io_uring/kbuf: defer release of mapped buffer rings

If a provided buffer ring is setup with IOU_PBUF_RING_MMAP, then the
kernel allocates the memory for it and the application is expected to
mmap(2) this memory. However, io_uring uses remap_pfn_range() for this
operation, so we cannot rely on normal munmap/release on freeing them
for us.

Stash an io_buf_free entry away for each of these, if any, and provide
a helper to free them post ->release().

Cc: stable@vger.kernel.org
Fixes: c56e022c0a ("io_uring: add support for user mapped provided buffer ring")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/io_uring_types.h

@@ -340,6 +340,9 @@ struct io_ring_ctx {
 
 	struct list_head	io_buffers_cache;
 
+	/* deferred free list, protected by ->uring_lock */
+	struct hlist_head	io_buf_list;
+
 	/* Keep this last, we don't need it for the fast path */
 	struct wait_queue_head		poll_wq;
 	struct io_restriction		restrictions;