Linux: drm/i915/gt: Unmask interrupts only when ACTIVE is true

In the Linux kernel, the following vulnerability has been resolved:

drm/i915/gt: Unmask interrupts only when ACTIVE is true

There is a small race window between intel_rps_boost() reading a stale ACTIVE state and queuing rps->work, where ACTIVE may be changed via asynchronous calls triggered by wakeref drop. This means that rps_work() could be triggered after the GT has been parked, so unmasking RPS PM interrupts would happen without holding PM wakelock. This sequence of events may be observed during live_rc6_ctx_wa() selftest:

1) live_rc6_ctx_wa() -> intel_engine_pm_put() -> wakeref_put()

ASYNC1) wakeref_put() callback -> __engine_park() -> intel_gt_pm_put_async() -> intel_gt_pm_put_async_untracked() -> intel_wakeref_put_async()

ASYNC2) wakeref_put_async() callback -> __gt_park() -> intel_rps_park() -> intel_rps_clear_active() [clear ACTIVE] WITH INTERRUPTS: -> rps_disable_interrupts() -> cancel_work_sync(rps->work) -> rps_reset_interrupts() [rps->pm_iir = 0] WITHOUT INTERRUPTS (TIMER mode): -> rps_stop_timer() -> cancel_work_sync(rps->work)

2) live_rc6_ctx_wa() -> intel_gt_wait_for_idle() -> intel_gt_retire_requests_timeout() -> dma_fence_wait_timeout() -> ops->wait() [i915_fence_wait()] -> i915_request_wait_timeout() -> intel_rps_boost() -> intel_rps_is_active() [check ACTIVE] [OPEN RACE WINDOW] -> queue_work(rps->work) -> retire_requests() -> i915_request_retire() -> intel_rps_dec_waiters() [from 1 to 0]

If ASYNC1 and ASYNC2 would execute during [OPEN RACE WINDOW], then:

3) rps_work() executes from the workqueue and may satisfy "(!pm_iir && !client_boost)" test (in TIMER mode rps->pm_iir is zeroed by an earlier rps_work() call or never gets a new value after previous intel_rps_unpark() event), so it jumps to "out" label, bypassing intel_rps_is_active() guard and executes gen6_gt_pm_unmask_irq();

4) GT is parked and runtime PM wakelock is dropped, so only a raw reference in __gt_park() -> intel_display_power_put_async() remains for 100ms, then the MMIO write in gen6_gt_pm_unmask_irq() detects missing wakelock via assert_rpm_wakelock_held() and emits "RPM wakelock ref not held during HW access" tainting kernel.

Commit 3e7abf814193 ("drm/i915: Extract GT render power state management") dropped the "if (rps->interrupts_enabled)" guard that gen6_pm_rps_work() had around this unmask in i915_irq.c. INTEL_RPS_ACTIVE brackets the same window, since it is set in intel_rps_unpark() before rps_enable_interrupts() and rps_start_timer() and cleared in intel_rps_park() before rps_stop_timer() and rps_disable_interrupts().

Skip unmasking when RPS is no longer active (reaching unmask operation is independent of INTERRUPT mode). This does not prevent RPS operation from resuming on the next unpark. In interrupt mode, intel_rps_unpark() calls rps_enable_interrupts(), which enables and unmasks the PM interrupts. In timer mode, it calls rps_start_timer(), restarting periodic engine-busyness evaluation. The timer generates software RPS events and queues rps_work() directly, independently of PM interrupt delivery. Neither mode requires the late worker to unmask interrupts while the GT is parked

(cherry picked from commit ca4a6deadad15b91ee083afe445fa6bdbfb68206)