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

Caleb Sander Mateos
2025-02-12T20:46:07Z

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@purestorage.com>
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