CVE-2026-64426 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
io_uring/nop: fix file reference leak with IOSQE_FIXED_FILE
NOP file-acquisition support choses between a fixed (registered) file and a normal fget()'d file based on its own IORING_NOP_FIXED_FILE flag in sqe->nop_flags. However, a request's REQ_F_FIXED_FILE is set independently from the generic IOSQE_FIXED_FILE sqe flag during request init, before the issue handler runs.
If a NOP is submitted with IOSQE_FIXED_FILE set (so REQ_F_FIXED_FILE is set) but without IORING_NOP_FIXED_FILE, io_nop() takes the normal path and grabs a real reference via io_file_get_normal(). On completion, io_put_file() only drops the reference when REQ_F_FIXED_FILE is clear, so the fget()'d file is never released and leaks:
BUG: memory leak unreferenced object 0xffff88800f42c240 (size 176): kmem_cache_alloc_noprof+0x358/0x440 alloc_empty_file+0x57/0x180 path_openat+0x44/0x1e50 do_file_open+0x121/0x200 do_sys_openat2+0xa7/0x150 __x64_sys_openat+0x82/0xf0
Decide between fixed and normal file acquisition from REQ_F_FIXED_FILE, the same way io_assign_file() does for every other opcode, and fold IORING_NOP_FIXED_FILE into REQ_F_FIXED_FILE at prep time.
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Analysis
by VulDB Data Team • 07/25/2026
The vulnerability under discussion involves a file reference leak in the Linux kernel's io_uring subsystem specifically affecting the NOP (no operation) file acquisition mechanism. This issue stems from a mismatch in how file references are managed between different flags within the io_uring framework, creating a scenario where file descriptors remain unreleased even after their intended use has completed.
The technical flaw manifests when a NOP operation is submitted with the IOSQE_FIXED_FILE flag set but without the IORING_NOP_FIXED_FILE flag. During request initialization, the system sets REQ_F_FIXED_FILE independently from the generic IOSQE_FIXED_FILE flag, creating a disconnect in reference management. When io_nop() executes, it follows the normal file acquisition path through io_file_get_normal() due to the absence of IORING_NOP_FIXED_FILE, resulting in an actual fget() call that acquires a reference to the file descriptor.
The operational impact of this vulnerability is significant as it leads to memory leaks within the kernel's file reference management system. The reference counting mechanism fails to properly release the acquired file descriptor because io_put_file() only drops references when REQ_F_FIXED_FILE is cleared, which never occurs in this scenario. This creates a dangling reference that prevents the underlying file structure from being freed, leading to resource exhaustion over time.
The root cause aligns with CWE-404, which describes improper resource management where resources are not properly released or recycled, and demonstrates characteristics of CWE-825, involving the removal of expired references. The vulnerability operates within the ATT&CK framework under the technique T1059.006 for system binary exploitation and T1562.001 for privilege escalation through resource exhaustion attacks, as sustained memory leaks can lead to system instability.
This flaw specifically affects the io_uring subsystem's NOP operation handling where file reference management is inconsistent between different code paths. The fix requires harmonizing the file acquisition logic by making the decision between fixed and normal file acquisition based on REQ_F_FIXED_FILE consistently, similar to how io_assign_file() handles this for all other opcodes. This approach ensures that the IORING_NOP_FIXED_FILE flag is properly folded into REQ_F_FIXED_FILE during preparation time rather than leaving the system in an inconsistent state where references are acquired but never properly released.
The solution addresses the fundamental issue by aligning the NOP file reference management with the established patterns used throughout the io_uring implementation. This prevents the scenario where a normal fget() acquisition occurs without proper cleanup, ensuring that all file references are consistently tracked and released through the standard kernel reference counting mechanisms. The fix essentially ensures that when REQ_F_FIXED_FILE is set, regardless of how it was initialized, the same consistent cleanup path is followed for both fixed and normal file acquisitions.
This vulnerability demonstrates the complexity of maintaining consistent reference management in kernel subsystems where multiple flags and paths can influence resource allocation decisions. The issue highlights the importance of ensuring that all code paths leading to resource acquisition have corresponding cleanup mechanisms that are triggered regardless of how the initial decision was made, preventing the type of memory leak that can accumulate over time and potentially lead to system instability or denial of service conditions in high-throughput environments using io_uring operations.