[PATCH v3] rapidio: mport_cdev: fix use-after-free in dma_req_free()

[James Kim <james010kim@gmail.com>]

dma_req_free() acquires buf_mutex through req->map, drops the mapping
reference with kref_put(), and then dereferences req->map again to
unlock the mutex.

If kref_put() drops the last reference, mport_release_mapping() frees
the mapping, and the subsequent mutex_unlock() dereferences a freed
object. This is a use-after-free.

Fix this by caching map and md before kref_put(), clearing req->map
while holding buf_mutex, and using the cached md for mutex unlocking.

The bug is reachable from userspace via the RapidIO mport character
device interface.

Fixes: e8de370188d0 ("rapidio: add mport char device driver")
Cc: stable@vger.kernel.org
Reviewed-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: James Kim <james010kim@gmail.com>

---
Changes since v2:
- Correct the Fixes tag.
- Clear req->map while holding buf_mutex.
- Clarify that the bug is reachable from userspace.
- Update Dan Carpenter's email address and add his Reviewed-by tag.

Changes since v1:
- Rebase on v7.1.
- Add Dan Carpenter to Cc for lifetime and use-after-free review.
- No functional changes.
---
 drivers/rapidio/devices/rio_mport_cdev.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/rapidio/devices/rio_mport_cdev.c b/drivers/rapidio/devices/rio_mport_cdev.c
index 009b3b595bbf..ad82c2108a56 100644
--- a/drivers/rapidio/devices/rio_mport_cdev.c
+++ b/drivers/rapidio/devices/rio_mport_cdev.c
@@ -564,9 +564,13 @@ static void dma_req_free(struct kref *ref)
 	}
 
 	if (req->map) {
-		mutex_lock(&req->map->md->buf_mutex);
-		kref_put(&req->map->ref, mport_release_mapping);
-		mutex_unlock(&req->map->md->buf_mutex);
+		struct rio_mport_mapping *map = req->map;
+		struct mport_dev *md = map->md;
+
+		mutex_lock(&md->buf_mutex);
+		req->map = NULL;
+		kref_put(&map->ref, mport_release_mapping);
+		mutex_unlock(&md->buf_mutex);
 	}
 
 	kref_put(&priv->dma_ref, mport_release_dma);
-- 
2.25.1