CVE-2025-38170 — linux / linux_kernel
In the Linux kernel, the following vulnerability has been resolved:
arm64/fpsimd: Discard stale CPU state when handling SME traps
The logic for handling SME traps manipulates saved FPSIMD/SVE/SME state incorrectly, and a race with preemption can result in a task having TIF_SME set and TIF_FOREIGN_FPSTATE clear even though the live CPU state is stale (e.g. with SME traps enabled). This can result in warnings from do_sme_acc() where SME traps are not expected while TIF_SME is set:
| /* With TIF_SME userspace shouldn't generate any traps */ | if (test_and_set_thread_flag(TIF_SME)) | WARN_ON(1);
This is very similar to the SVE issue we fixed in commit:
751ecf6afd6568ad ("arm64/sve: Discard stale CPU state when handling SVE traps")
The race can occur when the SME trap handler is preempted before and after manipulating the saved FPSIMD/SVE/SME state, starting and ending on the same CPU, e.g.
| void do_sme_acc(unsigned long esr, struct pt_regs regs) | { | // Trap on CPU 0 with TIF_SME clear, SME traps enabled | // task->fpsimd_cpu is 0. | // per_cpu_ptr(&fpsimd_last_state, 0) is task. | | ... | | // Preempted; migrated from CPU 0 to CPU 1. | // TIF_FOREIGN_FPSTATE is set. | | get_cpu_fpsimd_context(); | | / With TIF_SME userspace shouldn't generate any traps */ | if (test_and_set_thread_flag(TIF_SME)) | WARN_ON(1); | | if (!test_thread_flag(TIF_FOREIGN_FPSTATE)) { | unsigned long vq_minus_one = | sve_vq_from_vl(task_get_sme_vl(current)) - 1; | sme_set_vq(vq_minus_one); | | fpsimd_bind_task_to_cpu(); | } | | put_cpu_fpsimd_context(); | | // Preempted; migrated from CPU 1 to CPU 0. | // task->fpsimd_cpu is still 0 | // If per_cpu_ptr(&fpsimd_last_state, 0) is still task then: | // - Stale HW state is reused (with SME traps enabled) | // - TIF_FOREIGN_FPSTATE is cleared | // - A return to userspace skips HW state restore | }
Fix the case where the state is not live and TIF_FOREIGN_FPSTATE is set by calling fpsimd_flush_task_state() to detach from the saved CPU state. This ensures that a subsequent context switch will not reuse the stale CPU state, and will instead set TIF_FOREIGN_FPSTATE, forcing the new state to be reloaded from memory prior to a return to userspace.
Note: this was originallly posted as [1].
[ Rutland: rewrite commit message ]
| Software | From | Fixed in |
|---|---|---|
| linux / linux_kernel | 5.19 | 6.1.142 |
| linux / linux_kernel | 6.2 | 6.6.94 |
| linux / linux_kernel | 6.7 | 6.12.34 |
| linux / linux_kernel | 6.13 | 6.15.3 |
| debian / debian_linux | 11.0 | 11.0.x |
- https://git.kernel.org/stable/c/43be952e885476dafb74aa832c0847b2f4f650c6
- https://git.kernel.org/stable/c/6103f9ba51a59afb5a0f32299c837377c5a5a693
- https://git.kernel.org/stable/c/c4a4786d93e99517d6f10ed56b9ffba4ce88d3b3
- https://git.kernel.org/stable/c/d3eaab3c70905c5467e5c4ea403053d67505adeb
- https://git.kernel.org/stable/c/de89368de3894a8db27caeb8fd902ba1c49f696a
- https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html
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