CVE-2022-50506

In the Linux kernel, the following vulnerability has been resolved:

drbd: only clone bio if we have a backing device

Commit c347a787e34cb (drbd: set ->bibdev in drbdreqnew) moved a biosetdev call (which has since been removed) to "earlier", from drbdrequestprepare to drbdreq_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.