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

Overview

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.

Affected

• Linux >= f7f510ec195781c857ab76366a3e1c59e1caae42 < 3aa3e89cf80721c8d382b4c1a2b70a0449dad4a5
• Linux >= f7f510ec195781c857ab76366a3e1c59e1caae42 < 63335e7b638ae70028ae285bb95153874a8bc852
• Linux >= f7f510ec195781c857ab76366a3e1c59e1caae42 < 2e788948ff2a13358a303af112497a63201c5739
• Linux >= f7f510ec195781c857ab76366a3e1c59e1caae42 < fde19b0d4eeabae042519313c843fe6f27d41e9d
• Linux >= f7f510ec195781c857ab76366a3e1c59e1caae42 < 81dd21b5f0c299cc7b5bf84f04a61938559d20e6
• Linux >= f7f510ec195781c857ab76366a3e1c59e1caae42 < 285e17c44e3873a73460f294acbd64018ff64385
• Linux >= f7f510ec195781c857ab76366a3e1c59e1caae42 < 92d5736a62040ec1cfff23ea57e6599301690ad5
• Linux >= f7f510ec195781c857ab76366a3e1c59e1caae42 < e3046eeada299f917a8ad883af4434bfb86556b1
• Linux 2.6.26

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.