[PATCH 2/2] io_uring/uring_cmd: switch sqe to async_data on EAGAIN

[Caleb Sander Mateos <csander@xxxxxxxxxxxxxxx>]

5eff57fa9f3a ("io_uring/uring_cmd: defer SQE copying until it's needed")
moved the unconditional memcpy() of the uring_cmd SQE to async_data
to 2 cases when the request goes async:
- If REQ_F_FORCE_ASYNC is set to force the initial issue to go async
- If ->uring_cmd() returns -EAGAIN in the initial non-blocking issue

Unlike the REQ_F_FORCE_ASYNC case, in the EAGAIN case, io_uring_cmd()
copies the SQE to async_data but neglects to update the io_uring_cmd's
sqe field to point to async_data. As a result, sqe still points to the
slot in the userspace-mapped SQ. At the end of io_submit_sqes(), the
kernel advances the SQ head index, allowing userspace to reuse the slot
for a new SQE. If userspace reuses the slot before the io_uring worker
reissues the original SQE, the io_uring_cmd's SQE will be corrupted.

Introduce a helper io_uring_cmd_cache_sqes() to copy the original SQE to
the io_uring_cmd's async_data and point sqe there. Use it for both the
REQ_F_FORCE_ASYNC and EAGAIN cases. This ensures the uring_cmd doesn't
read from the SQ slot after it has been returned to userspace.

Signed-off-by: Caleb Sander Mateos <csander@xxxxxxxxxxxxxxx>
Fixes: 5eff57fa9f3a ("io_uring/uring_cmd: defer SQE copying until it's needed")
---
 io_uring/uring_cmd.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
index cfb22e1de0e7..bcfca18395c4 100644
--- a/io_uring/uring_cmd.c
+++ b/io_uring/uring_cmd.c
@@ -166,10 +166,19 @@ void io_uring_cmd_done(struct io_uring_cmd *ioucmd, ssize_t ret, u64 res2,
 		io_req_task_work_add(req);
 	}
 }
 EXPORT_SYMBOL_GPL(io_uring_cmd_done);
 
+static void io_uring_cmd_cache_sqes(struct io_kiocb *req)
+{
+	struct io_uring_cmd *ioucmd = io_kiocb_to_cmd(req, struct io_uring_cmd);
+	struct io_uring_cmd_data *cache = req->async_data;
+
+	memcpy(cache->sqes, ioucmd->sqe, uring_sqe_size(req->ctx));
+	ioucmd->sqe = cache->sqes;
+}
+
 static int io_uring_cmd_prep_setup(struct io_kiocb *req,
 				   const struct io_uring_sqe *sqe)
 {
 	struct io_uring_cmd *ioucmd = io_kiocb_to_cmd(req, struct io_uring_cmd);
 	struct io_uring_cmd_data *cache;
@@ -177,18 +186,14 @@ static int io_uring_cmd_prep_setup(struct io_kiocb *req,
 	cache = io_uring_alloc_async_data(&req->ctx->uring_cache, req);
 	if (!cache)
 		return -ENOMEM;
 	cache->op_data = NULL;
 
-	if (!(req->flags & REQ_F_FORCE_ASYNC)) {
-		/* defer memcpy until we need it */
-		ioucmd->sqe = sqe;
-		return 0;
-	}
-
-	memcpy(cache->sqes, sqe, uring_sqe_size(req->ctx));
-	ioucmd->sqe = cache->sqes;
+	ioucmd->sqe = sqe;
+	/* defer memcpy until we need it */
+	if (unlikely(req->flags & REQ_F_FORCE_ASYNC))
+		io_uring_cmd_cache_sqes(req);
 	return 0;
 }
 
 int io_uring_cmd_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
@@ -251,11 +256,11 @@ 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)
-			memcpy(cache->sqes, ioucmd->sqe, uring_sqe_size(req->ctx));
+			io_uring_cmd_cache_sqes(req);
 		return -EAGAIN;
 	} else if (ret == -EIOCBQUEUED) {
 		return -EIOCBQUEUED;
 	}
 
-- 
2.45.2