CVE-2026-43364

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

ublk: fix NULL pointer dereference in ublkctrlset_size()

ublkctrlsetsize() unconditionally dereferences ub->ubdisk via setcapacityand_notify() without checking if it is NULL.

ub->ubdisk is NULL before UBLKCMDSTARTDEV completes (it is only assigned in ublkctrlstartdev()) and after UBLKCMDSTOPDEV runs (ublkdetachdisk() sets it to NULL). Since the UBLKCMDUPDATESIZE handler performs no state validation, a user can trigger a NULL pointer dereference by sending UPDATESIZE to a device that has been added but not yet started, or one that has been stopped.

Fix this by checking ub->ub_disk under ub->mutex before dereferencing it, and returning -ENODEV if the disk is not available.