powerpc: Fix reschedule bug in KUAP-unlocked user copy

schedule must not be explicitly called while KUAP is unlocked, because
the AMR register will not be saved across the context switch on
64s (preemption is allowed because that is driven by interrupts which do
save the AMR).

exit_vmx_usercopy() runs inside an unlocked user access region, and it
calls preempt_enable() which will call schedule() if need_resched() was
set while non-preemptible. This can cause tasks to run unprotected when
the should not, and can cause the user copy to be improperly blocked
when scheduling back to it.

Fix this by avoiding the explicit resched for preempt kernels by
generating an interrupt to reschedule the context if need_resched() got
set.

Reported-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221013151647.1857994-3-npiggin@gmail.com

arch/powerpc/lib/vmx-helper.c

@@ -36,7 +36,17 @@ int exit_vmx_usercopy(void)
 {
 	disable_kernel_altivec();
 	pagefault_enable();
-	preempt_enable();
+	preempt_enable_no_resched();
+	/*
+	 * Must never explicitly call schedule (including preempt_enable())
+	 * while in a kuap-unlocked user copy, because the AMR register will
+	 * not be saved and restored across context switch. However preempt
+	 * kernels need to be preempted as soon as possible if need_resched is
+	 * set and we are preemptible. The hack here is to schedule a
+	 * decrementer to fire here and reschedule for us if necessary.
+	 */
+	if (IS_ENABLED(CONFIG_PREEMPT) && need_resched())
+		set_dec(1);
 	return 0;
 }