CVE-2026-93232: mm/hugetlb: fix boot panic with CONFIG_DEBUG_VM and HVO bootmem pages

[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/hugetlb: fix boot panic with CONFIG_DEBUG_VM and HVO bootmem pages

Patch series "mm: Refactor bootmem gigantic hugepage allocation", v4.

This series is split out from the earlier larger series "mm: Generalize
HVO for HugeTLB and device DAX" [1].  It collects the first 19 patches of
that series as a standalone set of fixes and preparatory cleanups around
bootmem HugeTLB handling, sparse initialization ordering, and related
vmemmap setup.

The first patches fix a few bugs found while reviewing the existing code,
including incorrect bootmem HVO handling, wrong vmemmap registration
arguments, a powerpc compound-vmemmap tracking bug, and too-late
initialization of gigantic bootmem HugeTLB struct pages.

The rest of the series reorders early memory initialization so the
relevant zone state is available before sparse and HugeTLB boot-time setup
runs, then simplifies the remaining bootmem gigantic hugepage allocation
path and removes code made obsolete by that rework.

At a high level:
  - patches [1-4] fix boot-time and arch-specific bugs
  - patches [5-12] reorder and simplify sparse/mm/hugetlb early init
  - patches [13-19] refactor bootmem gigantic hugepage allocation and
    remove obsolete helpers and state


This patch (of 19):

Commit 622026e87c40 ("mm/hugetlb: remove fake head pages") switched
HVO to reuse per-zone shared tail pages from zone->vmemmap_tails[].

Those shared tail pages were initialized in hugetlb_vmemmap_init(), but
bootmem HugeTLB folios are prepared earlier from
gather_bootmem_prealloc().  With hugetlb_free_vmemmap=on,
prep_and_add_bootmem_folios() can access pageblock flags on bootmem
HugeTLB pages whose mirrored tail struct pages already point to the shared
tail page.  On CONFIG_DEBUG_VM kernels, get_pfnblock_bitmap_bitidx() then
dereferences the still-uninitialized shared tail page and can panic during
boot.

Initialize zone->vmemmap_tails[] from gather_bootmem_prealloc(), before
bootmem HugeTLB folios are processed, and drop the later initialization
from hugetlb_vmemmap_init().

This bug only affects CONFIG_DEBUG_VM kernels, where the relevant
assertion is evaluated.

The Linux kernel CVE team has assigned CVE-2026-93232 to this issue.


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

	Issue introduced in 7.1 with commit 622026e87c4019e609010811757e31193cc23847 and fixed in 7.2.4 with commit 2ddf429e25cf8415d9c308f07e64012026bd4d77
	Issue introduced in 7.1 with commit 622026e87c4019e609010811757e31193cc23847 and fixed in 7.3-rc1 with commit c0caeceb0c3899dc42844d3979093b27d1434108

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-2026-93232
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/hugetlb.c
	mm/hugetlb_vmemmap.c
	mm/sparse-vmemmap.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/2ddf429e25cf8415d9c308f07e64012026bd4d77
	https://git.kernel.org/stable/c/c0caeceb0c3899dc42844d3979093b27d1434108