io_uring/kbuf: fix memory leak if io_buffer_add_list fails

io_register_pbuf_ring() ignores the return value of io_buffer_add_list(),
which can fail if xa_store() returns an error (e.g., -ENOMEM). When this
happens, the function returns 0 (success) to the caller, but the
io_buffer_list structure is neither added to the xarray nor freed.

In practice this requires failure injection to hit, hence not a real
issue. But it should get fixed up none the less.

Fixes: c7fb194 ("io_uring: add support for ring mapped supplied buffers")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

io_uring/kbuf.c

@@ -669,8 +669,9 @@ int io_register_pbuf_ring(struct io_ring_ctx *ctx, void __user *arg)
 	bl->buf_ring = br;
 	if (reg.flags & IOU_PBUF_RING_INC)
 		bl->flags |= IOBL_INC;
-	io_buffer_add_list(ctx, bl, reg.bgid);
-	return 0;
+	ret = io_buffer_add_list(ctx, bl, reg.bgid);
+	if (!ret)
+		return 0;
 fail:
 	io_free_region(ctx->user, &bl->region);
 	kfree(bl);