CVE-2026-43286: mm/hugetlb: restore failed global reservations to subpool

[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: restore failed global reservations to subpool

Commit a833a693a490 ("mm: hugetlb: fix incorrect fallback for subpool")
fixed an underflow error for hstate->resv_huge_pages caused by incorrectly
attributing globally requested pages to the subpool's reservation.

Unfortunately, this fix also introduced the opposite problem, which would
leave spool->used_hpages elevated if the globally requested pages could
not be acquired.  This is because while a subpool's reserve pages only
accounts for what is requested and allocated from the subpool, its "used"
counter keeps track of what is consumed in total, both from the subpool
and globally.  Thus, we need to adjust spool->used_hpages in the other
direction, and make sure that globally requested pages are uncharged from
the subpool's used counter.

Each failed allocation attempt increments the used_hpages counter by how
many pages were requested from the global pool.  Ultimately, this renders
the subpool unusable, as used_hpages approaches the max limit.

The issue can be reproduced as follows:
1. Allocate 4 hugetlb pages
2. Create a hugetlb mount with max=4, min=2
3. Consume 2 pages globally
4. Request 3 pages from the subpool (2 from subpool + 1 from global)
	4.1 hugepage_subpool_get_pages(spool, 3) succeeds.
		used_hpages += 3
	4.2 hugetlb_acct_memory(h, 1) fails: no global pages left
		used_hpages -= 2
5. Subpool now has used_hpages = 1, despite not being able to
   successfully allocate any hugepages. It believes it can now only
   allocate 3 more hugepages, not 4.

With each failed allocation attempt incrementing the used counter, the
subpool eventually reaches a point where its used counter equals its
max counter.  At that point, any future allocations that try to
allocate hugeTLB pages from the subpool will fail, despite the subpool
not having any of its hugeTLB pages consumed by any user.

Once this happens, there is no way to make the subpool usable again,
since there is no way to decrement the used counter as no process is
really consuming the hugeTLB pages.

The underflow issue that the original commit fixes still remains fixed
as well.

Without this fix, used_hpages would keep on leaking if
hugetlb_acct_memory() fails.

The Linux kernel CVE team has assigned CVE-2026-43286 to this issue.


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

	Issue introduced in 6.15 with commit a833a693a490ecff8ba377654c6d4d333718b6b1 and fixed in 6.18.16 with commit 5eac1322a7b14b8cd05ec896618278b90fba7f39
	Issue introduced in 6.15 with commit a833a693a490ecff8ba377654c6d4d333718b6b1 and fixed in 6.19.6 with commit f055897c975d079a90af873c791ab58cf0f6f2a5
	Issue introduced in 6.15 with commit a833a693a490ecff8ba377654c6d4d333718b6b1 and fixed in 7.0 with commit 1d3f9bb4c8af70304d19c22e30f5d16a2d589bb5
	Issue introduced in 6.14.8 with commit adb5c2e55524e3a96b02c3904b0bb6d5a5404d21

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-2026-43286
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


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/5eac1322a7b14b8cd05ec896618278b90fba7f39
	https://git.kernel.org/stable/c/f055897c975d079a90af873c791ab58cf0f6f2a5
	https://git.kernel.org/stable/c/1d3f9bb4c8af70304d19c22e30f5d16a2d589bb5