CVE-2025-71291 in Linux
Summary
by MITRE • 05/06/2026
In the Linux kernel, the following vulnerability has been resolved:
misc: bcm_vk: Fix possible null-pointer dereferences in bcm_vk_read()
In the function bcm_vk_read(), the pointer entry is checked, indicating that it can be NULL. If entry is NULL and rc is set to -EMSGSIZE, the following code may cause null-pointer dereferences:
struct vk_msg_blk tmp_msg = entry->to_h_msg[0];
set_msg_id(&tmp_msg, entry->usr_msg_id); tmp_msg.size = entry->to_h_blks - 1;
To prevent these possible null-pointer dereferences, copy to_h_msg, usr_msg_id, and to_h_blks from iter into temporary variables, and return these temporary variables to the application instead of accessing them through a potentially NULL entry.
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Analysis
by VulDB Data Team • 07/23/2026
The vulnerability identified in the Linux kernel's bcm_vk module represents a critical null-pointer dereference issue within the bcm_vk_read() function that could potentially lead to system instability or privilege escalation. This flaw resides in the miscellaneous device driver component responsible for handling virtual keyboard communications and demonstrates a classic programming error where insufficient null pointer validation leads to arbitrary code execution risks. The vulnerability specifically manifests when the entry pointer becomes NULL during message processing, creating a dangerous execution path that bypasses normal error handling mechanisms.
The technical implementation flaw occurs in the function's message processing logic where the code attempts to access members of a potentially NULL pointer without proper validation. The vulnerability stems from the assumption that entry->to_h_msg[0] and other related fields can be safely accessed even when entry itself is NULL. According to CWE-476, this represents a null pointer dereference vulnerability where the program fails to check for NULL values before dereferencing pointers. The condition where rc is set to -EMSGSIZE does not adequately protect against subsequent NULL pointer access patterns, creating a path where the kernel may attempt to read from memory locations that have not been properly allocated or validated.
Operationally, this vulnerability could be exploited by malicious actors to cause kernel crashes, leading to denial of service conditions, or potentially leverage the instability for privilege escalation attacks. The bcm_vk driver typically operates in kernel space where such flaws can result in complete system compromise, especially when combined with other kernel vulnerabilities. Attackers could craft specific input sequences that trigger the NULL pointer dereference, potentially causing the kernel to panic and reboot the system. This type of vulnerability aligns with ATT&CK technique T1068 which involves exploiting vulnerabilities in operating system kernels to gain elevated privileges.
The recommended mitigation strategy involves implementing proper null pointer validation before accessing structure members and utilizing temporary variable copying mechanisms as suggested in the fix. This approach ensures that data is safely extracted from potentially invalid pointers before being processed further. The solution demonstrates a defensive programming pattern where temporary variables are used to store critical data elements, preventing direct pointer dereferencing. This methodology aligns with secure coding practices outlined in the CERT Secure Coding Standards and provides a robust solution that eliminates the direct NULL pointer access while maintaining the intended functionality of the driver. The fix essentially transforms the vulnerable code path into a controlled error handling scenario that properly returns error codes rather than attempting to execute operations on invalid memory references.