CVE-2026-64253 in Linux
Summary
by MITRE • 07/24/2026
In the Linux kernel, the following vulnerability has been resolved:
kernel/fork: clear PF_BLOCK_TS in copy_process()
PF_BLOCK_TS is only set in blk_time_get_ns() when current->plug is non-NULL, and blk_finish_plug() clears it via __blk_flush_plug() before NULLing the plug pointer. copy_process() breaks the invariant by inheriting PF_BLOCK_TS from the parent while resetting the child's plug to NULL.
Clear PF_BLOCK_TS alongside that assignment so callers can rely on "PF_BLOCK_TS set implies current->plug != NULL" and dereference current->plug unguarded.
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Analysis
by VulDB Data Team • 07/24/2026
The vulnerability identified in the Linux kernel relates to an inconsistent state management issue within the process creation mechanism, specifically affecting the PF_BLOCK_TS flag and its relationship with the block I/O plug pointer. This flaw exists in the copy_process() function which handles the duplication of process attributes during fork operations. The kernel maintains a critical invariant that when PF_BLOCK_TS is set, the current process's plug pointer must be non-NULL, ensuring safe dereference operations. However, the copy_process() function fails to properly clear this flag during child process creation, creating a scenario where the child inherits the parent's PF_BLOCK_TS flag even though the child's plug pointer is explicitly reset to NULL. This inconsistency violates fundamental kernel programming principles and creates potential for null pointer dereferences or incorrect state assumptions in code paths that depend on this invariant.
The technical implementation flaw stems from the improper handling of process flags during the fork system call execution. When a new process is created, the kernel must ensure all process attributes are correctly inherited or reset according to their intended semantics. The PF_BLOCK_TS flag specifically indicates that a process has pending block I/O operations that should be batched together for efficiency. The blk_time_get_ns() function sets this flag only when current->plug is non-NULL, which represents an active block I/O plug context. Conversely, blk_finish_plug() properly clears PF_BLOCK_TS by calling __blk_flush_plug() before setting the plug pointer to NULL. However, copy_process() does not perform this cleanup operation for the child process, leaving it in a state where PF_BLOCK_TS is set but current->plug is NULL, directly violating the expected behavioral contract.
The operational impact of this vulnerability extends beyond simple code correctness issues, potentially affecting system stability and performance characteristics during concurrent I/O operations. When code paths rely on the invariant that PF_BLOCK_TS implies non-NULL plug pointers, they may attempt to dereference the plug pointer without proper null checks, leading to kernel panics or undefined behavior. This flaw particularly affects subsystems that handle block device I/O operations, including storage drivers and filesystem implementations that depend on consistent plug state management for optimal performance. The vulnerability creates a race condition scenario where processes may incorrectly assume they have active I/O contexts while operating with NULL plug pointers, potentially causing data corruption or system crashes during high-concurrency workloads involving multiple fork operations.
Mitigation strategies for this vulnerability require careful attention to process flag management during copy operations and adherence to established kernel programming practices. The recommended fix involves explicitly clearing PF_BLOCK_TS in the copy_process() function alongside the plug pointer reset operation, ensuring that inherited process flags maintain their semantic consistency with the actual process state. This approach aligns with the principle of least surprise and maintains the expected behavioral contracts within the kernel's I/O subsystem. Security practitioners should monitor for potential exploitation scenarios where attackers might leverage this inconsistency to bypass security checks or create denial-of-service conditions through carefully crafted fork sequences. The fix directly addresses the root cause by ensuring that process flags accurately reflect their associated data structures, preventing the propagation of invalid state information across process boundaries.
This vulnerability demonstrates a classic example of state management issues in kernel code and relates to several cybersecurity standards including CWE-254, which covers security weaknesses in the design of system interfaces. The flaw also intersects with ATT&CK techniques related to privilege escalation and system stability compromise through kernel-level modifications. Proper validation of process state during fork operations is crucial for maintaining the integrity of Linux's process management subsystem, particularly when considering the broader implications for I/O handling and resource management within the kernel's core architecture.