CVE-2022-48700 in Linux
Summary
by MITRE • 05/03/2024
In the Linux kernel, the following vulnerability has been resolved:
vfio/type1: Unpin zero pages
There's currently a reference count leak on the zero page. We increment the reference via pin_user_pages_remote(), but the page is later handled as an invalid/reserved page, therefore it's not accounted against the user and not unpinned by our put_pfn().
Introducing special zero page handling in put_pfn() would resolve the leak, but without accounting of the zero page, a single user could still create enough mappings to generate a reference count overflow.
The zero page is always resident, so for our purposes there's no reason to keep it pinned. Therefore, add a loop to walk pages returned from pin_user_pages_remote() and unpin any zero pages.
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Analysis
by VulDB Data Team • 09/29/2025
This vulnerability exists within the linux kernel's vfio/type1 driver implementation where a reference count leak occurs with zero pages during memory management operations. The issue stems from how the kernel handles user pages when mapping memory for virtual function input/output devices. When pin_user_pages_remote() is invoked to pin user pages, it increments the reference count on the zero page which is subsequently identified as invalid or reserved by the system. However, because these pages are marked as invalid, they do not get properly accounted against the user and therefore remain pinned indefinitely, creating a memory leak that can accumulate over time.
The technical flaw manifests in the improper handling of zero pages within the memory management subsystem where the pin_user_pages_remote() function successfully acquires references to pages but fails to release them when those pages are later determined to be invalid or reserved. This creates an unbounded reference count growth since the zero page remains pinned even though it should be treated as a special case due to its always-resident nature. The vulnerability is particularly concerning because it can be exploited by a single user to potentially cause a reference count overflow, which would lead to system instability and denial of service conditions.
The operational impact of this vulnerability extends beyond simple memory leaks to potentially compromise system stability and resource availability. Attackers could leverage this flaw by creating multiple mappings that generate sufficient reference count increments to eventually overflow the counter, leading to system crashes or unpredictable behavior. The issue affects virtualization environments where vfio/type1 drivers are used for device assignment, making it particularly relevant for cloud computing platforms, containerized environments, and systems utilizing hardware virtualization technologies.
The mitigation strategy involves implementing a specific handling mechanism within the put_pfn() function that explicitly walks through pages returned by pin_user_pages_remote() and unpin any zero pages encountered. This approach addresses both the immediate reference count leak and prevents potential overflow conditions by ensuring zero pages are never pinned unnecessarily. The solution aligns with common security practices for memory management in kernel space where special handling is required for system-resident pages like zero pages that should not consume user-level pinning resources. This fix follows established patterns in kernel security where zero pages require special treatment due to their guaranteed residency and system-critical nature, preventing both resource exhaustion and potential exploitation vectors that could arise from improper reference counting.
This vulnerability relates to CWE-401 which covers memory leak conditions in software systems, specifically focusing on the improper handling of reference counts. The issue also connects to ATT&CK technique T1499.004 which involves network denial of service attacks through resource exhaustion, as the reference count overflow could lead to system instability similar to resource exhaustion attacks. The fix implements proper memory management practices that align with kernel security guidelines for preventing reference count manipulation vulnerabilities and maintaining system stability under various load conditions.