[PATCH] netfs: Fix AIO error handling when doing write-through

[David Howells]

If an error occurs whilst we're doing an AIO write in write-through mode,
we may end up calling ->ki_complete() *and* returning an error from
->write_iter().  This can result in either a UAF (the ->ki_complete() func
pointer may get overwritten, for example) or a refcount underflow in
io_submit() as ->ki_complete is called twice.

Fix this by making netfs_end_writethrough() - and thus
netfs_perform_write() - unconditionally return -EIOCBQUEUED if we're doing
an AIO write and wait for completion if we're not.

Fixes: 288ace2f57c9 ("netfs: New writeback implementation")
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Jeff Layton <jlayton@xxxxxxxxxx>
cc: Enzo Matsumiya <ematsumiya@xxxxxxx>
cc: netfs@xxxxxxxxxxxxxxx
cc: v9fs@xxxxxxxxxxxxxxx
cc: linux-afs@xxxxxxxxxxxxxxxxxxx
cc: linux-cifs@xxxxxxxxxxxxxxx
cc: linux-fsdevel@xxxxxxxxxxxxxxx
---
 fs/netfs/write_issue.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/netfs/write_issue.c b/fs/netfs/write_issue.c
index e190043bc0da..acbfd1f5ee9d 100644
--- a/fs/netfs/write_issue.c
+++ b/fs/netfs/write_issue.c
@@ -636,7 +636,12 @@ int netfs_end_writethrough(struct netfs_io_request *wreq, struct writeback_contr
 
 	mutex_unlock(&ictx->wb_lock);
 
-	ret = wreq->error;
+	if (wreq->iocb) {
+		ret = -EIOCBQUEUED;
+	} else {
+		wait_on_bit(&wreq->flags, NETFS_RREQ_IN_PROGRESS, TASK_UNINTERRUPTIBLE);
+		ret = wreq->error;
+	}
 	netfs_put_request(wreq, false, netfs_rreq_trace_put_return);
 	return ret;
 }