[PATCH 2/3] iio: buffer: Tie IIO dma fence lock lifetime to the fence

[Lars-Peter Clausen <lars@metafoo.de>]

The `iio_dma_fence` implementation currently uses a lock embedded in the
`iio_dmabuf_priv`. But the `iio_dma_fence` can outlive the
`iio_dmabuf_priv`, which can cause a use-after-free.

Tie the lifetime of the lock to the lifetime of the fence by embedding them
in the same struct.

We can't just hold a reference to the `iio_dmabuf_priv` from the
`iio_dma_fence` since `iio_buffer_dmabuf_release()` might sleep and the
fence release callback is not allowed to sleep.

Note that the `dma_fence` framework now has an internal lock that gets used
when the passing `NULL` for `lock` in `dma_fence_init()`, but in order to
allow this patch to be backportable use an external lock.

Reported-by: codex:gpt-5.6
Fixes: 3e26d9f08fbe ("iio: core: Add new DMABUF interface infrastructure")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
---
 drivers/iio/industrialio-buffer.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
index 04b3916f89e2d..31c06bffdce8a 100644
--- a/drivers/iio/industrialio-buffer.c
+++ b/drivers/iio/industrialio-buffer.c
@@ -47,9 +47,6 @@ struct iio_dmabuf_priv {
 
 	u64 context;
 
-	/* Spinlock used for locking the dma_fence */
-	spinlock_t lock;
-
 	struct dma_buf_attachment *attach;
 	struct sg_table *sgt;
 	enum dma_data_direction dir;
@@ -58,6 +55,7 @@ struct iio_dmabuf_priv {
 
 struct iio_dma_fence {
 	struct dma_fence base;
+	spinlock_t lock; /* protects base */
 	struct iio_dmabuf_priv *priv;
 	struct work_struct work;
 };
@@ -1706,7 +1704,6 @@ static int iio_buffer_attach_dmabuf(struct iio_dev_buffer_pair *ib,
 	if (!priv)
 		return -ENOMEM;
 
-	spin_lock_init(&priv->lock);
 	priv->context = dma_fence_context_alloc(1);
 
 	dmabuf = dma_buf_get(fd);
@@ -1896,6 +1893,8 @@ static int iio_buffer_enqueue_dmabuf(struct iio_dev_buffer_pair *ib,
 		goto err_attachment_put;
 	}
 
+	spin_lock_init(&fence->lock);
+
 	fence->priv = priv;
 
 	seqno = atomic_add_return(1, &priv->seqno);
@@ -1906,7 +1905,7 @@ static int iio_buffer_enqueue_dmabuf(struct iio_dev_buffer_pair *ib,
 	 * the dma_fence.
 	 */
 	dma_fence_init(&fence->base, &iio_buffer_dma_fence_ops,
-		       &priv->lock, priv->context, seqno);
+		       &fence->lock, priv->context, seqno);
 
 	ret = iio_dma_resv_lock(dmabuf, nonblock);
 	if (ret)
-- 
2.47.3