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Subject[4.19-stable PATCH 1/2] dax: Don't access a freed inode
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Date
From: Matthew Wilcox <willy@infradead.org>

commit 55e56f06ed71d9441f3abd5b1d3c1a870812b3fe upstream.

After we drop the i_pages lock, the inode can be freed at any time.
The get_unlocked_entry() code has no choice but to reacquire the lock,
so it can't be used here. Create a new wait_entry_unlocked() which takes
care not to acquire the lock or dereference the address_space in any way.

Fixes: c2a7d2a11552 ("filesystem-dax: Introduce dax_lock_mapping_entry()")
Cc: <stable@vger.kernel.org>
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
fs/dax.c | 69 +++++++++++++++++++++++++++++---------------------------------
1 file changed, 32 insertions(+), 37 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 3a2682a6c832..415605fafaeb 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -229,8 +229,8 @@ static void put_unlocked_mapping_entry(struct address_space *mapping,
*
* Must be called with the i_pages lock held.
*/
-static void *__get_unlocked_mapping_entry(struct address_space *mapping,
- pgoff_t index, void ***slotp, bool (*wait_fn)(void))
+static void *get_unlocked_mapping_entry(struct address_space *mapping,
+ pgoff_t index, void ***slotp)
{
void *entry, **slot;
struct wait_exceptional_entry_queue ewait;
@@ -240,8 +240,6 @@ static void *__get_unlocked_mapping_entry(struct address_space *mapping,
ewait.wait.func = wake_exceptional_entry_func;

for (;;) {
- bool revalidate;
-
entry = __radix_tree_lookup(&mapping->i_pages, index, NULL,
&slot);
if (!entry ||
@@ -256,30 +254,39 @@ static void *__get_unlocked_mapping_entry(struct address_space *mapping,
prepare_to_wait_exclusive(wq, &ewait.wait,
TASK_UNINTERRUPTIBLE);
xa_unlock_irq(&mapping->i_pages);
- revalidate = wait_fn();
+ schedule();
finish_wait(wq, &ewait.wait);
xa_lock_irq(&mapping->i_pages);
- if (revalidate) {
- put_unlocked_mapping_entry(mapping, index, entry);
- return ERR_PTR(-EAGAIN);
- }
}
}

-static bool entry_wait(void)
+/*
+ * The only thing keeping the address space around is the i_pages lock
+ * (it's cycled in clear_inode() after removing the entries from i_pages)
+ * After we call xas_unlock_irq(), we cannot touch xas->xa.
+ */
+static void wait_entry_unlocked(struct address_space *mapping, pgoff_t index,
+ void ***slotp, void *entry)
{
+ struct wait_exceptional_entry_queue ewait;
+ wait_queue_head_t *wq;
+
+ init_wait(&ewait.wait);
+ ewait.wait.func = wake_exceptional_entry_func;
+
+ wq = dax_entry_waitqueue(mapping, index, entry, &ewait.key);
+ prepare_to_wait_exclusive(wq, &ewait.wait, TASK_UNINTERRUPTIBLE);
+ xa_unlock_irq(&mapping->i_pages);
schedule();
+ finish_wait(wq, &ewait.wait);
+
/*
- * Never return an ERR_PTR() from
- * __get_unlocked_mapping_entry(), just keep looping.
+ * Entry lock waits are exclusive. Wake up the next waiter since
+ * we aren't sure we will acquire the entry lock and thus wake
+ * the next waiter up on unlock.
*/
- return false;
-}
-
-static void *get_unlocked_mapping_entry(struct address_space *mapping,
- pgoff_t index, void ***slotp)
-{
- return __get_unlocked_mapping_entry(mapping, index, slotp, entry_wait);
+ if (waitqueue_active(wq))
+ __wake_up(wq, TASK_NORMAL, 1, &ewait.key);
}

static void unlock_mapping_entry(struct address_space *mapping, pgoff_t index)
@@ -398,19 +405,6 @@ static struct page *dax_busy_page(void *entry)
return NULL;
}

-static bool entry_wait_revalidate(void)
-{
- rcu_read_unlock();
- schedule();
- rcu_read_lock();
-
- /*
- * Tell __get_unlocked_mapping_entry() to take a break, we need
- * to revalidate page->mapping after dropping locks
- */
- return true;
-}
-
bool dax_lock_mapping_entry(struct page *page)
{
pgoff_t index;
@@ -446,14 +440,15 @@ bool dax_lock_mapping_entry(struct page *page)
}
index = page->index;

- entry = __get_unlocked_mapping_entry(mapping, index, &slot,
- entry_wait_revalidate);
+ entry = __radix_tree_lookup(&mapping->i_pages, index,
+ NULL, &slot);
if (!entry) {
xa_unlock_irq(&mapping->i_pages);
break;
- } else if (IS_ERR(entry)) {
- xa_unlock_irq(&mapping->i_pages);
- WARN_ON_ONCE(PTR_ERR(entry) != -EAGAIN);
+ } else if (slot_locked(mapping, slot)) {
+ rcu_read_unlock();
+ wait_entry_unlocked(mapping, index, &slot, entry);
+ rcu_read_lock();
continue;
}
lock_slot(mapping, slot);
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