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Port exploit to aarch64
- Replace x86_64 shellcode/ELF in shell_elf[] with aarch64 equivalent (e_machine=0xb7, MOVZ/SVC instructions, syscall numbers 144/146/159/221). - Update verify_byte() check at post-write to look for the aarch64 MOVZ opcode signature (0x80 0xd2) instead of the x86 (0x31 0xff). - Update su_marker[] to match the first 8 bytes of the aarch64 shellcode. Tested on Kali aarch64 6.19.11+kali-arm64; xfrm-ESP leg lands cleanly. rxrpc leg is x86-only (oopses on aarch64 in flush_dcache_page).
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README.md
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README.md
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# Dirty Frag: Universal Linux LPE
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# Dirty Frag (aarch64 port)
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<p align="center">
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> Port of [V4bel/dirtyfrag](https://github.com/V4bel/dirtyfrag) to 64-bit ARM (aarch64).
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<img src="assets/tux.png" width="400" alt="tux">
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> The original vulnerability discovery, write-up, and exploit are by
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</p>
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> **[Hyunwoo Kim (@v4bel)](https://x.com/v4bel)**. This repository only adapts the
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> embedded payload and verification logic so the exploit lands on aarch64 Linux.
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# Abstract
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## Abstract
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Dirty Frag chains two page-cache write vulnerabilities — `xfrm-ESP Page-Cache Write
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(CVE-2026-43284)` and `RxRPC Page-Cache Write (CVE-2026-43500)` — into a universal
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local privilege escalation. The upstream PoC ships with an x86_64-only payload
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(both the planted ELF and the verification logic). On aarch64 distributions the
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underlying kernel bugs are still present and the **xfrm-ESP primitive itself works
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unmodified** — only the userspace logic prematurely declares failure because it
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checks for x86 byte signatures.
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This document describes the Dirty Frag vulnerability class, first discovered and reported by [Hyunwoo Kim (@v4bel)](https://x.com/v4bel), which can obtain root privileges on major Linux distributions by chaining the `xfrm-ESP Page-Cache Write (CVE-2026-43284)` vulnerability and the `RxRPC Page-Cache Write (CVE-2026-43500)` vulnerability.
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This port replaces:
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Dirty Frag is a case that extends the bug class to which [Dirty Pipe](https://dirtypipe.cm4all.com/) and [Copy Fail](https://copy.fail/) belong. Because it is a deterministic logic bug that does not depend on a timing window, no race condition is required, the kernel does not panic when the exploit fails, and the success rate is very high.
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1. the 192-byte embedded x86_64 root-shell ELF with an aarch64 equivalent
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(`e_machine = 0xb7`, `MOVZ` / `SVC #0` shellcode, aarch64 syscall numbers
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`setgid=144`, `setuid=146`, `setgroups=159`, `execve=221`);
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2. the post-write `verify_byte()` check, which previously looked for the x86
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prologue `0x31 0xff` at the entry offset — now checks for the aarch64
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`MOVZ` opcode bytes `0x80 0xd2` at offsets `0x7a/0x7b`;
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3. the `su_marker[]` array used by `su_already_patched()` — now holds the first
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eight bytes of the aarch64 shellcode (`00 00 80 d2 08 12 80 d2`).
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For detailed technical information and the timeline, [see here](assets/write-up.md).
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These are the only three changes against upstream. The kernel-side primitive
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(`xfrm-ESP` page-cache write via `splice` + `vmsplice`) is unchanged.
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- `xfrm-ESP Page-Cache Write (CVE-2026-43284)` was patched in mainline [f4c50a4034e6](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f4c50a4034e62ab75f1d5cdd191dd5f9c77fdff4).
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## What works / what doesn't on aarch64
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- `RxRPC Page-Cache Write (CVE-2026-43500)` was patched in mainline [aa54b1d27fe0](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=aa54b1d27fe0c2b78e664a34fd0fdf7cd1960d71).
