulnerability in Linux Linux

Severity: highType: vulnerabilityCVE-2025-21642

In the Linux kernel, the following vulnerability has been resolved: mptcp: sysctl: sched: avoid using current->nsproxy Using the 'net' structure via 'current' is not recommended for different reasons. First, if the goal is to use it to read or write per-netns data, this is inconsistent with how the "generic" sysctl entries are doing: directly by only using pointers set to the table entry, e.g. table->data. Linked to that, the per-netns data should always be obtained from the table linked to the netns it had been created for, which may not coincide with the reader's or writer's netns. Another reason is that access to current->nsproxy->netns can oops if attempted when current->nsproxy had been dropped when the current task is exiting. This is what syzbot found, when using acct(2): Oops: general protection fault, probably for non-canonical address 0xdffffc0000000005: 0000 [#1] PREEMPT SMP KASAN PTI KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f] CPU: 1 UID: 0 PID: 5924 Comm: syz-executor Not tainted 6.13.0-rc5-syzkaller-00004-gccb98ccef0e5 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024 RIP: 0010:proc_scheduler+0xc6/0x3c0 net/mptcp/ctrl.c:125 Code: 03 42 80 3c 38 00 0f 85 fe 02 00 00 4d 8b a4 24 08 09 00 00 48 b8 00 00 00 00 00 fc ff df 49 8d 7c 24 28 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 cc 02 00 00 4d 8b 7c 24 28 48 8d 84 24 c8 00 00 RSP: 0018:ffffc900034774e8 EFLAGS: 00010206 RAX: dffffc0000000000 RBX: 1ffff9200068ee9e RCX: ffffc90003477620 RDX: 0000000000000005 RSI: ffffffff8b08f91e RDI: 0000000000000028 RBP: 0000000000000001 R08: ffffc90003477710 R09: 0000000000000040 R10: 0000000000000040 R11: 00000000726f7475 R12: 0000000000000000 R13: ffffc90003477620 R14: ffffc90003477710 R15: dffffc0000000000 FS: 0000000000000000(0000) GS:ffff8880b8700000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007fee3cd452d8 CR3: 000000007d116000 CR4: 00000000003526f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: <TASK> proc_sys_call_handler+0x403/0x5d0 fs/proc/proc_sysctl.c:601 __kernel_write_iter+0x318/0xa80 fs/read_write.c:612 __kernel_write+0xf6/0x140 fs/read_write.c:632 do_acct_process+0xcb0/0x14a0 kernel/acct.c:539 acct_pin_kill+0x2d/0x100 kernel/acct.c:192 pin_kill+0x194/0x7c0 fs/fs_pin.c:44 mnt_pin_kill+0x61/0x1e0 fs/fs_pin.c:81 cleanup_mnt+0x3ac/0x450 fs/namespace.c:1366 task_work_run+0x14e/0x250 kernel/task_work.c:239 exit_task_work include/linux/task_work.h:43 [inline] do_exit+0xad8/0x2d70 kernel/exit.c:938 do_group_exit+0xd3/0x2a0 kernel/exit.c:1087 get_signal+0x2576/0x2610 kernel/signal.c:3017 arch_do_signal_or_restart+0x90/0x7e0 arch/x86/kernel/signal.c:337 exit_to_user_mode_loop kernel/entry/common.c:111 [inline] exit_to_user_mode_prepare include/linux/entry-common.h:329 [inline] __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline] syscall_exit_to_user_mode+0x150/0x2a0 kernel/entry/common.c:218 do_syscall_64+0xda/0x250 arch/x86/entry/common.c:89 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7fee3cb87a6a Code: Unable to access opcode bytes at 0x7fee3cb87a40. RSP: 002b:00007fffcccac688 EFLAGS: 00000202 ORIG_RAX: 0000000000000037 RAX: 0000000000000000 RBX: 00007fffcccac710 RCX: 00007fee3cb87a6a RDX: 0000000000000041 RSI: 0000000000000000 RDI: 0000000000000003 RBP: 0000000000000003 R08: 00007fffcccac6ac R09: 00007fffcccacac7 R10: 00007fffcccac710 R11: 0000000000000202 R12: 00007fee3cd49500 R13: 00007fffcccac6ac R14: 0000000000000000 R15: 00007fee3cd4b000 </TASK> Modules linked in: ---[ end trace 0000000000000000 ]--- RIP: 0010:proc_scheduler+0xc6/0x3c0 net/mptcp/ctrl.c:125 Code: 03 42 80 3c 38 00 0f 85 fe 02 00 00 4d 8b a4 24 08 09 00 00 48 b8 00 00 00 00 00 fc ---truncated---

AI Analysis

Technical Summary

CVE-2025-21642 is a vulnerability identified in the Linux kernel, specifically within the Multipath TCP (mptcp) subsystem's sysctl scheduler code. The root cause of the vulnerability lies in the improper use of the 'current' task structure's nsproxy pointer to access network namespace data. The vulnerability arises because the code attempts to use current->nsproxy->netns, which is unsafe and can lead to a kernel oops (crash) if the current task's nsproxy has been dropped during task exit. This improper access can cause a general protection fault due to null pointer dereference or invalid memory access, as demonstrated by the syzbot fuzzing tool's detection of a kernel crash triggered by the acct(2) system call. The vulnerability is related to how per-network namespace data is accessed and managed in the kernel's sysctl interface, where the correct approach is to use pointers tied to the specific network namespace rather than relying on the current task's nsproxy. The kernel crash trace indicates that the fault occurs in the proc_scheduler function within net/mptcp/ctrl.c, triggered during sysctl write operations. This vulnerability is a logic and memory safety flaw that can cause denial of service (DoS) by crashing the kernel. There is no indication that this vulnerability allows privilege escalation or arbitrary code execution, but the kernel panic can disrupt system availability. The vulnerability affects multiple Linux kernel versions as identified by commit hashes, and it was publicly disclosed on January 19, 2025. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

Potential Impact

For European organizations, this vulnerability poses a risk primarily to systems running vulnerable Linux kernel versions with Multipath TCP enabled or the affected sysctl scheduler code in use. The impact is mainly a denial of service condition caused by kernel crashes, which can disrupt critical infrastructure, cloud services, and enterprise systems relying on Linux servers. Organizations using Linux-based network appliances, cloud instances, or container hosts could experience unexpected reboots or service interruptions. Given the widespread use of Linux in European data centers, telecommunications, and government systems, the disruption could affect availability of services, leading to operational downtime and potential financial losses. While no direct data breach or privilege escalation is indicated, the instability caused by repeated crashes could be exploited by attackers to cause persistent denial of service or to mask other malicious activities. The vulnerability is particularly relevant for environments with high network namespace usage or advanced networking features such as Multipath TCP, which are common in cloud and telecom infrastructures in Europe.

Mitigation Recommendations

European organizations should prioritize updating their Linux kernels to versions where this vulnerability is patched. Since the issue involves kernel-level code, applying official kernel updates or backported patches from trusted Linux distributions is the most effective mitigation. For environments where immediate patching is not feasible, disabling Multipath TCP or restricting sysctl write access related to the affected scheduler parameters can reduce exposure. Additionally, organizations should implement kernel crash monitoring and automated recovery mechanisms to minimize downtime in case of exploitation. Network namespace usage should be audited to ensure that applications and services do not rely on unsafe access patterns. Security teams should also monitor for unusual kernel oops or system crashes that could indicate attempted exploitation. Finally, maintaining strict access controls and limiting privileged user interactions with sysctl interfaces can reduce the risk of triggering the vulnerability.