ovl: fix incorrect fdput() on aio completion

ovl_{read,write}_iter() always call fdput(real) to put one or zero
refcounts of the real file, but for aio, whether it was submitted or not,
ovl_aio_put() also calls fdput(), which is not balanced.  This is only a
problem in the less common case when FDPUT_FPUT flag is set.

To fix the problem use get_file() to take file refcount and use fput()
instead of fdput() in ovl_aio_put().

Fixes: 2406a307 ("ovl: implement async IO routines")
Cc: <stable@vger.kernel.org> # v5.6
Reviewed-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>

fs/overlayfs/file.c

@@ -19,7 +19,6 @@ struct ovl_aio_req {
 	struct kiocb iocb;
 	refcount_t ref;
 	struct kiocb *orig_iocb;
-	struct fd fd;
 };
 
 static struct kmem_cache *ovl_aio_request_cachep;
@@ -280,7 +279,7 @@ static rwf_t ovl_iocb_to_rwf(int ifl)
 static inline void ovl_aio_put(struct ovl_aio_req *aio_req)
 {
 	if (refcount_dec_and_test(&aio_req->ref)) {
-		fdput(aio_req->fd);
+		fput(aio_req->iocb.ki_filp);
 		kmem_cache_free(ovl_aio_request_cachep, aio_req);
 	}
 }
@@ -342,10 +341,9 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 		if (!aio_req)
 			goto out;
 
-		aio_req->fd = real;
 		real.flags = 0;
 		aio_req->orig_iocb = iocb;
-		kiocb_clone(&aio_req->iocb, iocb, real.file);
+		kiocb_clone(&aio_req->iocb, iocb, get_file(real.file));
 		aio_req->iocb.ki_complete = ovl_aio_rw_complete;
 		refcount_set(&aio_req->ref, 2);
 		ret = vfs_iocb_iter_read(real.file, &aio_req->iocb, iter);
@@ -409,10 +407,9 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 		if (!aio_req)
 			goto out;
 
-		aio_req->fd = real;
 		real.flags = 0;
 		aio_req->orig_iocb = iocb;
-		kiocb_clone(&aio_req->iocb, iocb, real.file);
+		kiocb_clone(&aio_req->iocb, iocb, get_file(real.file));
 		aio_req->iocb.ki_flags = ifl;
 		aio_req->iocb.ki_complete = ovl_aio_rw_complete;
 		refcount_set(&aio_req->ref, 2);