CVE-2026-64253: kernel/fork: clear PF_BLOCK_TS in copy_process()

From: Greg Kroah-Hartman <[email protected]>

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

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

kernel/fork: clear PF_BLOCK_TS in copy_process()

PF_BLOCK_TS is only set in blk_time_get_ns() when current->plug is
non-NULL, and blk_finish_plug() clears it via __blk_flush_plug()
before NULLing the plug pointer.  copy_process() breaks the
invariant by inheriting PF_BLOCK_TS from the parent while resetting
the child's plug to NULL.

Clear PF_BLOCK_TS alongside that assignment so callers can rely on
"PF_BLOCK_TS set implies current->plug != NULL" and dereference
current->plug unguarded.

The Linux kernel CVE team has assigned CVE-2026-64253 to this issue.


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

	Issue introduced in 6.9 with commit 06b23f92af87a84d70881b2ecaa72e00f7838264 and fixed in 6.12.95 with commit ee0801aceabdf583392477baf69a290b09448b8f
	Issue introduced in 6.9 with commit 06b23f92af87a84d70881b2ecaa72e00f7838264 and fixed in 6.18.38 with commit 99e6c712cc300883b8cbf03347d5359ec1a4d6dd
	Issue introduced in 6.9 with commit 06b23f92af87a84d70881b2ecaa72e00f7838264 and fixed in 7.1.3 with commit 77bba61a20f1b3d206f4f90e10a7bb3cd90b9619
	Issue introduced in 6.9 with commit 06b23f92af87a84d70881b2ecaa72e00f7838264 and fixed in 7.2-rc1 with commit fd38b75c4b43295b10d69772a46d1c74dbd6fc81

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-2026-64253
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:
	kernel/fork.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/ee0801aceabdf583392477baf69a290b09448b8f
	https://git.kernel.org/stable/c/99e6c712cc300883b8cbf03347d5359ec1a4d6dd
	https://git.kernel.org/stable/c/77bba61a20f1b3d206f4f90e10a7bb3cd90b9619
	https://git.kernel.org/stable/c/fd38b75c4b43295b10d69772a46d1c74dbd6fc81