CVE-2026-43364: ublk: fix NULL pointer dereference in ublk_ctrl_set_size()

CVE-2026-43364: ublk: fix NULL pointer dereference in ublk_ctrl_set_size()

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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

ublk: fix NULL pointer dereference in ublk_ctrl_set_size()

ublk_ctrl_set_size() unconditionally dereferences ub->ub_disk via
set_capacity_and_notify() without checking if it is NULL.

ub->ub_disk is NULL before UBLK_CMD_START_DEV completes (it is only
assigned in ublk_ctrl_start_dev()) and after UBLK_CMD_STOP_DEV runs
(ublk_detach_disk() sets it to NULL). Since the UBLK_CMD_UPDATE_SIZE
handler performs no state validation, a user can trigger a NULL pointer
dereference by sending UPDATE_SIZE 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.

The Linux kernel CVE team has assigned CVE-2026-43364 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.16 with commit 98b995660bff011d8e00af03abd74ac7d1ac1390 and fixed in 6.18.20 with commit f13fe6794726755a43090cb680c4c58cea6aa5f1
	Issue introduced in 6.16 with commit 98b995660bff011d8e00af03abd74ac7d1ac1390 and fixed in 6.19.9 with commit c28d945bfa92e15147e93b73f95345b9bec979b0
	Issue introduced in 6.16 with commit 98b995660bff011d8e00af03abd74ac7d1ac1390 and fixed in 7.0 with commit 25966fc097691e5c925ad080f64a2f19c5fd940a

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-43364
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/block/ublk_drv.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/f13fe6794726755a43090cb680c4c58cea6aa5f1
	https://git.kernel.org/stable/c/c28d945bfa92e15147e93b73f95345b9bec979b0
	https://git.kernel.org/stable/c/25966fc097691e5c925ad080f64a2f19c5fd940a