io_uring/kbuf: switch to storing struct io_buffer_list locally

Currently the buffer list is stored in struct io_kiocb. The buffer list
can be of two types:

1) Classic/legacy buffer list. These don't need to get referenced after
   a buffer pick, and hence storing them in struct io_kiocb is perfectly
   fine.

2) Ring provided buffer lists. These DO need to be referenced after the
   initial buffer pick, as they need to get consumed later on. This can
   be either just incrementing the head of the ring, or it can be
   consuming parts of a buffer if incremental buffer consumptions has
   been configured.

For case 2, io_uring needs to be careful not to access the buffer list
after the initial pick-and-execute context. The core does recycling of
these, but it's easy to make a mistake, because it's stored in the
io_kiocb which does persist across multiple execution contexts. Either
because it's a multishot request, or simply because it needed some kind
of async trigger (eg poll) for retry purposes.

Add a struct io_buffer_list to struct io_br_sel, which is always on
stack for the various users of it. This prevents the buffer list from
leaking outside of that execution context, and additionally it enables
kbuf to not even pass back the struct io_buffer_list if the given
context isn't appropriately locked already.

This doesn't fix any bugs, it's simply a defensive measure to prevent
any issues with reuse of a buffer list.

Link: https://lore.kernel.org/r/20250821020750.598432-12-axboe@kernel.dk
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 6 insertions, 10 deletions

diff --git a/io_uring/kbuf.h b/io_uring/kbuf.h
index b1723c2620da..1a539969fc9c 100644
--- a/io_uring/kbuf.h
+++ b/io_uring/kbuf.h
@@ -63,11 +63,14 @@ struct buf_sel_arg {
 };
 
 /*
- * Return value from io_buffer_list selection. Just returns the error or
- * user address for now, will be extended to return the buffer list in the
- * future.
+ * Return value from io_buffer_list selection, to avoid stashing it in
+ * struct io_kiocb. For legacy/classic provided buffers, keeping a reference
+ * across execution contexts are fine. But for ring provided buffers, the
+ * list may go away as soon as ->uring_lock is dropped. As the io_kiocb
+ * persists, it's better to just keep the buffer local for those cases.
  */
 struct io_br_sel {
+	struct io_buffer_list *buf_list;
 	/*
 	 * Some selection parts return the user address, others return an error.
 	 */
@@ -107,13 +110,6 @@ struct io_mapped_region *io_pbuf_get_region(struct io_ring_ctx *ctx,
 static inline bool io_kbuf_recycle_ring(struct io_kiocb *req,
 					struct io_buffer_list *bl)
 {
-	/*
-	 * We don't need to recycle for REQ_F_BUFFER_RING, we can just clear
-	 * the flag and hence ensure that bl->head doesn't get incremented.
-	 * If the tail has already been incremented, hang on to it.
-	 * The exception is partial io, that case we should increment bl->head
-	 * to monopolize the buffer.
-	 */
 	if (bl) {
 		req->flags &= ~(REQ_F_BUFFER_RING|REQ_F_BUFFERS_COMMIT);
 		return true;