drbd: only clone bio if we have a backing device

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: drbd: only clone bio if we have a backing device Commit c347a787e34cb (drbd: set ->bi_bdev in drbd_req_new) moved a bio_set_dev call (which has since been removed) to "earlier", from drbd_request_prepare to drbd_req_new. The problem is that this accesses device->ldev->backing_bdev, which is not NULL-checked at this point. When we don't have an ldev (i.e. when the DRBD device is diskless), this leads to a null pointer deref. So, only allocate the private_bio if we actually have a disk. This is also a small optimization, since we don't clone the bio to only to immediately free it again in the diskless case.

References

• https://git.kernel.org/stable/c/05580a3bbf3cec677cb00a85dfeb21d6a9b48eaf
• https://git.kernel.org/stable/c/6d42ddf7f27b6723549ee6d4c8b1b418b59bf6b5