From c2c224932fd0ee6854d6ebfc8d059c2bcad86606 Mon Sep 17 00:00:00 2001 From: Alexander Gordeev Date: Thu, 4 Nov 2021 07:14:44 +0100 Subject: 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 Reviewed-by: Gerald Schaefer Signed-off-by: Alexander Gordeev Signed-off-by: Heiko Carstens --- arch/s390/mm/pgalloc.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'arch/s390') 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 { -- cgit v1.2.3-59-g8ed1b