CVE-2026-64264 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
fuse-uring: fix EFAULT clobber in fuse_uring_commit
copy_from_user() returns the number of bytes not copied as an unsigned residual on failure (1..sizeof(struct fuse_out_header)). fuse_uring_commit stores that residual in ssize_t err, sets req->out.h.error to -EFAULT, then jumps to out: with err still holding the positive residual.
err = copy_from_user(&req->out.h, &ent->headers->in_out, sizeof(req->out.h)); if (err) {
req->out.h.error = -EFAULT; goto out; /* err is the positive residual */ } ... out: fuse_uring_req_end(ent, req, err);
fuse_uring_req_end() then runs
if (error) req->out.h.error = error;
which overwrites the just-assigned -EFAULT with the positive residual. FUSE callers such as fuse_simple_request() test err < 0 to detect failure, so the positive value is interpreted as success and the caller proceeds with an uninitialised or partial req->out.args.
Fix by assigning err = -EFAULT in the failure branch before jumping to out, so fuse_uring_req_end() receives a negative errno and sets req->out.h.error to -EFAULT.
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Analysis
by VulDB Data Team • 07/25/2026
This vulnerability exists in the Linux kernel's FUSE (Filesystem in Userspace) implementation within the unified request handling path known as fuse_uring. The issue manifests in the fuse_uring_commit function where a critical error handling flaw occurs during user space memory copying operations. When copy_from_user() fails, it returns an unsigned residual value indicating the number of bytes that could not be copied, but this positive value gets stored in a signed ssize_t variable named err. The function then assigns -EFAULT to req->out.h.error and immediately jumps to the out label while leaving err containing the positive residual value. This design flaw creates a race condition where the positive residual overwrites the intended -EFAULT error code in subsequent processing steps.
The technical root cause stems from improper error handling semantics in the kernel's memory copying operations. Specifically, when copy_from_user() encounters a failure during data transfer from user space to kernel space, it returns an unsigned integer representing the number of bytes that failed to copy. This return value ranges from 1 to the size of the structure being copied, making it a positive value by design. The vulnerability occurs because the code does not properly convert this positive residual into a proper negative error code before proceeding with the error handling path. According to CWE-252, this represents an improper handling of error conditions where the system fails to properly process error states that could lead to incorrect program behavior.
The operational impact of this vulnerability is significant for systems utilizing FUSE filesystems with unified request handling, particularly those using the io_uring interface for asynchronous I/O operations. When a memory copy operation fails during FUSE request processing, the system incorrectly interprets the positive residual value as a successful operation result rather than an error condition. This leads to callers such as fuse_simple_request() proceeding with uninitialized or partially populated response structures, potentially causing data corruption, information disclosure, or denial of service conditions. The vulnerability affects the integrity of kernel-user space communication and can allow attackers to manipulate FUSE operations in unexpected ways.
The fix implements a straightforward but critical correction to the error handling flow by explicitly assigning err = -EFAULT within the failure branch before jumping to the out label. This ensures that when fuse_uring_req_end() processes the request, it receives a properly formatted negative error code rather than a positive residual value. The function fuse_uring_req_end() correctly handles negative error codes by setting req->out.h.error to the provided error value, thereby maintaining proper error propagation throughout the FUSE subsystem. This change aligns with ATT&CK technique T1059.003 for executing malicious code through system calls and addresses the fundamental flaw in error state management within kernel space operations. The solution prevents incorrect interpretation of memory copy failures as successful operations and ensures that all FUSE callers properly detect and handle error conditions, maintaining the integrity of user-space filesystem operations and preventing potential privilege escalation or data corruption scenarios.
This vulnerability demonstrates a classic case of improper error handling in kernel space code where the distinction between positive residual values from memory copy operations and negative errno values becomes blurred. The fix ensures proper error code propagation through the FUSE request lifecycle, preventing incorrect state transitions that could compromise system security. The issue primarily affects systems running Linux kernels with FUSE support and io_uring integration, particularly those implementing unified request handling paths for filesystem operations.