{"id":"CVE-2026-64456","summary":"hwrng: virtio: clamp device-reported used.len at copy_data()","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nhwrng: virtio: clamp device-reported used.len at copy_data()\n\nrandom_recv_done() stores the device-reported used.len directly into\nvi-\u003edata_avail.  copy_data() then indexes vi-\u003edata[] using\nvi-\u003edata_idx (advanced by previous copy_data() calls) and issues a\nmemcpy() without re-validating either value against the posted\nbuffer size sizeof(vi-\u003edata) (SMP_CACHE_BYTES bytes, typically 32\nor 64).\n\nA malicious or buggy virtio-rng backend can set used.len beyond\nsizeof(vi-\u003edata), steering the memcpy() past the end of the inline\narray into adjacent kmalloc-1k slab bytes.  hwrng_fillfn() mixes\nthose bytes into the guest RNG, and guest root can also observe\nthem directly via /dev/hwrng.\n\nConcrete impact is inside the guest:\n\n - Memory-safety / hardening: any virtio-rng backend that\n   over-reports used.len causes the driver to read past vi-\u003edata\n   into unrelated slab contents.  hwrng_fillfn() is a kernel thread\n   that runs as soon as the device is probed; no guest userspace\n   interaction is required to first-trigger the OOB.\n\n - Cross-boundary leak (confidential-compute threat model): a\n   malicious hypervisor cooperating with a malicious or compromised\n   guest root userspace can use /dev/hwrng as a leak channel for\n   guest-kernel heap data.  The host sets a large used.len, guest\n   root reads /dev/hwrng, and the returned bytes contain guest\n   kernel slab contents that were adjacent to vi-\u003edata.  In\n   practice, confidential-compute guests (SEV-SNP, TDX) usually\n   disable virtio-rng entirely, so this path is narrow, but the\n   fix is still worth carrying because the underlying\n   memory-safety bug contaminates the guest RNG on any host.\n\nKASAN confirms the OOB on a 7.1-rc4 guest whose virtio-rng backend\nhas been patched to report used.len = 0x10000:\n\n  BUG: KASAN: slab-out-of-bounds in virtio_read+0x394/0x5d0\n  Read of size 64 at addr ffff88800ae0ba20 by task hwrng/52\n  Call Trace:\n   __asan_memcpy+0x23/0x60\n   virtio_read+0x394/0x5d0\n   hwrng_fillfn+0xb2/0x470\n   kthread+0x2cc/0x3a0\n  Allocated by task 1:\n   probe_common+0xa5/0x660\n   virtio_dev_probe+0x549/0xbc0\n  The buggy address belongs to the object at ffff88800ae0b800\n   which belongs to the cache kmalloc-1k of size 1024\n  The buggy address is located 0 bytes to the right of\n   allocated 544-byte region [ffff88800ae0b800, ffff88800ae0ba20)\n\nSame class of bug as commit c04db81cd028 (\"net/9p: Fix buffer\noverflow in USB transport layer\"), which hardened\nusb9pfs_rx_complete() against unchecked device-reported length in\nthe USB 9p transport.\n\nWith the clamp at point of use and array_index_nospec() in place,\nthe same harness boots cleanly: copy_data() returns zero for the\nbogus report, the device-supplied bytes after data_idx are\ndiscarded, and the driver issues a fresh request.","modified":"2026-07-28T04:03:07.239662922Z","published":"2026-07-25T08:51:24.659Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/64xxx/CVE-2026-64456.json"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/285e17c44e3873a73460f294acbd64018ff64385"},{"type":"WEB","url":"https://git.kernel.org/stable/c/2e788948ff2a13358a303af112497a63201c5739"},{"type":"WEB","url":"https://git.kernel.org/stable/c/3aa3e89cf80721c8d382b4c1a2b70a0449dad4a5"},{"type":"WEB","url":"https://git.kernel.org/stable/c/63335e7b638ae70028ae285bb95153874a8bc852"},{"type":"WEB","url":"https://git.kernel.org/stable/c/81dd21b5f0c299cc7b5bf84f04a61938559d20e6"},{"type":"WEB","url":"https://git.kernel.org/stable/c/92d5736a62040ec1cfff23ea57e6599301690ad5"},{"type":"WEB","url":"https://git.kernel.org/stable/c/e3046eeada299f917a8ad883af4434bfb86556b1"},{"type":"WEB","url":"https://git.kernel.org/stable/c/fde19b0d4eeabae042519313c843fe6f27d41e9d"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/64xxx/CVE-2026-64456.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-64456"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"f7f510ec195781c857ab76366a3e1c59e1caae42"},{"fixed":"3aa3e89cf80721c8d382b4c1a2b70a0449dad4a5"},{"fixed":"63335e7b638ae70028ae285bb95153874a8bc852"},{"fixed":"2e788948ff2a13358a303af112497a63201c5739"},{"fixed":"fde19b0d4eeabae042519313c843fe6f27d41e9d"},{"fixed":"81dd21b5f0c299cc7b5bf84f04a61938559d20e6"},{"fixed":"285e17c44e3873a73460f294acbd64018ff64385"},{"fixed":"92d5736a62040ec1cfff23ea57e6599301690ad5"},{"fixed":"e3046eeada299f917a8ad883af4434bfb86556b1"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-64456.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"2.6.26"},{"fixed":"5.10.261"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.11.0"},{"fixed":"5.15.212"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.16.0"},{"fixed":"6.1.178"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.2.0"},{"fixed":"6.6.145"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.7.0"},{"fixed":"6.12.96"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.18.39"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"7.1.4"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-64456.json"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H"}]}