3.1 KiB
CVE-2025-38170
Description
In the Linux kernel, the following vulnerability has been resolved:arm64/fpsimd: Discard stale CPU state when handling SME trapsThe logic for handling SME traps manipulates saved FPSIMD/SVE/SME stateincorrectly, and a race with preemption can result in a task havingTIF_SME set and TIF_FOREIGN_FPSTATE clear even though the live CPU stateis stale (e.g. with SME traps enabled). This can result in warnings fromdo_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 andafter manipulating the saved FPSIMD/SVE/SME state, starting and ending onthe 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 setby calling fpsimd_flush_task_state() to detach from the saved CPUstate. This ensures that a subsequent context switch will not reuse thestale CPU state, and will instead set TIF_FOREIGN_FPSTATE, forcing thenew state to be reloaded from memory prior to a return to userspace.Note: this was originallly posted as [1].[ Rutland: rewrite commit message ]
POC
Reference
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