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Commit 3446c13b authored by Martin Schwidefsky's avatar Martin Schwidefsky
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s390/mm: four page table levels vs. fork 



The fork of a process with four page table levels is broken since
git commit 6252d702 "[S390] dynamic page tables."

All new mm contexts are created with three page table levels and
an asce limit of 4TB. If the parent has four levels dup_mmap will
add vmas to the new context which are outside of the asce limit.
The subsequent call to copy_page_range will walk the three level
page table structure of the new process with non-zero pgd and pud
indexes. This leads to memory clobbers as the pgd_index *and* the
pud_index is added to the mm->pgd pointer without a pgd_deref
in between.

The init_new_context() function is selecting the number of page
table levels for a new context. The function is used by mm_init()
which in turn is called by dup_mm() and mm_alloc(). These two are
used by fork() and exec(). The init_new_context() function can
distinguish the two cases by looking at mm->context.asce_limit,
for fork() the mm struct has been copied and the number of page
table levels may not change. For exec() the mm_alloc() function
set the new mm structure to zero, in this case a three-level page
table is created as the temporary stack space is located at
STACK_TOP_MAX = 4TB.

This fixes CVE-2016-2143.

Reported-by: default avatarMarcin Kościelnicki <koriakin@0x04.net>
Reviewed-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
Cc: stable@vger.kernel.org
Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
parent 7a76aa95

arch/s390/include/asm/mmu_context.h

+11 −5
Original line number Diff line number Diff line
@@ -15,17 +15,25 @@ 
static inline int init_new_context(struct task_struct *tsk,

				   struct mm_struct *mm)

{

	spin_lock_init(&mm->context.list_lock);

	INIT_LIST_HEAD(&mm->context.pgtable_list);

	INIT_LIST_HEAD(&mm->context.gmap_list);

	cpumask_clear(&mm->context.cpu_attach_mask);

	atomic_set(&mm->context.attach_count, 0);

	mm->context.flush_mm = 0;

	mm->context.asce_bits = _ASCE_TABLE_LENGTH | _ASCE_USER_BITS;

	mm->context.asce_bits |= _ASCE_TYPE_REGION3;

#ifdef CONFIG_PGSTE

	mm->context.alloc_pgste = page_table_allocate_pgste;

	mm->context.has_pgste = 0;

	mm->context.use_skey = 0;

#endif

	if (mm->context.asce_limit == 0) {

		/* context created by exec, set asce limit to 4TB */

		mm->context.asce_bits = _ASCE_TABLE_LENGTH |

			_ASCE_USER_BITS | _ASCE_TYPE_REGION3;

		mm->context.asce_limit = STACK_TOP_MAX;

	} else if (mm->context.asce_limit == (1UL << 31)) {

		mm_inc_nr_pmds(mm);

	}

	crst_table_init((unsigned long *) mm->pgd, pgd_entry_type(mm));

	return 0;

}
@@ -111,8 +119,6 @@ static inline void activate_mm(struct mm_struct *prev, 
static inline void arch_dup_mmap(struct mm_struct *oldmm,

				 struct mm_struct *mm)

{

	if (oldmm->context.asce_limit < mm->context.asce_limit)

		crst_table_downgrade(mm, oldmm->context.asce_limit);

}



static inline void arch_exit_mmap(struct mm_struct *mm)

arch/s390/include/asm/pgalloc.h

+19 −5
Original line number Diff line number Diff line
@@ -100,12 +100,26 @@ static inline void pud_populate(struct mm_struct *mm, pud_t *pud, pmd_t *pmd) 


static inline pgd_t *pgd_alloc(struct mm_struct *mm)

{

	spin_lock_init(&mm->context.list_lock);

	INIT_LIST_HEAD(&mm->context.pgtable_list);

	INIT_LIST_HEAD(&mm->context.gmap_list);

	return (pgd_t *) crst_table_alloc(mm);

	unsigned long *table = crst_table_alloc(mm);



	if (!table)

		return NULL;

	if (mm->context.asce_limit == (1UL << 31)) {

		/* Forking a compat process with 2 page table levels */

		if (!pgtable_pmd_page_ctor(virt_to_page(table))) {

			crst_table_free(mm, table);

			return NULL;

		}

	}

	return (pgd_t *) table;

}



static inline void pgd_free(struct mm_struct *mm, pgd_t *pgd)

{

	if (mm->context.asce_limit == (1UL << 31))

		pgtable_pmd_page_dtor(virt_to_page(pgd));

	crst_table_free(mm, (unsigned long *) pgd);

}

#define pgd_free(mm, pgd) crst_table_free(mm, (unsigned long *) pgd)



static inline void pmd_populate(struct mm_struct *mm,

				pmd_t *pmd, pgtable_t pte)