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> [!NOTE]
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| Leg | x86_64 | aarch64 | Notes |
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> At the time this document was first made public (2026-05-07), the embargo had been broken due to external factors, so no patch or CVE existed yet. After consultation with the maintainers on linux-distros@vs.openwall.org at that time, the Dirty Frag document was published at their request. For the disclosure timeline, refer to the technical details.
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| ----------------------------- | ------ | -------------- | -------------------------------------------------------------------------------------------------------------------------------------------------- |
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| `xfrm-ESP` page-cache write | works | **works** | Uses real `struct page*` via `splice` from the target file. Cache-coherency differences do not affect the write path. |
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| `RxRPC` page-cache write | works | **kernel oops**| Supplies a fabricated `struct page*` that x86 `flush_dcache_page()` ignores (no-op) but aarch64 dereferences (real cache flush) → translation fault. |
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# Exploiting
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Because the xfrm-ESP leg succeeds on aarch64, the rxrpc fallback is unnecessary
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and the exploit obtains a root shell through the same flow as upstream.
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## One-line special
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If you only have access to a distribution that blocks unprivileged user-namespace
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creation (e.g., Ubuntu under AppArmor), the xfrm-ESP leg will not run. On those
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systems the rxrpc fallback is currently not portable to aarch64 — see
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*Limitations* below.
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## Exploiting
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```
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```
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git clone https://github.com/V4bel/dirtyfrag.git && cd dirtyfrag && gcc -O0 -Wall -o exp exp.c -lutil && ./exp
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git clone https://github.com/<your-user>/dirtyfrag-aarch64.git && \
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cd dirtyfrag-aarch64 && \
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gcc -O0 -Wall -o exp exp.c -lutil && \
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./exp
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```
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```
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This PoC is provided as accurate information following consultation with linux-distros. Do not use it on systems that you are not authorized to test.
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Pass `-v` or set `DIRTYFRAG_VERBOSE=1` to see the patching progress.
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This PoC is for authorized testing only. Do not run it on systems you do not
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own or are not contracted to test.
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## Cleanup
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## Cleanup
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⚠️ **Important:** After running this exploit, the page cache is contaminated. To clear the polluted page cache and ensure system stability, either run:
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After running the exploit, the page cache for `/usr/bin/su` is polluted. Either
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drop the caches or reboot:
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```bash
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```
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echo 3 > /proc/sys/vm/drop_caches
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echo 3 | sudo tee /proc/sys/vm/drop_caches
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```
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```
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or reboot the system.
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The on-disk binary is not modified — only the in-memory page cache is.
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# Affected Versions
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## Tested
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- **CVE-2026-43284**: xfrm-ESP Page-Cache Write vulnerability is in scope from [cac2661c53f3 (2017-01-17)](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cac2661c53f3) up to [f4c50a4034e6 (2026-05-05)](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f4c50a4034e62ab75f1d5cdd191dd5f9c77fdff4).
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- Kali ARM (Apple Silicon, VMware Fusion), kernel `6.19.11+kali-arm64`, glibc
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- **CVE-2026-43500**: RxRPC Page-Cache Write vulnerability is in scope from [2dc334f1a63a (2023-06-08)](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2dc334f1a63a) up to [aa54b1d27fe0 (2026-05-10)](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=aa54b1d27fe0c2b78e664a34fd0fdf7cd1960d71).
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built for `aarch64-linux-gnu`. Both modules `esp4` and `xfrm_user` autoload
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cleanly on first use; unprivileged user namespaces are enabled by default.
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In other words, the effective lifetime of the vulnerabilities is about 9 years.
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If you test this on additional aarch64 distributions (Raspberry Pi OS, Ubuntu
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Server for ARM, openSUSE aarch64, Fedora aarch64, Amazon Linux 2023 aarch64,
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Oracle Linux ARM, etc.) please open an issue or PR with the kernel version and
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result.
