arm64: entry.S: Do not preempt from IRQ before all cpufeatures are enabled

Preempting from IRQ-return means that the task has its PSTATE saved on the stack, which will get restored when the task is resumed and does the actual IRQ return. However, enabling some CPU features requires modifying the PSTATE. This means that, if a task was scheduled out during an IRQ-return before all CPU features are enabled, the task might restore a PSTATE that does not include the feature enablement changes once scheduled back in. * Task 1: PAN == 0 ---| |--------------- | |<- return from IRQ, PSTATE.PAN = 0 | <- IRQ | +--------+ <- preempt() +-- ^ | reschedule Task 1, PSTATE.PAN == 1 * Init: --------------------+------------------------ ^ | enable_cpu_features set PSTATE.PAN on all CPUs Worse than this, since PSTATE is untouched when task switching is done, a task missing the new bits in PSTATE might affect another task, if both do direct calls to schedule() (outside of IRQ/exception contexts). Fix this by preventing preemption on IRQ-return until features are enabled on all CPUs. This way the only PSTATE values that are saved on the stack are from synchronous exceptions. These are expected to be fatal this early, the exception is BRK for WARN_ON(), but as this uses do_debug_exception() which keeps IRQs masked, it shouldn't call schedule().

JTJulien Thierry committed 7 years ago