CVE-2026-93182: sched/fair: Fix overflow in update_tg_cfs_runnable()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

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

sched/fair: Fix overflow in update_tg_cfs_runnable()

A divide-by-zero crash is observed when running hackbench:

   [14697.488452] CPU: 112 UID: 0 PID: 124791 Comm: hackbench Not tainted 7.1.0-rc2+
   [14697.492627] RIP: 0010:propagate_entity_load_avg+0x35f/0x3e0
   [14697.506799]  <TASK>
   [14697.507411]  __dequeue_task+0x2b4/0xc70
   [14697.508677]  dequeue_task_fair+0x36/0x370
   [14697.509047]  dequeue_task+0x101/0x2f0
   [14697.509426]  __schedule+0x1b1/0x1a00
   [14697.510868]  anon_pipe_read+0x3da/0x450
   [14697.511400]  vfs_read+0x361/0x390
   [14697.512053]  __x64_sys_read+0x19/0x30

The divide-by-zero happens here:

  if (scale_load_down(gcfs_rq->load.weight)) {
           load_sum = div_u64(gcfs_rq->avg.load_sum,
                              scale_load_down(gcfs_rq->load.weight));
  }

gcfs_rq->load.weight is an insane large value and is truncated
to the lower 32 bits by div_u64, which happen to be 0.

Using AI for investigation, the cause is a u32 overflow in
update_tg_cfs_runnable(), and flat pickup became a victim when using
tg_tasks():

   u32 new_sum, divider;
   ...
   new_sum = se->avg.runnable_avg * divider; <-- boom

The following sequence shows how this triggers the crash:

   propagate_entity_load_avg()
     update_tg_cfs_runnable()     # u32 overflow corrupts runnable_sum

   __update_load_avg_cfs_rq()
     ___update_load_avg()         # computes insane runnable_avg
   update_tg_load_avg()           # propagates to tg->runnable_avg

   update_cfs_group()
     calc_concur_shares()
       tg_tasks()                 # long-to-int truncation, negative nr
     reweight_entity()            # corrupted se->load.weight
       update_load_add()          # corrupted cfs_rq->load.weight

   propagate_entity_load_avg()
     update_tg_cfs_load()
       div_u64()                  # divide-by-zero

Fix by widening new_sum from u32 to u64 (no need to force tg_tasks()
to return unsigned long after this fix)

The Linux kernel CVE team has assigned CVE-2026-93182 to this issue.


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

	Issue introduced in 5.17 with commit 95246d1ec80b8d19d882cd8eb7ad094e63b41bb8 and fixed in 6.1.188 with commit 39d707097a0820c6e87eff4195f0940e8ff1147e
	Issue introduced in 5.17 with commit 95246d1ec80b8d19d882cd8eb7ad094e63b41bb8 and fixed in 6.6.157 with commit 69fafae4eb2c9fc9bcff1bcbfcccf5d197434d75
	Issue introduced in 5.17 with commit 95246d1ec80b8d19d882cd8eb7ad094e63b41bb8 and fixed in 6.12.110 with commit 7cc237e3bc1733928313d2462b413ae441b0eef3
	Issue introduced in 5.17 with commit 95246d1ec80b8d19d882cd8eb7ad094e63b41bb8 and fixed in 6.18.52 with commit 18fdd0978ec3dda0ff6865cd273e47e022180710
	Issue introduced in 5.17 with commit 95246d1ec80b8d19d882cd8eb7ad094e63b41bb8 and fixed in 7.2.6 with commit f94e62d21faad5e244d0874598a4d0ebbc0ea405
	Issue introduced in 5.17 with commit 95246d1ec80b8d19d882cd8eb7ad094e63b41bb8 and fixed in 7.3-rc1 with commit 4f166adb5cb0525d9e32d45729fd8f28c80acbee

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-2026-93182
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/fair.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/39d707097a0820c6e87eff4195f0940e8ff1147e
	https://git.kernel.org/stable/c/69fafae4eb2c9fc9bcff1bcbfcccf5d197434d75
	https://git.kernel.org/stable/c/7cc237e3bc1733928313d2462b413ae441b0eef3
	https://git.kernel.org/stable/c/18fdd0978ec3dda0ff6865cd273e47e022180710
	https://git.kernel.org/stable/c/f94e62d21faad5e244d0874598a4d0ebbc0ea405
	https://git.kernel.org/stable/c/4f166adb5cb0525d9e32d45729fd8f28c80acbee