CVE-2026-43292: mm/vmalloc: prevent RCU stalls in kasan_release_vmalloc_node

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

mm/vmalloc: prevent RCU stalls in kasanreleasevmallocnode

When CONFIGPAGEOWNER is enabled, freeing KASAN shadow pages during vmalloc cleanup triggers expensive stack unwinding that acquires RCU read locks. Processing a large purgelist without rescheduling can cause the task to hold CPU for extended periods (10+ seconds), leading to RCU stalls and potential OOM conditions.

The issue manifests in purgevmapnode() -> kasanreleasevmallocnode() where iterating through hundreds or thousands of vmaparea entries and freeing their associated shadow pages causes:

rcu: INFO: rcupreempt detected stalls on CPUs/tasks: rcu: Tasks blocked on level-0 rcunode (CPUs 0-1): P6229/1:b..l ... task:kworker/0:17 state:R running task stack:28840 pid:6229 ... kasanreleasevmallocnode+0x1ba/0xad0 mm/vmalloc.c:2299 purgevmapnode+0x1ba/0xad0 mm/vmalloc.c:2299

Each call to kasanreleasevmalloc() can free many pages, and with pageowner tracking, each free triggers savestack() which performs stack unwinding under RCU read lock. Without yielding, this creates an unbounded RCU critical section.

Add periodic condresched() calls within the loop to allow: - RCU grace periods to complete - Other tasks to run - Scheduler to preempt when needed

The fix uses needresched() for immediate response under load, with a batch count of 32 as a guaranteed upper bound to prevent worst-case stalls even under light load.