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: ha…

Overview

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)

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.