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This can lead to a situation where a CPU uses the outdated ASCE to run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 ccbb53e2202404b85aae86e883d3e64405d2d305..e4462202200d74fa4e1ff77553e6dd8d2a7e2a4e 100644 (file)
--- 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