CVE-2021-46905 in Linux
Summary
by MITRE • 02/26/2024
In the Linux kernel, the following vulnerability has been resolved:
net: hso: fix NULL-deref on disconnect regression
Commit 8a12f8836145 ("net: hso: fix null-ptr-deref during tty device unregistration") fixed the racy minor allocation reported by syzbot, but introduced an unconditional NULL-pointer dereference on every disconnect instead.
Specifically, the serial device table must no longer be accessed after the minor has been released by hso_serial_tty_unregister().
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Analysis
by VulDB Data Team • 02/26/2024
The vulnerability identified as CVE-2021-46905 represents a critical null pointer dereference issue within the Linux kernel's hso (High Speed Onboard) network driver subsystem. This flaw manifests specifically during device disconnection operations and demonstrates the complex challenges inherent in kernel-level race condition fixes. The hso driver handles USB-based cellular modems and other high-speed onboard communication devices, making this vulnerability particularly concerning for embedded systems and mobile network equipment. The issue stems from a regression introduced during an earlier attempt to address a different race condition, highlighting how seemingly targeted fixes can inadvertently create new security weaknesses in kernel code.
The technical root cause of this vulnerability lies in the improper handling of device table access following minor number release operations within the hso_serial_tty_unregister() function. When a device disconnects, the system releases the minor number associated with the serial device, but subsequent code attempts to access the serial device table without proper null checks. This creates a classic null pointer dereference scenario where the kernel attempts to access memory at address zero or an invalid location, leading to immediate system instability and potential privilege escalation. The flaw operates at the kernel level and affects the tty subsystem's device management, where the minor allocation mechanism conflicts with device cleanup procedures. This type of vulnerability maps directly to CWE-476 which specifically addresses NULL pointer dereferences in software systems.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable privilege escalation attacks within the kernel space. When the null pointer dereference occurs during device disconnection, it can cause the kernel to panic and reboot the entire system, resulting in denial of service for connected devices. More critically, if an attacker can control the timing of device disconnection events, they may be able to exploit this vulnerability to gain elevated privileges within the kernel context. The hso driver's role in managing USB cellular modems means that systems relying on this technology for connectivity could experience complete service interruption, particularly in mobile or IoT environments where cellular connectivity is essential. This vulnerability also demonstrates the challenges of maintaining kernel stability when addressing race conditions, as the fix for one issue inadvertently introduced another. According to ATT&CK framework, this vulnerability could be leveraged for privilege escalation and denial of service operations within the kernel environment, representing a significant risk to system integrity and availability.
Mitigation strategies for CVE-2021-46905 require immediate kernel updates from vendors, as the fix involves correcting the serial device table access logic within the hso driver code. System administrators should prioritize patching affected kernel versions, particularly those running on embedded systems or mobile devices that utilize hso-compatible USB modems. The solution involves implementing proper null checks before accessing the serial device table after minor number release, ensuring that the driver properly handles the device disconnection sequence without attempting to access freed memory structures. Organizations should also monitor their kernel versions and apply security patches promptly, as this vulnerability affects the core networking subsystem and could be exploited to compromise entire networked systems. The fix aligns with security best practices for kernel development and emphasizes the importance of thorough testing when addressing race conditions in kernel space code, as the interaction between concurrent operations can create unexpected failure modes that require careful analysis and validation.