Affected: Linux

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

mm/kmemleak: avoid soft lockup when scanning task stacks

Patch series "mm/kmemleak: avoid soft lockup when scanning task", v3.

kmemleak_scan() scans every task stack under one rcu_read_lock() with no reschedule point, which can trip the soft lockup watchdog on hosts with very many threads.

That prints the following message, depending on the workload+host configuration:

watchdog: BUG: soft lockup - CPU#35 stuck for 22s! [kmemleak:537] scan_block kmemleak_scan kmemleak_scan_thread kthread

Patch 1 walks the tasks with find_ge_pid() so the scan reschedules between tasks

Patches 2-3 let the scan loops stop early once a scan is interrupted.

This patch (of 3):

kmemleak_scan() walks every thread and scans its kernel stack under a single rcu_read_lock() with no reschedule point. On a host with very many threads -- amplified by KASAN/lockdep in debug builds -- this loop can hog a CPU long enough to trip the soft lockup watchdog:

watchdog: BUG: soft lockup - CPU#35 stuck for 22s! [kmemleak:537] scan_block kmemleak_scan kmemleak_scan_thread kthread

A cond_resched() cannot be added directly: the loop runs inside an RCU read-side critical section.

Walk the tasks one PID at a time with find_ge_pid(), taking the RCU read lock only to look up and pin each task. The stack is then scanned with no lock held, so cond_resched() runs between tasks and the scan stops ea…