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

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

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

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

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

mm/vmalloc: prevent RCU stalls in kasan_release_vmalloc_node

When CONFIG_PAGE_OWNER is enabled, freeing KASAN shadow pages during
vmalloc cleanup triggers expensive stack unwinding that acquires RCU read
locks.  Processing a large purge_list 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 purge_vmap_node() -> kasan_release_vmalloc_node()
where iterating through hundreds or thousands of vmap_area entries and
freeing their associated shadow pages causes:

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

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

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

The fix uses need_resched() 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.

The Linux kernel CVE team has assigned CVE-2026-43292 to this issue.


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

	Fixed in 6.12.75 with commit 2efa9c02c9b4c0d6866aa445f11056809b25ca28
	Fixed in 6.18.16 with commit 1afe45f89d54b7183768ebbbbf14238ec187ab5c
	Fixed in 6.19.6 with commit b351fbe71091f7c8676c8ba597653d08b6719447
	Fixed in 7.0 with commit 5747435e0fd474c24530ef1a6822f47e7d264b27

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-2026-43292
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:
	mm/vmalloc.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/2efa9c02c9b4c0d6866aa445f11056809b25ca28
	https://git.kernel.org/stable/c/1afe45f89d54b7183768ebbbbf14238ec187ab5c
	https://git.kernel.org/stable/c/b351fbe71091f7c8676c8ba597653d08b6719447
	https://git.kernel.org/stable/c/5747435e0fd474c24530ef1a6822f47e7d264b27