io_uring: fix free of unallocated buffer list

in the error path of io_register_pbuf_ring, only free bl if it was
allocated.

Reported-by: Dipanjan Das <mail.dipanjan.das@gmail.com>
Fixes: c7fb194 ("io_uring: add support for ring mapped supplied buffers")
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/all/CANX2M5bXKw1NaHdHNVqssUUaBCs8aBpmzRNVEYEvV0n44P7ioA@mail.gmail.com/
Link: https://lore.kernel.org/all/CANX2M5YiZBXU3L6iwnaLs-HHJXRvrxM8mhPDiMDF9Y9sAvOHUA@mail.gmail.com/
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Dylan Yudaken

fs/io_uring.c

@@ -12931,7 +12931,7 @@ static int io_register_pbuf_ring(struct io_ring_ctx *ctx, void __user *arg)
 {
 	struct io_uring_buf_ring *br;
 	struct io_uring_buf_reg reg;
-	struct io_buffer_list *bl;
+	struct io_buffer_list *bl, *free_bl = NULL;
 	struct page **pages;
 	int nr_pages;
 
@@ -12963,7 +12963,7 @@ static int io_register_pbuf_ring(struct io_ring_ctx *ctx, void __user *arg)
 		if (bl->buf_nr_pages || !list_empty(&bl->buf_list))
 			return -EEXIST;
 	} else {
-		bl = kzalloc(sizeof(*bl), GFP_KERNEL);
+		free_bl = bl = kzalloc(sizeof(*bl), GFP_KERNEL);
 		if (!bl)
 			return -ENOMEM;
 	}
@@ -12972,7 +12972,7 @@ static int io_register_pbuf_ring(struct io_ring_ctx *ctx, void __user *arg)
 			     struct_size(br, bufs, reg.ring_entries),
 			     &nr_pages);
 	if (IS_ERR(pages)) {
-		kfree(bl);
+		kfree(free_bl);
 		return PTR_ERR(pages);
 	}