io_uring: truncate lengths larger than MAX_RW_COUNT on provide buffers

mainline inclusion
from mainline-v5.13-rc1
commit d1f82808
category: bugfix
bugzilla: NA
CVE: CVE-2021-3491

--------------------------------

Read and write operations are capped to MAX_RW_COUNT. Some read ops rely on
that limit, and that is not guaranteed by the IORING_OP_PROVIDE_BUFFERS.

Truncate those lengths when doing io_add_buffers, so buffer addresses still
use the uncapped length.

Also, take the chance and change struct io_buffer len member to __u32, so
it matches struct io_provide_buffer len member.

This fixes CVE-2021-3491, also reported as ZDI-CAN-13546.

Fixes: ddf0322d ("io_uring: add IORING_OP_PROVIDE_BUFFERS")
Reported-by: Billy Jheng Bing-Jhong (@st424204)
Signed-off-by: NThadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NXiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: NZhang Yi <yi.zhang@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

fs/io_uring.c

@@ -216,7 +216,7 @@ struct fixed_file_data {
 struct io_buffer {
 	struct list_head list;
 	__u64 addr;
-	__s32 len;
+	__u32 len;
 	__u16 bid;
 };
 
@@ -3523,7 +3523,7 @@ static int io_add_buffers(struct io_provide_buf *pbuf, struct io_buffer **head)
 			break;
 
 		buf->addr = addr;
-		buf->len = pbuf->len;
+		buf->len = min_t(__u32, pbuf->len, MAX_RW_COUNT);
 		buf->bid = bid;
 		addr += pbuf->len;
 		bid++;