CVE-2026-64130

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

mm/page_alloc: fix initialization of tags of the huge zero folio with init_on_free

__GFP_ZEROTAGS semantics are currently a bit weird, but effectively this

flag is only ever set alongside __GFP_ZERO and __GFP_SKIP_KASAN.

If we run with init_on_free, we will zero out pages during

__free_pages_prepare(), to skip zeroing on the allocation path.

However, when allocating with __GFP_ZEROTAG set, post_alloc_hook() will

consequently not only skip clearing page content, but also skip clearing

tag memory.

Not clearing tags through __GFP_ZEROTAGS is irrelevant for most pages that

will get mapped to user space through set_pte_at() later: set_pte_at() and

friends will detect that the tags have not been initialized yet

(PG_mte_tagged not set), and initialize them.

However, for the huge zero folio, which will be mapped through a PMD

marked as special, this initialization will not be performed, ending up

exposing whatever tags were still set for the pages.

The docs (Documentation/arch/arm64/memory-tagging-extension.rst) state

that allocation tags are set to 0 when a page is first mapped to user

space. That no longer holds with the huge zero folio when init_on_free is

enabled.

Fix it by decoupling __GFP_ZEROTAGS from __GFP_ZERO, passing to

tag_clear_highpages() whether we want to also clear page content.

Invert the meaning of the tag_clear_highpages() return value to have

clearer semantics.

Reproduced with the huge zero folio by modifying the check_buffer_fill

arm64/mte selftest to use a 2 MiB area, after making sure that pages have

a non-0 tag set when freeing (note that, during boot, we will not actually

initialize tags, but only set KASAN_TAG_KERNEL in the page flags).

$ ./check_buffer_fill

1..20

...

not ok 17 Check initial tags with private mapping, sync error mode and mmap memory

not ok 18 Check initial tags with private mapping, sync error mode and mmap/mprotect memory

...

This code needs more cleanups; we'll tackle that next, like

decoupling __GFP_ZEROTAGS from __GFP_SKIP_KASAN.

[[email protected]: s/__GPF_ZERO/__GFP_ZERO/, per David]