CVE-2024-50275

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.