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 stable@vger.kernel.org.

thanks,

greg k-h

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

From a38662084c8bdb829ff486468c7ea801c13fcc34 Mon Sep 17 00:00:00 2001

From: Martin Schwidefsky schwidefsky@de.ibm.com 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This can lead to a situation where a CPU uses the outdated ASCE to run a task.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.

Cc: stable@kernel.org # v3.15+ Fixes: 53e857f30867 ("s390/mm,tlb: race of lazy TLB flush vs. recreation") Reported-by: Heiko Carstens heiko.carstens@de.ibm.com Reviewed-by: Heiko Carstens heiko.carstens@de.ibm.com Signed-off-by: Martin Schwidefsky schwidefsky@de.ibm.com

diff --git a/arch/s390/include/asm/mmu_context.h b/arch/s390/include/asm/mmu_context.h index ccbb53e22024..e4462202200d 100644 --- a/arch/s390/include/asm/mmu_context.h +++ b/arch/s390/include/asm/mmu_context.h @@ -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