Commit a3866208 authored Jan 08, 2019 by Martin Schwidefsky

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This can lead to a situation where a CPU uses the outdated ASCE to run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: 53e857f3 ("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>

parent 03aa047e

arch/s390/include/asm/mmu_context.h

+2 −3

Original line number	Diff line number	Diff line
		@@ -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 */
		@@ -103,6 +101,7 @@ 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);
		}
		if (prev != next)
		cpumask_clear_cpu(cpu, &prev->context.cpu_attach_mask);
		}

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