CVE-2020-29374 in Linux
Summary
by MITRE • 11/28/2020
An issue was discovered in the Linux kernel before 5.7.3, related to mm/gup.c and mm/huge_memory.c. The get_user_pages (aka gup) implementation, when used for a copy-on-write page, does not properly consider the semantics of read operations and therefore can grant unintended write access, aka CID-17839856fd58.
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Analysis
by VulDB Data Team • 12/11/2020
The vulnerability described in CVE-2020-29374 represents a critical flaw in the Linux kernel's memory management subsystem that affects versions prior to 5.7.3. This issue resides within the get_user_pages (GUP) implementation located in mm/gup.c and mm/huge_memory.c files, fundamentally compromising the kernel's memory protection mechanisms. The vulnerability specifically manifests when GUP is utilized for copy-on-write pages, where the implementation fails to properly handle the semantic differences between read and write operations. This misconfiguration creates a scenario where processes can inadvertently gain write permissions on pages that should only allow read access, violating fundamental memory protection principles that are essential for system security and isolation.
The technical flaw stems from the improper handling of copy-on-write semantics within the GUP implementation, which is a core kernel function responsible for mapping user virtual addresses to kernel physical addresses. When a process attempts to access a page that is marked for copy-on-write, the kernel should ensure that read operations maintain read-only access while write operations trigger page splitting and copy mechanisms. However, the vulnerability allows the GUP implementation to grant write access even during read operations, effectively bypassing the copy-on-write protection mechanism that is designed to prevent unauthorized modifications to shared pages. This behavior creates a privilege escalation vector where malicious processes can modify memory regions they should not have write access to, potentially leading to data corruption, information disclosure, or even system compromise.
The operational impact of this vulnerability extends beyond simple memory access violations and represents a serious threat to system integrity and security. Attackers could exploit this flaw to gain unauthorized write access to memory regions, potentially allowing them to modify critical system data, inject malicious code into other processes, or manipulate kernel data structures. The vulnerability affects the core memory management functionality that underpins virtually all kernel operations, making it particularly dangerous as it could be leveraged to escalate privileges from user-level processes to kernel-level access. The implications are especially severe in multi-user environments or systems where different processes need to maintain strict memory isolation, as this flaw could enable one process to corrupt or manipulate another process's memory space without proper authorization.
From a cybersecurity perspective, this vulnerability aligns with CWE-284 (Improper Access Control) and represents a classic case of privilege escalation through improper memory management. The flaw demonstrates the critical importance of proper kernel memory protection mechanisms and the potential consequences when these protections are compromised. Organizations should prioritize immediate patching to kernel versions 5.7.3 and later, as this vulnerability can be exploited remotely or locally depending on system configuration. The ATT&CK framework categorizes this issue under privilege escalation techniques, specifically targeting the kernel-level memory management subsystem. Mitigation strategies should include comprehensive kernel updates, monitoring for unauthorized memory access patterns, and implementing additional security controls such as kernel lockdown modes and memory protection features that can help detect or prevent exploitation attempts. System administrators must also consider implementing process isolation measures and regularly auditing memory access patterns to identify potential exploitation attempts that may leverage this vulnerability.