s390/mm: fix asce_bits handling with dynamic pagetable levels
There is a race with multi-threaded applications between context switch and
pagetable upgrade. In switch_mm() a new user_asce is built from mm->pgd and
mm->context.asce_bits, w/o holding any locks. A concurrent mmap with a
pagetable upgrade on another thread in crst_table_upgrade() could already
have set new asce_bits, but not yet the new mm->pgd. This would result in a
corrupt user_asce in switch_mm(), and eventually in a kernel panic from a
translation exception.
Fix this by storing the complete asce instead of just the asce_bits, which
can then be read atomically from switch_mm(), so that it either sees the
old value or the new value, but no mixture. Both cases are OK. Having the
old value would result in a page fault on access to the higher level memory,
but the fault handler would see the new mm->pgd, if it was a valid access
after the mmap on the other thread has completed. So as worst-case scenario
we would have a page fault loop for the racing thread until the next time
slice.
Also remove dead code and simplify the upgrade/downgrade path, there are no
upgrades from 2 levels, and only downgrades from 3 levels for compat tasks.
There are also no concurrent upgrades, because the mmap_sem is held with
down_write() in do_mmap, so the flush and table checks during upgrade can
be removed.
Reported-by: Michael Munday <munday@ca.ibm.com>
Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
index c7b0451..2489b2e 100644
--- a/arch/s390/mm/init.c
+++ b/arch/s390/mm/init.c
@@ -89,7 +89,8 @@
asce_bits = _ASCE_TYPE_REGION3 | _ASCE_TABLE_LENGTH;
pgd_type = _REGION3_ENTRY_EMPTY;
}
- S390_lowcore.kernel_asce = (__pa(init_mm.pgd) & PAGE_MASK) | asce_bits;
+ init_mm.context.asce = (__pa(init_mm.pgd) & PAGE_MASK) | asce_bits;
+ S390_lowcore.kernel_asce = init_mm.context.asce;
clear_table((unsigned long *) init_mm.pgd, pgd_type,
sizeof(unsigned long)*2048);
vmem_map_init();
diff --git a/arch/s390/mm/mmap.c b/arch/s390/mm/mmap.c
index 45c4daa..89cf09e 100644
--- a/arch/s390/mm/mmap.c
+++ b/arch/s390/mm/mmap.c
@@ -174,7 +174,7 @@
if (!(flags & MAP_FIXED))
addr = 0;
if ((addr + len) >= TASK_SIZE)
- return crst_table_upgrade(current->mm, TASK_MAX_SIZE);
+ return crst_table_upgrade(current->mm);
return 0;
}
@@ -191,7 +191,7 @@
return area;
if (area == -ENOMEM && !is_compat_task() && TASK_SIZE < TASK_MAX_SIZE) {
/* Upgrade the page table to 4 levels and retry. */
- rc = crst_table_upgrade(mm, TASK_MAX_SIZE);
+ rc = crst_table_upgrade(mm);
if (rc)
return (unsigned long) rc;
area = arch_get_unmapped_area(filp, addr, len, pgoff, flags);
@@ -213,7 +213,7 @@
return area;
if (area == -ENOMEM && !is_compat_task() && TASK_SIZE < TASK_MAX_SIZE) {
/* Upgrade the page table to 4 levels and retry. */
- rc = crst_table_upgrade(mm, TASK_MAX_SIZE);
+ rc = crst_table_upgrade(mm);
if (rc)
return (unsigned long) rc;
area = arch_get_unmapped_area_topdown(filp, addr, len,
diff --git a/arch/s390/mm/pgalloc.c b/arch/s390/mm/pgalloc.c
index f6c3de2..e8b5962 100644
--- a/arch/s390/mm/pgalloc.c
