[PATCH] io_uring/uring_cmd: unconditionally copy SQEs at prep time

[Jens Axboe <axboe@kernel.dk>]

This isn't generally necessary, but conditions have been observed where
SQE data is accessed from the original SQE after prep has been done and
outside of the initial issue. Opcode prep handlers must ensure that any
SQE related data is stable beyond the prep phase, but uring_cmd is a bit
special in how it handles the SQE which makes it susceptible to reading
stale data. If the application has reused the SQE before the original
completes, then that can lead to data corruption.

Down the line we can relax this again once uring_cmd has been sanitized
a bit, and avoid unnecessarily copying the SQE.

Reported-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

Let's just do the unconditional copy for now. I kept it on top of the
other patches deliberately, as they tell a story of how we got there.
This will 100% cover all cases, obviously, and then we can focus on
future work on avoiding the copy when unnecessary without having any
rush on that front.

diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
index 8af7780407b7..b78d050aaa3f 100644
--- a/io_uring/uring_cmd.c
+++ b/io_uring/uring_cmd.c
@@ -186,9 +186,14 @@ static int io_uring_cmd_prep_setup(struct io_kiocb *req,
 	cache->op_data = NULL;
 
 	ioucmd->sqe = sqe;
-	/* defer memcpy until we need it */
-	if (unlikely(req->flags & REQ_F_FORCE_ASYNC))
-		io_uring_cmd_cache_sqes(req);
+	/*
+	 * Unconditionally cache the SQE for now - this is only needed for
+	 * requests that go async, but prep handlers must ensure that any
+	 * sqe data is stable beyond prep. Since uring_cmd is special in
+	 * that it doesn't read in per-op data, play it safe and ensure that
+	 * any SQE data is stable beyond prep. This can later get relaxed.
+	 */
+	io_uring_cmd_cache_sqes(req);
 	return 0;
 }
 
@@ -251,16 +256,8 @@ int io_uring_cmd(struct io_kiocb *req, unsigned int issue_flags)
 	}
 
 	ret = file->f_op->uring_cmd(ioucmd, issue_flags);
-	if (ret == -EAGAIN) {
-		struct io_uring_cmd_data *cache = req->async_data;
-
-		if (ioucmd->sqe != cache->sqes)
-			io_uring_cmd_cache_sqes(req);
-		return -EAGAIN;
-	} else if (ret == -EIOCBQUEUED) {
-		return -EIOCBQUEUED;
-	}
-
+	if (ret == -EAGAIN || ret == -EIOCBQUEUED)
+		return ret;
 	if (ret < 0)
 		req_set_fail(req);
 	io_req_uring_cleanup(req, issue_flags);
-- 
Jens Axboe