FAILED: patch "[PATCH] s390/mm: always force a load of the primary ASCE on context" failed to apply to 3.18-stable tree

The patch below does not apply to the 3.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From a38662084c8bdb829ff486468c7ea801c13fcc34 Mon Sep 17 00:00:00 2001
From: Martin Schwidefsky <[email protected]>
Date: Tue, 8 Jan 2019 12:44:57 +0100
Subject: [PATCH] s390/mm: always force a load of the primary ASCE on context
switch

The ASCE of an mm_struct can be modified after a task has been created, e.g. via crst_table_downgrade for a compat process. The active_mm logic to avoid the switch_mm call if the next task is a kernel thread can lead to a situation where switch_mm is called where 'prev == next' is true but 'prev->context.asce == next->context.asce' is not.

This can lead to a situation where a CPU uses the outdated ASCE to run a task. The result can be a crash, endless loops and really subtle problem due to TLBs being created with an invalid ASCE.

index ccbb53e22024..e4462202200d 100644
@@ -90,8 +90,6 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 {
 	int cpu = smp_processor_id();
 
-	if (prev == next)
-		return;
 	S390_lowcore.user_asce = next->context.asce;
 	cpumask_set_cpu(cpu, &next->context.cpu_attach_mask);
 	/* Clear previous user-ASCE from CR1 and CR7 */

@@ -103,7 +101,8 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 		__ctl_load(S390_lowcore.vdso_asce, 7, 7);
 		clear_cpu_flag(CIF_ASCE_SECONDARY);
 	}
-	cpumask_clear_cpu(cpu, &prev->context.cpu_attach_mask);
+	if (prev != next)
+		cpumask_clear_cpu(cpu, &prev->context.cpu_attach_mask);
 }
 
 #define finish_arch_post_lock_switch finish_arch_post_lock_switch