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Subject[PATCH] mm/z3fold: Fix z3fold_buddy_slots use after free
From
Running z3fold stress testing with address sanitization
showed zhdr->slots was being used after it was freed.

z3fold_free(z3fold_pool, handle)
free_handle(handle)
kmem_cache_free(pool->c_handle, zhdr->slots)
release_z3fold_page_locked_list(kref)
__release_z3fold_page(zhdr, true)
zhdr_to_pool(zhdr)
slots_to_pool(zhdr->slots) *BOOM*

Instead we split free_handle into two functions, release_handle()
and free_slots(). We use release_handle() in place of free_handle(),
and use free_slots() to call kmem_cache_free() after
__release_z3fold_page() is done.

Fixes: 7c2b8baa61fe ("mm/z3fold.c: add structure for buddy handles")
Signed-off-by: Henry Burns <henryburns@google.com>
---
mm/z3fold.c | 33 ++++++++++++++-------------------
1 file changed, 14 insertions(+), 19 deletions(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index f7993ff778df..e174d1549734 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -213,31 +213,24 @@ static inline struct z3fold_buddy_slots *handle_to_slots(unsigned long handle)
return (struct z3fold_buddy_slots *)(handle & ~(SLOTS_ALIGN - 1));
}

-static inline void free_handle(unsigned long handle)
+static inline void release_handle(unsigned long handle)
{
- struct z3fold_buddy_slots *slots;
- int i;
- bool is_free;
-
if (handle & (1 << PAGE_HEADLESS))
return;

WARN_ON(*(unsigned long *)handle == 0);
*(unsigned long *)handle = 0;
- slots = handle_to_slots(handle);
- is_free = true;
- for (i = 0; i <= BUDDY_MASK; i++) {
- if (slots->slot[i]) {
- is_free = false;
- break;
- }
- }
+}

- if (is_free) {
- struct z3fold_pool *pool = slots_to_pool(slots);
+/* At this point all of the slots should be empty */
+static inline void free_slots(struct z3fold_buddy_slots *slots)
+{
+ struct z3fold_pool *pool = slots_to_pool(slots);
+ int i;

- kmem_cache_free(pool->c_handle, slots);
- }
+ for (i = 0; i <= BUDDY_MASK; i++)
+ VM_BUG_ON(slots->slot[i]);
+ kmem_cache_free(pool->c_handle, slots);
}

static struct dentry *z3fold_do_mount(struct file_system_type *fs_type,
@@ -431,7 +424,8 @@ static inline struct z3fold_pool *zhdr_to_pool(struct z3fold_header *zhdr)
static void __release_z3fold_page(struct z3fold_header *zhdr, bool locked)
{
struct page *page = virt_to_page(zhdr);
- struct z3fold_pool *pool = zhdr_to_pool(zhdr);
+ struct z3fold_buddy_slots *slots = zhdr->slots;
+ struct z3fold_pool *pool = slots_to_pool(slots);

WARN_ON(!list_empty(&zhdr->buddy));
set_bit(PAGE_STALE, &page->private);
@@ -442,6 +436,7 @@ static void __release_z3fold_page(struct z3fold_header *zhdr, bool locked)
spin_unlock(&pool->lock);
if (locked)
z3fold_page_unlock(zhdr);
+ free_slots(slots);
spin_lock(&pool->stale_lock);
list_add(&zhdr->buddy, &pool->stale);
queue_work(pool->release_wq, &pool->work);
@@ -1009,7 +1004,7 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
return;
}

- free_handle(handle);
+ release_handle(handle);
if (kref_put(&zhdr->refcount, release_z3fold_page_locked_list)) {
atomic64_dec(&pool->pages_nr);
return;
--
2.22.0.410.gd8fdbe21b5-goog
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