+++ b/arch/s390/mm/pgalloc.c
@@ -76,81 +76,52 @@
__tlb_flush_local();
}
-int crst_table_upgrade(struct mm_struct *mm, unsigned long limit)
+int crst_table_upgrade(struct mm_struct *mm)
{
unsigned long *table, *pgd;
- unsigned long entry;
- int flush;
- BUG_ON(limit > TASK_MAX_SIZE);
- flush = 0;
-repeat:
+ /* upgrade should only happen from 3 to 4 levels */
+ BUG_ON(mm->context.asce_limit != (1UL << 42));
+
table = crst_table_alloc(mm);
if (!table)
return -ENOMEM;
+
spin_lock_bh(&mm->page_table_lock);
- if (mm->context.asce_limit < limit) {
- pgd = (unsigned long *) mm->pgd;
- if (mm->context.asce_limit <= (1UL << 31)) {
- entry = _REGION3_ENTRY_EMPTY;
- mm->context.asce_limit = 1UL << 42;
- mm->context.asce_bits = _ASCE_TABLE_LENGTH |
- _ASCE_USER_BITS |
- _ASCE_TYPE_REGION3;
- } else {
- entry = _REGION2_ENTRY_EMPTY;
- mm->context.asce_limit = 1UL << 53;
- mm->context.asce_bits = _ASCE_TABLE_LENGTH |
- _ASCE_USER_BITS |
- _ASCE_TYPE_REGION2;
- }
- crst_table_init(table, entry);
- pgd_populate(mm, (pgd_t *) table, (pud_t *) pgd);
- mm->pgd = (pgd_t *) table;
- mm->task_size = mm->context.asce_limit;
- table = NULL;
- flush = 1;
- }
+ pgd = (unsigned long *) mm->pgd;
+ crst_table_init(table, _REGION2_ENTRY_EMPTY);
+ pgd_populate(mm, (pgd_t *) table, (pud_t *) pgd);
+ mm->pgd = (pgd_t *) table;
+ mm->context.asce_limit = 1UL << 53;
+ mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH |
+ _ASCE_USER_BITS | _ASCE_TYPE_REGION2;
+ mm->task_size = mm->context.asce_limit;
spin_unlock_bh(&mm->page_table_lock);
- if (table)
- crst_table_free(mm, table);
- if (mm->context.asce_limit < limit)
- goto repeat;
- if (flush)
- on_each_cpu(__crst_table_upgrade, mm, 0);
+
+ on_each_cpu(__crst_table_upgrade, mm, 0);
return 0;
}
-void crst_table_downgrade(struct mm_struct *mm, unsigned long limit)
+void crst_table_downgrade(struct mm_struct *mm)
{
pgd_t *pgd;
+ /* downgrade should only happen from 3 to 2 levels (compat only) */
+ BUG_ON(mm->context.asce_limit != (1UL << 42));
+
if (current->active_mm == mm) {
clear_user_asce();
__tlb_flush_mm(mm);
}
- while (mm->context.asce_limit > limit) {
- pgd = mm->pgd;
- switch (pgd_val(*pgd) & _REGION_ENTRY_TYPE_MASK) {
- case _REGION_ENTRY_TYPE_R2:
- mm->context.asce_limit = 1UL << 42;
- mm->context.asce_bits = _ASCE_TABLE_LENGTH |
- _ASCE_USER_BITS |
- _ASCE_TYPE_REGION3;
- break;
- case _REGION_ENTRY_TYPE_R3:
- mm->context.asce_limit = 1UL << 31;
- mm->context.asce_bits = _ASCE_TABLE_LENGTH |
- _ASCE_USER_BITS |
- _ASCE_TYPE_SEGMENT;
- break;
- default:
- BUG();
- }
- mm->pgd = (pgd_t *) (pgd_val(*pgd) & _REGION_ENTRY_ORIGIN);
- mm->task_size = mm->context.asce_limit;
- crst_table_free(mm, (unsigned long *) pgd);
- }
+
+ pgd = mm->pgd;
+ mm->pgd = (pgd_t *) (pgd_val(*pgd) & _REGION_ENTRY_ORIGIN);
+ mm->context.asce_limit = 1UL << 31;
+ mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH |
+ _ASCE_USER_BITS | _ASCE_TYPE_SEGMENT;
+ mm->task_size = mm->context.asce_limit;
+ crst_table_free(mm, (unsigned long *) pgd);
+
if (current->active_mm == mm)
set_user_asce(mm);
}