CVE-2024-50275 Information
Description
In the Linux kernel the following vulnerability has been resolved:
arm64/sve: Discard stale CPU state when handling SVE traps
The logic for handling SVE traps manipulates saved FPSIMD/SVE state incorrectly and a race with preemption can result in a task having TIF_SVE set and TIF_FOREIGN_FPSTATE clear even though the live CPU state is stale (e.g. with SVE traps enabled). This has been observed to result in warnings from do_sve_acc() where SVE traps are not expected while TIF_SVE is set:
| if (test_and_set_thread_flag(TIF_SVE)) | WARN_ON(1); / SVE access shouldn’t have trapped /
Warnings of this form have been reported intermittently e.g.
https://lore.kernel.org/linux-arm-kernel/CA+G9fYtEGe_DhY2Ms7+L7NKsLYUomGsgqpdBj+QwDLeSg=JhGg@mail.gmail.com/ https://lore.kernel.org/linux-arm-kernel/000000000000511e9a060ce5a45c@google.com/
The race can occur when the SVE trap handler is preempted before and after manipulating the saved FPSIMD/SVE state starting and ending on the same CPU e.g.
| void do_sve_acc(unsigned long esr struct pt_regs regs)
|
| // Trap on CPU 0 with TIF_SVE clear SVE 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();
|
| if (test_and_set_thread_flag(TIF_SVE))
| WARN_ON(1); / SVE access shouldn’t have trapped /
|
| sve_init_regs()
| if (!test_thread_flag(TIF_FOREIGN_FPSTATE))
| …
| else
| fpsimd_to_sve(current);
| current->thread.fp_type = FP_STATE_SVE;
|
|
|
| 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 SVE 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.
Reference
https://git.kernel.org/stable/c/51d11ea0250d6ee461987403bbfd4b2abb5613a7 https://git.kernel.org/stable/c/fa9ce027b3ce37a2bb173bf2553b5caa438fd8c9 https://git.kernel.org/stable/c/751ecf6afd6568adc98f2a6052315552c0483d18
Share on: