{"id":"CVE-2026-11985","summary":"Cross-thread FPU register leak on ARM when FPU enabled without register sharing","details":"On the Zephyr ARM port, enabling the hardware FPU (CONFIG_FPU) forces the \"Floating point ABI\" choice, which defaults to CONFIG_FP_HARDABI. Both FP_HARDABI and FP_SOFTABI permit the compiler to emit hardware FP instructions in any function, even code that never uses floating-point types. However, the callee-saved FP registers (s16-s31 / d8-d15) are only saved and restored across a context switch when CONFIG_FPU_SHARING is enabled (arch/arm/core/cortex_m/swap_helper.S and arch/arm/core/cortex_a_r/swap_helper.S), and prior to this fix selecting an ABI did not enable FPU register sharing, which defaults off.\n\nIn a build that enables the FPU with the default ABI but leaves CONFIG_FPU_SHARING disabled, the kernel preserves no callee-saved FP register state across thread switches. The documented precondition for this \"unshared\" mode — that only a single thread ever executes FP instructions — is silently violated because the compiler may generate FP instructions in every thread.\n\nUnder CONFIG_USERSPACE, where threads are mutually isolated, this becomes an information-disclosure boundary crossing: a victim thread can leave secret-derived values in s16-s31, and a co-resident unprivileged thread can read those registers directly (FP register access is not privilege-gated), recovering data left behind by another thread. Without userspace the same defect causes cross-thread FP state corruption (a correctness fault). The leak is bounded to the 16 callee-saved single-precision registers and is opportunistic, so impact is low.\n\nThe fix makes FP_HARDABI and FP_SOFTABI select CONFIG_FPU_SHARING and tags every thread with K_FP_REGS at creation, so callee-saved FP state is always preserved across context switches whenever the compiler may emit FP instructions.","aliases":["GHSA-qxr9-wh3c-hvgv"],"modified":"2026-08-13T08:22:26.809198Z","published":"2026-08-11T04:56:59.359Z","database_specific":{"cna_assigner":"zephyr","cwe_ids":["CWE-200"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/11xxx/CVE-2026-11985.json"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/11xxx/CVE-2026-11985.json"},{"type":"ADVISORY","url":"https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-qxr9-wh3c-hvgv"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-11985"},{"type":"FIX","url":"https://github.com/zephyrproject-rtos/zephyr/commit/3d405326a7653cba6860280e45c8734f8d3fc423"},{"type":"PACKAGE","url":"https://github.com/zephyrproject-rtos/zephyr"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/zephyrproject-rtos/zephyr","events":[{"introduced":"d62e861a01a2f4455ae89d105aa9dbc4bef501c5"},{"fixed":"3d405326a7653cba6860280e45c8734f8d3fc423"}],"database_specific":{"extracted_events":[{"introduced":"1.13.0"},{"last_affected":"4.4.1"}],"source":["AFFECTED_FIELD","REFERENCES"]}}],"versions":["v4.4.0","v4.4.0-rc3","v4.4.0-rc2","v4.4.0-rc1","v4.3.0","v4.3.0-rc3","v4.3.0-rc2","v4.3.0-rc1","v4.2.0","v4.2.0-rc3","v4.2.0-rc2","v4.2.0-rc1","v4.1.0","v4.1.0-rc3","v4.1.0-rc2","v4.1.0-rc1","v4.0.0","v4.0.0-rc3","v4.0.0-rc2","v4.0.0-rc1","v3.7.0","v3.7.0-rc3","v3.7.0-rc2","v3.7.0-rc1","v3.6.0","v3.6.0-rc3","v3.6.0-rc2","v3.6.0-rc1","zephyr-v3.5.0","v3.5.0","v3.5.0-rc3","v3.5.0-rc2","v3.5.0-rc1","zephyr-v3.4.0","v3.4.0","v3.4.0-rc3","v3.4.0-rc2","v3.4.0-rc1","zephyr-v3.3.0","v3.3.0","v3.3.0-rc3","v3.3.0-rc2","v3.3.0-rc1","zephyr-v3.2.0","v3.2.0","v3.2.0-rc3","v3.2.0-rc2","v3.2.0-rc1","zephyr-v3.1.0","v3.1.0","v3.1.0-rc3","v3.1.0-rc2","v3.1.0-rc1","zephyr-v3.0.0","v3.0.0","v3.0.0-rc3","v3.0.0-rc2","v3.0.0-rc1","v2.7.99","v2.7.0-rc3","v2.7.0-rc2","v2.7.0-rc1","zephyr-v2.6.0","v2.6.0","v2.6.0-rc3","v2.6.0-rc2","v2.6.0-rc1","zephyr-v2.5.0","v2.5.0","v2.5.0-rc4","v2.5.0-rc3","v2.5.0-rc2","v2.5.0-rc1","zephyr-v2.4.0","v2.4.0","v2.4.0-rc3","v2.4.0-rc2","v2.4.0-rc1","zephyr-v2.3.0","v2.3.0","v2.3.0-rc2","v2.3.0-rc1","zephyr-v2.1.0","v2.1.0","zephyr-v2.2.0","v2.2.0","v2.2.0-rc3","v2.2.0-rc2","v2.2.0-rc1","v2.1.0-rc3","v2.1.0-rc2","v2.1.0-rc1","zephyr-v2.0.0","v2.0.0","v2.0.0-rc3","v2.0.0-rc2","v2.0.0-rc1","zephyr-v1.14.0","v1.14.0","v1.14.0-rc3","v1.14.0-rc2","v1.14.0-rc1","zephyr-v1.13.0","v1.13.0"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-11985.json","vanir_signatures_modified":"2026-08-13T08:22:26Z","vanir_signatures":[{"target":{"file":"arch/arm/core/cortex_a_r/thread.c"},"deprecated":false,"digest":{"line_hashes":["201286029314524494498128356808564192504","104347002957239262924125349064233115973","166051331415121378672551717649011919236"],"threshold":0.9},"id":"CVE-2026-11985-79939f1b","signature_type":"Line","signature_version":"v1","source":"https://github.com/zephyrproject-rtos/zephyr/commit/3d405326a7653cba6860280e45c8734f8d3fc423"},{"signature_version":"v1","source":"https://github.com/zephyrproject-rtos/zephyr/commit/3d405326a7653cba6860280e45c8734f8d3fc423","target":{"file":"arch/arm/core/cortex_m/thread.c"},"deprecated":false,"digest":{"line_hashes":["133213859919341387544139446249840634504","168053699243473771620553595147241923081","62889485891716143511019643449186801737","141840167206073541013171732426541765659"],"threshold":0.9},"id":"CVE-2026-11985-9a6f2ca0","signature_type":"Line"},{"source":"https://github.com/zephyrproject-rtos/zephyr/commit/3d405326a7653cba6860280e45c8734f8d3fc423","target":{"file":"arch/arm/core/cortex_a_r/thread.c","function":"arch_new_thread"},"deprecated":false,"digest":{"length":2198,"function_hash":"204756728203584525907142883130311346884"},"id":"CVE-2026-11985-eb9f7c8b","signature_type":"Function","signature_version":"v1"},{"signature_type":"Function","signature_version":"v1","source":"https://github.com/zephyrproject-rtos/zephyr/commit/3d405326a7653cba6860280e45c8734f8d3fc423","target":{"file":"arch/arm/core/cortex_m/thread.c","function":"arch_new_thread"},"deprecated":false,"digest":{"function_hash":"143836995697260138417716582391524082636","length":2158},"id":"CVE-2026-11985-f21a2147"}]}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N"}]}