Dirty Frag Deep Dive: Systemic Failure of splice() Zero-Copy Mechanism

Forum topic · 小凯 · 2026-05-08

Summary

Dirty Frag is a Linux kernel privilege escalation technique disclosed on 2026-05-08 by researcher Hyunwoo Kim (@v4bel). It chains two independent vulnerabilities — one in the xfrm ESP path (introduced in 2017) and one in RxRPC (2023) — to abuse the splice() zero-copy mechanism. splice() injects read-only page-cache pages of files such as /etc/passwd into socket buffer fragment slots; the ESP or RxRPC receive path then performs in-place decryption, directly modifying page cache. On subsequent execution, the kernel loads the tampered copy, yielding a deterministic, race-free, root shell across distributions. The ESP variant allows 4-byte precise writes but requires user namespaces (blocked by Ubuntu AppArmor), while the RxRPC variant writes 8 bytes without namespaces but needs rxrpc.ko (loaded by default on Ubuntu). Confirmed affected systems include Ubuntu 24/26, RHEL 10.1, Fedora 44, openSUSE Tumbleweed, CentOS Stream 10, AlmaLinux 10, and WSL2. An ESP patch was merged into the netdev tree (commit f4c50a4034e6) on 2026-05-07, but the RxRPC patch remains unmerged. Disabling esp4/esp6 mitigates temporarily but breaks StrongSwan/Libreswan IPsec VPN tunnels.

Disclosure date: 2026-05-08 | Discoverer: Hyunwoo Kim (@v4bel) | Current status: zero patches applied broadly, no CVE assigned yet

TL;DR

Dirty Frag chains two independent vulnerabilities — xfrm-ESP (introduced in 2017) and RxRPC (introduced in 2023) — abusing the splice() zero-copy mechanism to inject read-only file page-cache pages into kernel-writable network decryption paths, achieving deterministic, race-free, cross-distribution root privilege escalation.

Vulnerability Family Tree

| Vulnerability | Year | Target | Key mechanism | |------|------|------|------| | Dirty COW | 2016 | Private memory | COW race | | Dirty Pipe | 2022 | Page cache | pipe_buffer flags | | Copy Fail | 2026.04 | Page cache | AF_ALG scatterlist | | Dirty Frag | 2026.05 | Page cache | skb frag |

Core Mechanism

1. splice() places page-cache page references of a file into the socket buffer's skb frag slots 2. ESP/RxRPC receive path performs in-place decryption, directly modifying the page cache 3. When the file is subsequently executed, the kernel loads the tampered in-memory copy, triggering a root shell

Two Complementary Vulnerabilities

xfrm-ESP: precise 4-byte write; requires user namespaces (blocked by Ubuntu AppArmor)
RxRPC: 8-byte write, no namespace required, but needs rxrpc.ko (loaded by default on Ubuntu)
Combined effect: covers nearly all distributions

Confirmed Affected Systems

Ubuntu 24/26, RHEL 10.1, Fedora 44, openSUSE Tumbleweed, CentOS Stream 10, AlmaLinux 10, WSL2

Temporary Mitigation (Side-Effect Warning)

Blocking/unloading esp4/esp6 mitigates the ESP variant.

⚠️ Disabling esp4/esp6 will completely break StrongSwan/Libreswan IPsec VPN tunnels.

Patch Status

ESP patch: merged into the netdev tree (2026-05-07), commit f4c50a4034e6, based on Kuan-Ting Chen's shared-frag approach
RxRPC patch: still not merged upstream

Deeper Takeaway

There is a systemic mismatch between splice()'s zero-copy assumption (read-only pass-through) and in-place write operations in kernel subsystems. As long as code paths exist that pass page references via splice() and later write to them in place, new "Dirty" variants will continue to appear.

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