CVE-2026-43286

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 subpools 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 subpools 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 subpools 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.

Weakness

The product does not sufficiently track and release allocated memory after it has been used, making the memory unavailable for reallocation and reuse.

Affected Software

Potential Mitigations

• Choose a language or tool that provides automatic memory management, or makes manual memory management less error-prone.
• For example, glibc in Linux provides protection against free of invalid pointers.
• When using Xcode to target OS X or iOS, enable automatic reference counting (ARC) [REF-391].
• To help correctly and consistently manage memory when programming in C++, consider using a smart pointer class such as std::auto_ptr (defined by ISO/IEC ISO/IEC 14882:2003), std::shared_ptr and std::unique_ptr (specified by an upcoming revision of the C++ standard, informally referred to as C++ 1x), or equivalent solutions such as Boost.

References

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