CVE の説明

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---

CVE の情報

メトリクス

メトリクス	スコア	深刻度	CVSS ベクター	出典
V3.1	7.8	HIGH	CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H 詳細情報 Base: Exploitabilty Metrics The Exploitability metrics reflect the characteristics of the thing that is vulnerable, which we refer to formally as the vulnerable component. Attack Vector This metric reflects the context by which vulnerability exploitation is possible. Local The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Attack Complexity This metric describes the conditions beyond the attacker’s control that must exist in order to exploit the vulnerability. Low Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success when attacking the vulnerable component. Privileges Required This metric describes the level of privileges an attacker must possess before successfully exploiting the vulnerability. Low The attacker requires privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges has the ability to access only non-sensitive resources. User Interaction This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable component. None The vulnerable system can be exploited without interaction from any user. Base: Scope Metrics The Scope metric captures whether a vulnerability in one vulnerable component impacts resources in components beyond its security scope. Scope Formally, a security authority is a mechanism (e.g., an application, an operating system, firmware, a sandbox environment) that defines and enforces access control in terms of how certain subjects/actors (e.g., human users, processes) can access certain restricted objects/resources (e.g., files, CPU, memory) in a controlled manner. All the subjects and objects under the jurisdiction of a single security authority are considered to be under one security scope. If a vulnerability in a vulnerable component can affect a component which is in a different security scope than the vulnerable component, a Scope change occurs. Intuitively, whenever the impact of a vulnerability breaches a security/trust boundary and impacts components outside the security scope in which vulnerable component resides, a Scope change occurs. Unchanged An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority. Base: Impact Metrics The Impact metrics capture the effects of a successfully exploited vulnerability on the component that suffers the worst outcome that is most directly and predictably associated with the attack. Analysts should constrain impacts to a reasonable, final outcome which they are confident an attacker is able to achieve. Confidentiality Impact This metric measures the impact to the confidentiality of the information resources managed by a software component due to a successfully exploited vulnerability. High There is a total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server. Integrity Impact This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. High There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component. Availability Impact This metric measures the impact to the availability of the impacted component resulting from a successfully exploited vulnerability. High There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable). Temporal Metrics The Temporal metrics measure the current state of exploit techniques or code availability, the existence of any patches or workarounds, or the confidence in the description of a vulnerability. Environmental Metrics These metrics enable the analyst to customize the CVSS score depending on the importance of the affected IT asset to a user’s organization, measured in terms of Confidentiality, Integrity, and Availability.	416baaa9-dc9f-4396-8d5f-8c081fb06d67