CVE-2026-74632 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
mm/huge_memory: fix huge_zero_pfn race
Patch series "mm/huge_memory: fix huge_zero_pfn race", v2.
There is a subtle race in the reference-counted huge_zero_folio implementation.
The fast path atomic logic fails to account for the fact that the shrinker (which drops the final huge_zero_refcount pin) can overwrite huge_zero_pfn with the ~0UL sentinel value in shrink_huge_zero_folio_scan() after a racing get_huge_zero_folio() installed a valid value there.
This results in huge_zero_folio being correctly set but huge_zero_pfn being set incorrectly and thus is_huge_zero_pfn() and consequently is_huge_zero_pmd() will misidentify the huge zero folio as being an ordinary THP folio.
This can result in the huge zero folio being split and otherwise treated incorrectly.
The solution to this is very subtle as there is an atomic fast path, and thus ordering in weakly ordered architectures has to be treated very carefully.
The first commit fixes the issue by introducing a spinlock around huge_zero_[pfn, folio, refcount] write, with careful consideration paid to
load/store ordering in the fast path. It is placed first and kept as small as possible so that it can be backported on its own.
The second commit is a pure cleanup which reworks the CONFIG_PERSISTENT_HUGE_ZERO_FOLIO logic to better separate the persistent logic from the dynamically allocated one.
This patch (of 2):
If !CONFIG_PERSISTENT_HUGE_ZERO_FOLIO, the huge_zero_folio is refcounted by huge_zero_refcount and returned by mm_get_huge_zero_folio().
When the caller is done with the huge zero page, its reference count is decremented. Only a shrinker can set the reference count to zero.
A race can unfortunately occur between a shrinker decrementing the reference count to zero and a concurrent page fault.
This is because shrink_huge_zero_folio_scan() might, if very unlucky, be preempted between setting huge_zero_refcount to zero and writing an invalid value.
During this time get_huge_zero_folio() could write to huge_zero_pfn before shrink_huge_zero_folio_scan() resumes.
In this event the huge zero folio will be persistently misidentified causing the THP code path to be entered inappropriately for the huge zero folio:
CPU 0 CPU 1 =======================================|================================= shrink_huge_zero_folio_scan() | atomic_cmpxchg() sets refcount to 0 | xchg() sets huge_zero_folio to NULL | get_huge_zero_folio() | | atomic_inc_not_zero() -> zero preempted for a long time | Allocate new huge zero folio | | Write valid huge_zero_folio v | Write valid huge_zero_pfn Overwrite huge_zero_pfn with ~0UL <--- Invalid overwrite!
This results in is_huge_zero_pfn() and is_huge_zero_pmd() incorrectly returning false for a huge zero page which could result in issues like the huge zero folio being incorrectly split.
Note that the issue is with huge_zero_pfn not huge_zero_folio, as get_huge_zero_folio() uses cmpxchg() gated on huge_zero_folio being NULL with a retry loop and shrink_huge_zero_folio_scan() uses xchg() to set huge_zero_folio.
Fix the issue by introducing a spinlock, huge_zero_lock, to prevent concurrent write of huge_zero_folio, huge_zero_pfn and huge_zero_refcount.
There needs to be significant care taken here to ensure correctness:
The fast path in get_huge_zero_folio() uses atomic_inc_not_zero(), which is outside of the critical section, and means huge zero allocation is gated on zero huge_zero_refcount.
The fast path doesn't use huge_zero_lock, so the critical section is irrelevant to it.
So invariants are required - huge_zero_refcount MUST:
* Only be set in the huge_zero_lock critical section to ensure serialisation of huge_zero_pfn, huge_zero_folio and ---truncated---
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Analysis
by VulDB Data Team • 08/22/2026
The Linux kernel contains a subtle race condition within the memory management subsystem specifically affecting the handling of huge zero pages. This vulnerability arises from improper synchronization between reference counting mechanisms and page fault handlers when dealing with transparent huge pages. The core issue lies in the interaction between the shrinker logic, which manages the lifecycle of the huge_zero_folio by dropping its final reference count pin, and concurrent operations that attempt to access or allocate new instances of this folio during a race window.
The technical flaw centers on an atomic fast path implementation that fails to maintain strict ordering guarantees in weakly ordered architectures. Specifically, there is a critical section where the shrinker function shrink_huge_zero_folio_scan() may preemptively set huge_zero_refcount to zero and subsequently overwrite huge_zero_pfn with a sentinel value representing invalidity. However, if this operation races against get_huge_zero_folio(), which uses atomic operations to install valid values for both huge_zero_folio and huge_zero_pfn, the final state of huge_zero_pfn can be incorrectly overwritten by the shrinker after the fault handler has already written a valid page frame number. This results in a scenario where huge_zero_folio is correctly set but huge_zero_pfn holds an invalid or stale value.
This inconsistency leads to significant operational impacts because subsequent checks such as is_huge_zero_pfn() and is_huge_zero_pmd() rely on the validity of these fields to identify whether a page belongs to the special huge zero folio pool. When misidentified, the kernel treats the huge zero folio as an ordinary transparent huge page rather than recognizing it as part of the optimized zero-page infrastructure. Consequently, this can trigger inappropriate code paths that may attempt to split or otherwise manipulate the huge zero folio incorrectly, potentially leading to memory corruption, performance degradation due to unnecessary allocations and splits, or system instability depending on the specific context in which the error occurs.
The resolution involves introducing a spinlock named huge_zero_lock to serialize writes to the trio of variables: huge_zero_pfn, huge_zero_folio, and huge_zero_refcount. This synchronization primitive ensures that only one execution path can modify these shared resources at any given time, thereby eliminating the race condition. The implementation carefully considers load-store ordering in fast paths where atomic operations are used outside the critical section to maintain performance while ensuring correctness. Additionally, a secondary cleanup refactors the logic for persistent huge zero folios to better separate it from dynamically allocated variants, improving code clarity and maintainability without altering the core security fix.
From a classification perspective, this vulnerability aligns with CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization, commonly known as a race condition. In terms of attack surface and detection, while not directly exploitable for privilege escalation in most standard configurations, it represents an integrity violation within the kernel memory management subsystem that could be leveraged in conjunction with other vulnerabilities to cause denial-of-service conditions through resource exhaustion or unexpected page splitting behaviors. Security analysts should monitor for patches addressing mm/huge_memory synchronization issues and ensure systems are updated to versions containing this fix to maintain the integrity of transparent huge page handling mechanisms.