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.
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.
Published:
2026-07-25
Score:
n/a
EPSS:
n/a
KEV:
No
Impact:
n/a
Action:
n/a
Analysis
No analysis available yet.
Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).
| Vendor | Product | Default status | Versions | |||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Linux | Linux | unaffected |
|
|||||||||||||||||||||||||||||||||
| Linux | Linux | affected |
|
No data.
No data.
No data available yet.
Remediation
No remediation available yet.
Tracking
Sign in to view the affected projects.
Advisories
No advisories yet.
References
History
Sat, 25 Jul 2026 09:30:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| 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. | |
| Title | hwrng: virtio: clamp device-reported used.len at copy_data() | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
|
MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-07-25T08:51:24.659Z
Reserved: 2026-07-19T15:36:31.789Z
Link: CVE-2026-64456
Vulnrichment
No data.
NVD
No data.
Redhat
No data.
OpenCVE Enrichment
No data.
Weaknesses
No weakness.