Cross-thread FPU register leak on ARM when FPU enabled without register sharing

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-11985
StatePUBLISHED
Assignerzephyr
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-08-11 05:17:11 UTC
Updated2026-08-11 05:17:11 UTC
DescriptionOn 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. In 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. Under 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. The 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.

Risk And Classification

Primary CVSS: v3.1 3.6 LOW from [email protected]

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N

Problem Types: CWE-200 | CWE-200 info-leak

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary3.6LOWCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N
3.1CNACVSS3.6LOWCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N

CVSS v3.1 Breakdown

Attack Vector
Local
Attack Complexity
High
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Zephyrproject Zephyr affected 1.13.0 4.4.1 semver Not specified

References

ReferenceSourceLinkTags
github.com/zephyrproject-rtos/zephyr/commit/3d405326a7653cba6860280e45c8... [email protected] github.com
github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-qxr9-wh3c-... [email protected] github.com
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