powerpc: Skip stack validation checking alternate stacks if they are not allocated

Stack validation in early boot can just bail out of checking alternate
stacks if they are not validated yet. Checking against a NULL stack
could cause NULLish pointer values to be considered valid.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221216115930.2667772-5-npiggin@gmail.com

arch/powerpc/kernel/process.c

@@ -2117,6 +2117,9 @@ static inline int valid_irq_stack(unsigned long sp, struct task_struct *p,
 	unsigned long stack_page;
 	unsigned long cpu = task_cpu(p);
 
+	if (!hardirq_ctx[cpu] || !softirq_ctx[cpu])
+		return 0;
+
 	stack_page = (unsigned long)hardirq_ctx[cpu];
 	if (sp >= stack_page && sp <= stack_page + THREAD_SIZE - nbytes)
 		return 1;
@@ -2138,6 +2141,14 @@ static inline int valid_emergency_stack(unsigned long sp, struct task_struct *p,
 	if (!paca_ptrs)
 		return 0;
 
+	if (!paca_ptrs[cpu]->emergency_sp)
+		return 0;
+
+# ifdef CONFIG_PPC_BOOK3S_64
+	if (!paca_ptrs[cpu]->nmi_emergency_sp || !paca_ptrs[cpu]->mc_emergency_sp)
+		return 0;
+#endif
+
 	stack_page = (unsigned long)paca_ptrs[cpu]->emergency_sp - THREAD_SIZE;
 	if (sp >= stack_page && sp <= stack_page + THREAD_SIZE - nbytes)
 		return 1;