CVE-2025-21718 in Linux
Summary
by MITRE • 02/27/2025
In the Linux kernel, the following vulnerability has been resolved:
net: rose: fix timer races against user threads
Rose timers only acquire the socket spinlock, without checking if the socket is owned by one user thread.
Add a check and rearm the timers if needed.
BUG: KASAN: slab-use-after-free in rose_timer_expiry+0x31d/0x360 net/rose/rose_timer.c:174 Read of size 2 at addr ffff88802f09b82a by task swapper/0/0
CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.13.0-rc5-syzkaller-00172-gd1bf27c4e176 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024 Call Trace: __dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120 print_address_description mm/kasan/report.c:378 [inline]
print_report+0x169/0x550 mm/kasan/report.c:489 kasan_report+0x143/0x180 mm/kasan/report.c:602 rose_timer_expiry+0x31d/0x360 net/rose/rose_timer.c:174 call_timer_fn+0x187/0x650 kernel/time/timer.c:1793 expire_timers kernel/time/timer.c:1844 [inline]
__run_timers kernel/time/timer.c:2418 [inline]
__run_timer_base+0x66a/0x8e0 kernel/time/timer.c:2430 run_timer_base kernel/time/timer.c:2439 [inline]
run_timer_softirq+0xb7/0x170 kernel/time/timer.c:2449 handle_softirqs+0x2d4/0x9b0 kernel/softirq.c:561 __do_softirq kernel/softirq.c:595 [inline]
invoke_softirq kernel/softirq.c:435 [inline]
__irq_exit_rcu+0xf7/0x220 kernel/softirq.c:662 irq_exit_rcu+0x9/0x30 kernel/softirq.c:678 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1049 [inline]
sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1049
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Analysis
by VulDB Data Team • 05/25/2026
The vulnerability identified as CVE-2025-21718 affects the Linux kernel's ROSE (Remote Operations Signaling Environment) network protocol implementation, specifically within the timer management subsystem. This issue manifests as a use-after-free condition that occurs when ROSE timers attempt to access socket resources that have already been freed by user threads. The problem stems from insufficient synchronization mechanisms that fail to account for the possibility that a socket may be concurrently accessed by both kernel timer threads and user-space processes. The kernel's ROSE implementation only acquires a socket spinlock during timer operations but does not verify whether the socket remains valid and owned by a user thread, creating a race condition that can lead to memory corruption.
The technical flaw resides in the rose_timer_expiry function located in net/rose/rose_timer.c at line 174, where KASAN (Kernel Address Sanitizer) detects a slab-use-after-free error during memory access. The stack trace reveals that the problematic access occurs during timer expiration when the kernel attempts to read a 2-byte value from address ffff88802f09b82a, which has already been freed by the swapper/0 process. This memory access pattern indicates that a timer callback is executing against a socket structure that has been deallocated by a user thread, violating the fundamental principle of proper resource lifecycle management in concurrent kernel code. The underlying issue is consistent with CWE-416, which describes use-after-free vulnerabilities, and represents a classic race condition scenario where kernel and user threads operate on shared resources without proper coordination.
The operational impact of this vulnerability is significant as it can lead to system instability, potential privilege escalation, and denial of service conditions within network operations that utilize the ROSE protocol. Attackers could potentially exploit this race condition to cause kernel memory corruption, leading to system crashes or arbitrary code execution in kernel space. The vulnerability affects systems running Linux kernel versions where the ROSE protocol is enabled and actively used for network communications. Given that ROSE is primarily used in amateur radio and specialized network environments, the attack surface may be limited, but the potential for system compromise remains high due to the nature of kernel memory corruption vulnerabilities. The issue aligns with ATT&CK technique T1068, which involves the exploitation of privileges to gain unauthorized access to system resources.
The mitigation strategy involves implementing proper checks within the ROSE timer subsystem to verify socket validity before timer expiration and to rearm timers appropriately when necessary. The fix requires adding validation logic that ensures the socket remains allocated and properly owned before any timer operations proceed. This approach addresses the root cause by preventing timer callbacks from accessing freed memory structures and maintains proper synchronization between kernel timer threads and user-space processes. The solution should be integrated into the existing timer management code path to ensure that all timer expiration events properly validate socket state before proceeding with memory operations. This vulnerability demonstrates the critical importance of proper resource management in kernel space and highlights the necessity of comprehensive testing for race conditions in concurrent kernel subsystems, particularly those involving timer-based operations and network protocol implementations.