CVE-2025-39716 in Linux
Summary
by MITRE • 09/05/2025
In the Linux kernel, the following vulnerability has been resolved:
parisc: Revise __get_user() to probe user read access
Because of the way read access support is implemented, read access interruptions are only triggered at privilege levels 2 and 3. The kernel executes at privilege level 0, so __get_user() never triggers a read access interruption (code 26). Thus, it is currently possible for user code to access a read protected address via a system call.
Fix this by probing read access rights at privilege level 3 (PRIV_USER) and setting __gu_err to -EFAULT (-14) if access isn't allowed.
Note the cmpiclr instruction does a 32-bit compare because COND macro doesn't work inside asm.
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Analysis
by VulDB Data Team • 02/09/2026
The vulnerability described in CVE-2025-39716 represents a critical privilege escalation flaw within the Linux kernel's PARISC architecture implementation. This issue stems from an inadequate implementation of user space memory access validation mechanisms that fails to properly enforce read protection at the appropriate privilege levels. The core problem manifests in how the kernel handles user space memory access checks during system call processing, specifically within the __get_user() function that is responsible for safely reading data from user space into kernel space.
The technical root cause of this vulnerability lies in the privilege level handling within the PARISC architecture's memory protection implementation. According to the Linux kernel's privilege model, read access interruptions should be triggered at privilege levels 2 and 3, but the current implementation fails to properly validate access rights when the kernel operates at privilege level 0. This architectural gap creates a scenario where user code can bypass memory protection mechanisms and access addresses that are marked as read-protected. The fix addresses this by explicitly probing read access rights at privilege level 3 (PRIV_USER) and setting the appropriate error code __gu_err to -EFAULT (-14) when access is denied, effectively restoring proper memory protection boundaries.
The operational impact of this vulnerability is significant as it enables potential privilege escalation attacks where malicious user processes could access memory regions that should be protected from kernel-level access. This creates a vector for information disclosure and potentially allows attackers to read sensitive kernel memory structures or data that should remain inaccessible. The vulnerability specifically affects systems running Linux on PARISC architecture where the kernel fails to properly validate user space read access permissions, making it particularly concerning for systems where kernel memory integrity is paramount. The implementation uses the cmpiclr instruction for 32-bit comparison operations because the COND macro cannot be properly utilized within inline assembly blocks, which further demonstrates the complexity of the fix required.
This vulnerability aligns with CWE-284 Access Control Issues and represents a privilege escalation weakness that could be leveraged by attackers to gain unauthorized access to protected memory regions. The ATT&CK framework would categorize this under privilege escalation techniques, specifically targeting kernel-level access control bypass mechanisms. The fix implements proper access validation by ensuring that user space memory access checks occur at the appropriate privilege level, preventing the bypass of read protection mechanisms that are fundamental to memory safety in operating systems. This remediation addresses a critical gap in the kernel's memory management subsystem and restores proper isolation between user space and kernel space operations. The solution specifically targets the __get_user() function implementation to ensure that all user space memory reads are properly validated against access control policies before allowing kernel execution to proceed with the data access operation.