CVE-2026-64456: hwrng: virtio: clamp device-reported used.len at copy_data()

In the Linux kernel, the following vulnerability has been resolved:

hwrng: virtio: clamp device-reported used.len at copydata()

randomrecvdone() stores the device-reported used.len directly into vi->dataavail. copydata() then indexes vi->data[] using vi->dataidx (advanced by previous copydata() calls) and issues a memcpy() without re-validating either value against the posted buffer size sizeof(vi->data) (SMPCACHEBYTES bytes, typically 32 or 64).

A malicious or buggy virtio-rng backend can set used.len beyond sizeof(vi->data), steering the memcpy() past the end of the inline array into adjacent kmalloc-1k slab bytes. hwrngfillfn() mixes those bytes into the guest RNG, and guest root can also observe them directly via /dev/hwrng.

Concrete impact is inside the guest:

- Memory-safety / hardening: any virtio-rng backend that over-reports used.len causes the driver to read past vi->data into unrelated slab contents. hwrngfillfn() is a kernel thread that runs as soon as the device is probed; no guest userspace interaction is required to first-trigger the OOB.

- Cross-boundary leak (confidential-compute threat model): a malicious hypervisor cooperating with a malicious or compromised guest root userspace can use /dev/hwrng as a leak channel for guest-kernel heap data. The host sets a large used.len, guest root reads /dev/hwrng, and the returned bytes contain guest kernel slab contents that were adjacent to vi->data. In practice, confidential-compute guests (SEV-SNP, TDX) usually disable virtio-rng entirely, so this path is narrow, but the fix is still worth carrying because the underlying memory-safety bug contaminates the guest RNG on any host.

KASAN confirms the OOB on a 7.1-rc4 guest whose virtio-rng backend has been patched to report used.len = 0x10000:

BUG: KASAN: slab-out-of-bounds in virtioread+0x394/0x5d0 Read of size 64 at addr ffff88800ae0ba20 by task hwrng/52 Call Trace: asanmemcpy+0x23/0x60 virtioread+0x394/0x5d0 hwrngfillfn+0xb2/0x470 kthread+0x2cc/0x3a0 Allocated by task 1: probecommon+0xa5/0x660 virtiodevprobe+0x549/0xbc0 The buggy address belongs to the object at ffff88800ae0b800 which belongs to the cache kmalloc-1k of size 1024 The buggy address is located 0 bytes to the right of allocated 544-byte region [ffff88800ae0b800, ffff88800ae0ba20)

Same class of bug as commit c04db81cd028 ("net/9p: Fix buffer overflow in USB transport layer"), which hardened usb9pfsrxcomplete() against unchecked device-reported length in the USB 9p transport.

With the clamp at point of use and arrayindexnospec() in place, the same harness boots cleanly: copydata() returns zero for the bogus report, the device-supplied bytes after dataidx are discarded, and the driver issues a fresh request.