CVE-2026-89518: sched_ext: Fix this_rq() assumptions in dispatch kfuncs

CVE-2026-89518: sched_ext: Fix this_rq() assumptions in dispatch kfuncs

[Greg Kroah-Hartman]

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

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

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

sched_ext: Fix this_rq() assumptions in dispatch kfuncs

Under core scheduling, dispatch runs from within the core-wide pick and can
target a sibling rq, so ops.dispatch() may execute on a CPU different from
the dispatched rq's. Several kfunc paths assumed the two always coincide:

- scx_dsq_move() decided whether an rq lock is held by testing this_rq()'s
  rq flags and lock-danced accordingly. A dispatch for a sibling took the
  unlocked-context branch and acquired the source rq lock on top of the
  already held dispatched rq lock which could deadlock.

- scx_bpf_sub_dispatch() dispatched this_rq() with its stashed
  sub_dispatch_prev, which is NULL when dispatching for a sibling.

- finish_dispatch(), scx_bpf_dsq_reenq() and scx_bpf_dsq_nr_queued()
  resolved SCX_DSQ_LOCAL to this CPU's local DSQ rather than the dispatched
  rq's. The latter two are callable from other rq-locked operations too,
  where SCX_DSQ_LOCAL now likewise resolves to the op's rq. This changes
  behavior also without core scheduling, e.g. for ops.enqueue() running a
  remote wakeup on the waking CPU, and is intended: which CPU happens to
  execute an operation is incidental, the op's rq is what it is operating
  on, and the resolution now matches the insert side where SCX_DSQ_LOCAL
  dispatches land on the task's rq.

Use the rq tracked by scx_locked_rq(), which is set to the dispatched rq
around ops invocations and NULL in unlocked contexts.

The Linux kernel CVE team has assigned CVE-2026-89518 to this issue.


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

	Issue introduced in 6.19 with commit 4c95380701f58b8112f0b891de8d160e4199e19d and fixed in 7.2.4 with commit 6d1890d3c6137ab523799765ae2de62cc05f116d
	Issue introduced in 6.19 with commit 4c95380701f58b8112f0b891de8d160e4199e19d and fixed in 7.3-rc1 with commit 3dd52416e44a70bc993adb96d2e0d71b9ea21359

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-2026-89518
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/sched/ext/ext.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/6d1890d3c6137ab523799765ae2de62cc05f116d
	https://git.kernel.org/stable/c/3dd52416e44a70bc993adb96d2e0d71b9ea21359