In the Linux kernel, the following vulnerability has been resolved: hwrng: virtio: clamp device-reported used.len at copy_data() random_recv_done()… (CVE-2026-64456)
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.
AI Analysis
Technical Summary
The Linux kernel's virtio-rng driver stored the device-reported used.len value directly without validating it against the buffer size, leading to an out-of-bounds read in copy_data(). A malicious or buggy virtio-rng backend could set used.len beyond the size of the internal data buffer, causing memcpy() to read past the buffer into adjacent kmalloc slab memory. This memory is then mixed into the guest RNG output and can be directly read by guest root via /dev/hwrng. This creates a memory safety issue and a potential cross-boundary information leak in confidential compute environments. The vulnerability was confirmed by KASAN testing and fixed by clamping the used.len value and adding array_index_nospec() to prevent speculative out-of-bounds access.
Potential Impact
The vulnerability allows a malicious or buggy virtio-rng backend to cause an out-of-bounds read of guest kernel memory, contaminating the guest's random number generator output with adjacent slab data. In confidential compute scenarios, a malicious hypervisor combined with compromised guest root can use this to leak guest kernel heap contents. The issue compromises memory safety and can lead to information disclosure within the guest. However, many confidential compute guests disable virtio-rng, limiting the attack surface. No known exploits are reported in the wild.
Mitigation Recommendations
A fix is available that clamps the device-reported used.len at the point of use and applies array_index_nospec() to prevent out-of-bounds access. Users should apply the official Linux kernel patches that address this issue. Since this is a kernel vulnerability, upgrading to a patched kernel version is the recommended remediation. No additional mitigation is required if the kernel is updated. Patch status is not explicitly confirmed in the provided data; check the vendor advisory for current remediation guidance.
In the Linux kernel, the following vulnerability has been resolved: hwrng: virtio: clamp device-reported used.len at copy_data() random_recv_done()… (CVE-2026-64456)
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.
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The Linux kernel's virtio-rng driver stored the device-reported used.len value directly without validating it against the buffer size, leading to an out-of-bounds read in copy_data(). A malicious or buggy virtio-rng backend could set used.len beyond the size of the internal data buffer, causing memcpy() to read past the buffer into adjacent kmalloc slab memory. This memory is then mixed into the guest RNG output and can be directly read by guest root via /dev/hwrng. This creates a memory safety issue and a potential cross-boundary information leak in confidential compute environments. The vulnerability was confirmed by KASAN testing and fixed by clamping the used.len value and adding array_index_nospec() to prevent speculative out-of-bounds access.
Potential Impact
The vulnerability allows a malicious or buggy virtio-rng backend to cause an out-of-bounds read of guest kernel memory, contaminating the guest's random number generator output with adjacent slab data. In confidential compute scenarios, a malicious hypervisor combined with compromised guest root can use this to leak guest kernel heap contents. The issue compromises memory safety and can lead to information disclosure within the guest. However, many confidential compute guests disable virtio-rng, limiting the attack surface. No known exploits are reported in the wild.
Mitigation Recommendations
A fix is available that clamps the device-reported used.len at the point of use and applies array_index_nospec() to prevent out-of-bounds access. Users should apply the official Linux kernel patches that address this issue. Since this is a kernel vulnerability, upgrading to a patched kernel version is the recommended remediation. No additional mitigation is required if the kernel is updated. Patch status is not explicitly confirmed in the provided data; check the vendor advisory for current remediation guidance.
Technical Details
- Gcve Source
- db.gcve.eu
- Osv Id
- GHSA-7vrc-rff8-p548
- Osv Schema Version
- 1.4.0
- Aliases
- ["CVE-2026-64456"]
- Ecosystems
- []
- Database Specific Severity
- null
- Cvss Version
- null
Threat ID: 6a6542089c2644c7f8082fc7
Added to database: 07/25/2026, 23:08:56 UTC
Last enriched: 07/25/2026, 23:20:58 UTC
Last updated: 09/07/2026, 10:52:10 UTC
Views: 47
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