CVE-2026-80946 in Linux
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
fuse: copy request headers via a stack buffer for io-uring
The fuse-io-uring transport copies req->in.h out to the ring in fuse_uring_copy_to_ring() and req->out.h back in fuse_uring_commit(). Both headers live inside the fuse_request slab object, whose cache (fuse_req_cachep) is created without a usercopy whitelist, so copying them directly to/from userspace trips CONFIG_HARDENED_USERCOPY and panics:
usercopy: Kernel memory exposure attempt detected from SLUB object 'fuse_request' (offset 56, size 40)! kernel BUG at mm/usercopy.c:102! Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
RIP: 0010:usercopy_abort (mm/usercopy.c:90) Call Trace: __check_heap_object (mm/slub.c:8268) __check_object_size (mm/usercopy.c:197 mm/usercopy.c:258 mm/usercopy.c:223) copy_header_to_ring (fs/fuse/dev_uring.c:618) fuse_uring_prepare_send (fs/fuse/dev_uring.c:776 fs/fuse/dev_uring.c:785) fuse_uring_send_in_task (fs/fuse/dev_uring.c:1306) tctx_task_work_run (io_uring/tw.c:96) task_work_run (kernel/task_work.c:233) io_run_task_work (io_uring/tw.h:84) io_cqring_wait (io_uring/wait.c:278) __do_sys_io_uring_enter (io_uring/io_uring.c:2685) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
Bounce both headers through an on-stack copy so the usercopy touches stack memory, not the slab object.
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Analysis
by VulDB Data Team • 09/12/2026
The Linux kernel's Filesystem in Userspace subsystem contains a critical security flaw within its io-uring transport implementation that triggers a kernel panic when interacting with hardened usercopy protections. This vulnerability arises from an improper handling of data structures during asynchronous I/O operations, specifically involving the copying of request headers between kernel space and userspace via the io_uring interface. The fuse-io-uring mechanism is designed to allow efficient communication between the FUSE daemon running in userspace and the kernel module by utilizing ring buffers for passing requests and responses. However, the implementation directly copies header structures from a slab object allocated on behalf of the request into the user-accessible ring buffer without adequate safeguards against direct memory exposure checks enforced by modern security hardening features.
The technical root cause lies in the allocation characteristics of the fuse_request structure, which resides in the fuse_req_cachep SLUB cache. This particular kernel slab cache was created without a designated usercopy whitelist, meaning that any attempt to copy data directly from this object into userspace is flagged as an invalid operation by CONFIG_HARDENED_USERCOPY. When the function fuse_uring_copy_to_ring attempts to move req->in.h out to the ring and subsequently when fuse_uring_commit copies req->out.h back in, it performs a direct memory copy originating from the slab object. Because these headers are part of the kernel-internal data structure rather than explicitly whitelisted for user exposure, the kernel's security subsystem detects this as an attempt to expose internal kernel memory to userspace, resulting in a fatal bug check and system crash.
The operational impact of this vulnerability is severe, primarily manifesting as a denial-of-service condition through local privilege escalation or system instability. An attacker with access to FUSE mounts can trigger the vulnerable code path by submitting specific io_uring requests that invoke the affected copy operations. The immediate consequence is a kernel panic, which crashes the entire operating system and results in complete service unavailability for all processes running on the host. While the primary effect is availability loss due to the crash, the nature of the flaw involves an attempted exposure of kernel memory layout details through error messages or potential side channels before the panic occurs, although the hardening mechanism effectively blocks the actual data leak by aborting the operation and triggering the bug check.
This vulnerability aligns with CWE-200 Information Exposure and CWE-787 Out-of-bounds Write in the context of improper validation of memory access boundaries enforced by security features. It also relates to ATT&CK technique T1530 Data from Local System, as it involves an attempt to read kernel memory that is not intended for user consumption. The issue highlights a common pitfall in systems programming where performance optimizations bypass necessary security abstractions, specifically the separation between internal kernel data structures and those safe for direct userspace interaction via io_uring rings.
The resolution implemented by the Linux kernel maintainers involves modifying the copy logic to use an intermediate stack buffer as a bounce zone. Instead of copying headers directly from or to the slab object, the code now first copies the header into a local variable on the kernel stack and then performs the usercopy operation using that stack memory. Since stack memory is generally whitelisted for usercopy operations in this context, this approach satisfies CONFIG_HARDENED_USERCOPY requirements while maintaining functional correctness. This mitigation ensures that no direct references to non-whitelisted slab objects are passed to userspace copying functions, thereby eliminating the trigger condition for the panic and restoring secure operation of FUSE over io_uring. Administrators should apply kernel updates containing this patch immediately to prevent potential denial-of-service attacks against systems relying on FUSE with io-uring support.