diff options
author | Alexander Gordeev <agordeev@linux.ibm.com> | 2021-11-04 09:14:44 +0300 |
---|---|---|
committer | Heiko Carstens <hca@linux.ibm.com> | 2021-12-16 21:58:08 +0300 |
commit | c2c224932fd0ee6854d6ebfc8d059c2bcad86606 (patch) | |
tree | 7b419a30e348faaa2a7638929a166b543b97a1dd /arch/s390/mm/pgalloc.c | |
parent | cb22cd2d8ff3201f04f167efa86cc561dccffffe (diff) |
s390/mm: fix 2KB pgtable release race
There is a race on concurrent 2KB-pgtables release paths when
both upper and lower halves of the containing parent page are
freed, one via page_table_free_rcu() + __tlb_remove_table(),
and the other via page_table_free(). The race might lead to a
corruption as result of remove of list item in page_table_free()
concurrently with __free_page() in __tlb_remove_table().
Let's assume first the lower and next the upper 2KB-pgtables are
freed from a page. Since both halves of the page are allocated
the tracking byte (bits 24-31 of the page _refcount) has value
of 0x03 initially:
CPU0 CPU1
---- ----
page_table_free_rcu() // lower half
{
// _refcount[31..24] == 0x03
...
atomic_xor_bits(&page->_refcount,
0x11U << (0 + 24));
// _refcount[31..24] <= 0x12
...
table = table | (1U << 0);
tlb_remove_table(tlb, table);
}
...
__tlb_remove_table()
{
// _refcount[31..24] == 0x12
mask = _table & 3;
// mask <= 0x01
...
page_table_free() // upper half
{
// _refcount[31..24] == 0x12
...
atomic_xor_bits(
&page->_refcount,
1U << (1 + 24));
// _refcount[31..24] <= 0x10
// mask <= 0x10
...
atomic_xor_bits(&page->_refcount,
mask << (4 + 24));
// _refcount[31..24] <= 0x00
// mask <= 0x00
...
if (mask != 0) // == false
break;
fallthrough;
...
if (mask & 3) // == false
...
else
__free_page(page); list_del(&page->lru);
^^^^^^^^^^^^^^^^^^ RACE! ^^^^^^^^^^^^^^^^^^^^^
} ...
}
The problem is page_table_free() releases the page as result of
lower nibble unset and __tlb_remove_table() observing zero too
early. With this update page_table_free() will use the similar
logic as page_table_free_rcu() + __tlb_remove_table(), and mark
the fragment as pending for removal in the upper nibble until
after the list_del().
In other words, the parent page is considered as unreferenced and
safe to release only when the lower nibble is cleared already and
unsetting a bit in upper nibble results in that nibble turned zero.
Cc: stable@vger.kernel.org
Suggested-by: Vlastimil Babka <vbabka@suse.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Diffstat (limited to 'arch/s390/mm/pgalloc.c')
-rw-r--r-- | arch/s390/mm/pgalloc.c | 4 |
1 files changed, 3 insertions, 1 deletions
diff --git a/arch/s390/mm/pgalloc.c b/arch/s390/mm/pgalloc.c index 82a2617362d5..457c80924542 100644 --- a/arch/s390/mm/pgalloc.c +++ b/arch/s390/mm/pgalloc.c @@ -244,13 +244,15 @@ void page_table_free(struct mm_struct *mm, unsigned long *table) /* Free 2K page table fragment of a 4K page */ bit = ((unsigned long) table & ~PAGE_MASK)/(PTRS_PER_PTE*sizeof(pte_t)); spin_lock_bh(&mm->context.lock); - mask = atomic_xor_bits(&page->_refcount, 1U << (bit + 24)); + mask = atomic_xor_bits(&page->_refcount, 0x11U << (bit + 24)); mask >>= 24; if (mask & 3) list_add(&page->lru, &mm->context.pgtable_list); else list_del(&page->lru); spin_unlock_bh(&mm->context.lock); + mask = atomic_xor_bits(&page->_refcount, 0x10U << (bit + 24)); + mask >>= 24; if (mask != 0) return; } else { |