CVE-2026-98368 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
esp: downgrade zerocopy managed frags before mutating skb frags
On the out-of-place output path (esp->inplace == false) ESP rewrites the skb frag array: esp_output_head() appends a trailer frag and esp_output_tail() replaces the frags with a destination page, both referenced with get_page().
When the skb carries zerocopy managed frags (SKBFL_MANAGED_FRAG_REFS) the payload frags are owned by the ubuf and must not be referenced or unreferenced individually, but ESP mutates the frag array without ever downgrading the skb. This breaks the managed-frag invariant two ways:
- esp_ssg_unref() walks the source scatterlist and drops a page reference for every frag, including the ubuf-owned payload frags, pushing their refcount below the GUP pin bias while the pages are still pinned, i.e. a use-after-free of the zerocopy pages;
- esp_output_tail() installs its destination page as frag 0 with get_page() but leaves SKBFL_MANAGED_FRAG_REFS set, so skb_release_data() takes the skip_unref branch and never drops that reference, leaking the x->xfrag page at packet rate.
Fix this the way every other frag-mutating site does (__ip_append_data(), __ip6_append_data(), tcp_sendmsg_locked()) and call skb_zcopy_downgrade_managed() before ESP touches the frag array: it takes a real reference on each existing frag and clears SKBFL_MANAGED_FRAG_REFS, so the per-frag unref in esp_ssg_unref() and the frag release in skb_release_data() are both balanced and no mixed-ownership frag array is left behind.
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Analysis
by VulDB Data Team • 10/06/2026
The vulnerability identified within the Linux kernel's IPsec ESP implementation stems from a critical violation of memory management invariants regarding zero-copy managed fragments. In network stack operations, particularly those involving high-throughput packet processing, the kernel utilizes zerocopy mechanisms to allow user-space applications to send data without copying it into kernel buffers. This is achieved by managing fragment references through specific flags such as SKBFL_MANAGED_FRAG_REFS, which indicate that the payload fragments are owned by a userspace buffer (ubuf) and must not be individually referenced or unreferenced in standard ways while they remain pinned via Get User Pages operations. The ESP output path, specifically when operating in out-of-place mode where esp->inplace is false, involves rewriting the socket buffer fragment array to append trailers and replace fragments with destination pages using get_page calls. However, prior to this fix, the code mutated these frag arrays without first downgrading the zerocopy managed status of the skb. This oversight breaks the fundamental invariant that manages-frag references must be handled exclusively by the ubuf owner until explicitly downgraded, leading to severe memory safety violations and resource leaks.
The operational impact manifests in two distinct failure modes due to this improper handling of fragment ownership. First, during the unreference phase via esp_ssg_unref(), the kernel walks the source scatterlist and drops a page reference for every frag entry. Because the code failed to downgrade the managed status before mutation, it incorrectly attempts to drop references on ubuf-owned payload frags. This action pushes their reference count below the bias established by GUP pins while those pages are still actively pinned in memory. Consequently, this results in a use-after-free condition where kernel components may access or free zerocopy pages that are still in active use by userspace applications, potentially leading to data corruption, system crashes, or exploitation for arbitrary code execution if an attacker can influence the packet flow. Second, during the output tail processing via esp_output_tail(), the function installs a destination page as fragment zero using get_page but leaves the SKBFL_MANAGED_FRAG_REFS flag set. This causes skb_release_data to take a skip_unref branch, meaning it never drops the reference acquired for the new frag. At packet rate, this results in a significant memory leak of x->xfrag pages, which can lead to resource exhaustion and denial of service conditions under sustained traffic loads.
To resolve these issues, the fix aligns ESP fragment handling with established best practices found elsewhere in the kernel networking stack, such as in __ip_append_data(), __ip6_append_data(), and tcp_sendmsg_locked(). The core remediation involves calling skb_zcopy_downgrade_managed() before any mutation of the frag array occurs. This function performs two critical operations: it takes a real reference on each existing fragment to ensure they remain valid independently of the userspace buffer, and it clears the SKBFL_MANAGED_FRAG_REFS flag. By doing so, the kernel transitions the fragments from managed zerocopy ownership to standard kernel-managed references. This ensures that subsequent per-fragment unreference operations in esp_ssg_unref() are balanced correctly against the acquired references, and that skb_release_data properly drops all associated references without skipping them due to incorrect flags. The result is a clean separation of concerns where no mixed-ownership frag array remains after processing, thereby eliminating both the use-after-free vulnerability and the memory leak.
From a classification perspective, this vulnerability maps directly to CWE-416 Use After Free, as the premature decrementing of reference counts on pinned pages allows access to freed or invalid memory regions. Additionally, it relates to CWE-789 Memory Leak due to Reference Count Imbalance, where references are acquired but never released under specific code paths triggered by high packet rates. In terms of attack vectors and defensive mapping, this flaw is relevant to ATT&CK technique T1499 Endpoint Denial of Service via resource exhaustion, as the memory leak can degrade system stability over time. It also touches upon aspects of privilege escalation potential if the use-after-free condition can be exploited to corrupt kernel structures, aligning with general principles of improper access control and memory management found in CWE-823 Use of Pointer Subtraction or Related Operations when misapplied to reference counting logic. Mitigation requires applying the specific kernel patch that enforces the downgrade of managed fragments prior to ESP processing, ensuring compliance with standard network stack fragment lifecycle rules. System administrators should prioritize updating affected kernels and monitoring for signs of memory pressure or unexpected crashes in systems handling high volumes of IPsec traffic.