btrfs: fix use-after-free waiting for encoded read endios

Fix a use-after-free in the I/O completion path for encoded reads by
using a completion instead of a wait_queue for synchronizing the
destruction of 'struct btrfs_encoded_read_private'.

Fixes: 1881fba ("btrfs: add BTRFS_IOC_ENCODED_READ ioctl")
CC: [email protected] # 6.1+
Reviewed-by: Filipe Manana <[email protected]>
Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Johannes Thumshirn <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: morbidrsa

fs/btrfs/inode.c

@@ -9078,9 +9078,9 @@ static ssize_t btrfs_encoded_read_inline(
 }
 
 struct btrfs_encoded_read_private {
-	wait_queue_head_t wait;
+	struct completion done;
 	void *uring_ctx;
-	atomic_t pending;
+	refcount_t pending_refs;
 	blk_status_t status;
 };
 
@@ -9099,14 +9099,14 @@ static void btrfs_encoded_read_endio(struct btrfs_bio *bbio)
 		 */
 		WRITE_ONCE(priv->status, bbio->bio.bi_status);
 	}
-	if (atomic_dec_and_test(&priv->pending)) {
+	if (refcount_dec_and_test(&priv->pending_refs)) {
 		int err = blk_status_to_errno(READ_ONCE(priv->status));
 
 		if (priv->uring_ctx) {
 			btrfs_uring_read_extent_endio(priv->uring_ctx, err);
 			kfree(priv);
 		} else {
-			wake_up(&priv->wait);
+			complete(&priv->done);
 		}
 	}
 	bio_put(&bbio->bio);
@@ -9126,8 +9126,8 @@ int btrfs_encoded_read_regular_fill_pages(struct btrfs_inode *inode,
 	if (!priv)
 		return -ENOMEM;
 
-	init_waitqueue_head(&priv->wait);
-	atomic_set(&priv->pending, 1);
+	init_completion(&priv->done);
+	refcount_set(&priv->pending_refs, 1);
 	priv->status = 0;
 	priv->uring_ctx = uring_ctx;
 
@@ -9140,7 +9140,7 @@ int btrfs_encoded_read_regular_fill_pages(struct btrfs_inode *inode,
 		size_t bytes = min_t(u64, disk_io_size, PAGE_SIZE);
 
 		if (bio_add_page(&bbio->bio, pages[i], bytes, 0) < bytes) {
-			atomic_inc(&priv->pending);
+			refcount_inc(&priv->pending_refs);
 			btrfs_submit_bbio(bbio, 0);
 
 			bbio = btrfs_bio_alloc(BIO_MAX_VECS, REQ_OP_READ, fs_info,
@@ -9155,11 +9155,11 @@ int btrfs_encoded_read_regular_fill_pages(struct btrfs_inode *inode,
 		disk_io_size -= bytes;
 	} while (disk_io_size);
 
-	atomic_inc(&priv->pending);
+	refcount_inc(&priv->pending_refs);
 	btrfs_submit_bbio(bbio, 0);
 
 	if (uring_ctx) {
-		if (atomic_dec_return(&priv->pending) == 0) {
+		if (refcount_dec_and_test(&priv->pending_refs)) {
 			ret = blk_status_to_errno(READ_ONCE(priv->status));
 			btrfs_uring_read_extent_endio(uring_ctx, ret);
 			kfree(priv);
@@ -9168,8 +9168,8 @@ int btrfs_encoded_read_regular_fill_pages(struct btrfs_inode *inode,
 
 		return -EIOCBQUEUED;
 	} else {
-		if (atomic_dec_return(&priv->pending) != 0)
-			io_wait_event(priv->wait, !atomic_read(&priv->pending));
+		if (!refcount_dec_and_test(&priv->pending_refs))
+			wait_for_completion_io(&priv->done);
 		/* See btrfs_encoded_read_endio() for ordering. */
 		ret = blk_status_to_errno(READ_ONCE(priv->status));
 		kfree(priv);