CVE-2020-27784 in Linux
Summary
by MITRE • 09/01/2022
A vulnerability was found in the Linux kernel, where accessing a deallocated instance in printer_ioctl() printer_ioctl() tries to access of a printer_dev instance. However, use-after-free arises because it had been freed by gprinter_free().
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Analysis
by VulDB Data Team • 10/11/2022
The vulnerability identified as CVE-2020-27784 represents a critical use-after-free condition within the Linux kernel's printer driver subsystem. This flaw exists in the printer_ioctl() function which attempts to access a printer_dev instance that has already been deallocated by the gprinter_free() function. The fundamental issue stems from improper memory management where the kernel fails to properly track the lifecycle of printer device instances, creating a scenario where freed memory can still be accessed by subsequent operations. Such memory corruption vulnerabilities are particularly dangerous as they can lead to arbitrary code execution or system instability.
The technical implementation of this vulnerability involves the printer driver's ioctl interface which handles device control operations. When a printer device is freed through gprinter_free(), the associated printer_dev structure is removed from memory but references to this structure may persist within the system's data structures or function pointers. The printer_ioctl() function subsequently attempts to access this freed memory location, triggering undefined behavior that can be exploited by malicious actors. This type of vulnerability falls under CWE-416 which specifically addresses use-after-free conditions in software development, where a pointer is used after the memory it points to has been freed.
The operational impact of CVE-2020-27784 extends beyond simple system crashes, as it provides potential attack vectors for privilege escalation and system compromise. An attacker who can control the printer device operations or has access to the ioctl interface can potentially manipulate the freed memory location to execute arbitrary code with kernel privileges. This represents a significant security risk in environments where untrusted users have access to printer device interfaces. The vulnerability aligns with ATT&CK technique T1068 which covers 'Exploitation for Privilege Escalation' and demonstrates how memory corruption flaws can be leveraged to gain elevated system access. The impact is particularly severe in multi-user systems or server environments where printer drivers are frequently accessed and may be exposed to untrusted input.
Mitigation strategies for this vulnerability require immediate kernel updates from vendors such as Red Hat, SUSE, and other Linux distributions that have released patches addressing the specific memory management issue. System administrators should prioritize applying these patches to prevent exploitation, as the vulnerability is exploitable in the kernel context. Additionally, implementing proper memory management practices in kernel code including reference counting, proper locking mechanisms, and validation of memory pointers before access can prevent similar issues. The fix typically involves ensuring that printer_ioctl() functions properly validate whether the printer_dev instance is still valid before accessing it, and that proper synchronization exists between the gprinter_free() function and any operations that might reference the freed structure. Organizations should also consider implementing kernel module signing and runtime protection mechanisms to detect and prevent exploitation attempts.