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This Dirty Frag has been tested on the following distribution versions.
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## Affected versions
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- Ubuntu 24.04.4: 6.17.0-23-generic
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Same scope as upstream — these are kernel bugs, not architecture-specific:
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- RHEL 10.1: 6.12.0-124.49.1.el10_1.x86_64
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- openSUSE Tumbleweed: 7.0.2-1-default
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- CentOS Stream 10: 6.12.0-224.el10.x86_64
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- AlmaLinux 10: 6.12.0-124.52.3.el10_1.x86_64
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- Fedora 44: 6.19.14-300.fc44.x86_64
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- ...
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# Mitigation
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- **CVE-2026-43284** (xfrm-ESP): `cac2661c53f3` (2017-01-17) → `f4c50a4034e6` (2026-05-05)
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- **CVE-2026-43500** (RxRPC): `2dc334f1a63a` (2023-06-08) → `aa54b1d27fe0` (2026-05-10)
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1. Use the following command to remove the modules in which the vulnerabilities occur and clear the page cache.
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Kernels built before those fix commits are vulnerable. As of this port's
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```bash
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publication the patches are in mainline but have not yet been backported to
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sh -c "printf 'install esp4 /bin/false\ninstall esp6 /bin/false\ninstall rxrpc /bin/false\n' > /etc/modprobe.d/dirtyfrag.conf; rmmod esp4 esp6 rxrpc 2>/dev/null; echo 3 > /proc/sys/vm/drop_caches; true"
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every aarch64 distribution.
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## Mitigation
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Same as upstream:
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```
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sudo sh -c "printf 'install esp4 /bin/false\ninstall esp6 /bin/false\ninstall rxrpc /bin/false\n' \
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> /etc/modprobe.d/dirtyfrag.conf; rmmod esp4 esp6 rxrpc 2>/dev/null; \
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echo 3 > /proc/sys/vm/drop_caches; true"
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```
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```
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2. Once each distribution backports a patch, update accordingly.
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Update to a kernel that includes both fix commits as soon as your distribution
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ships the backport.
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# FAQ
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## Limitations
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## Why did you chain two vulnerabilities?
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- **rxrpc fallback is not ported.** On aarch64 the rxrpc primitive faults the
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kernel in `flush_dcache_page` because of how the bug constructs a fake
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`struct page*`. Making that leg work would require a different page-supply
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technique (real backing page that aliases the target file's page cache),
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which is a research problem, not a code change.
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- **Only `/usr/bin/su` is patched.** The `/etc/passwd` backdoor path used by
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the rxrpc leg is unavailable on aarch64 for the reason above. If your target
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distribution restricts unprivileged userns and you cannot use the xfrm-ESP
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leg either, this port will not give you root.
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xfrm-ESP Page-Cache Write provides a powerful arbitrary 4-byte STORE primitive like Copy Fail, and is included on most distributions, but it requires the privilege to create a namespace.
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## Credit
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Ubuntu sometimes blocks unprivileged user namespace creation through AppArmor policy. In such an environment, xfrm-ESP Page-Cache Write cannot be triggered. RxRPC Page-Cache Write does not require the privilege to create a namespace, but the `rxrpc.ko` module itself is not included in most distributions. However, on Ubuntu, the `rxrpc.ko` module is loaded by default.
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- Original vulnerability discovery, write-up, and exploit: **Hyunwoo Kim
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([@v4bel](https://x.com/v4bel))** — see upstream repo
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[V4bel/dirtyfrag](https://github.com/V4bel/dirtyfrag) and the technical
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write-up linked from there.
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- aarch64 port (this repo): payload + verification adapted to aarch64 only;
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all kernel-exploitation logic is upstream's.
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Chaining the two variants makes the blind spots cover each other, allowing root privileges to be obtained on every major distribution. For details, refer to the technical details document.
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## License
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## Another "branded" "Dirty" series?
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The upstream repository does not ship an explicit license file. This fork
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preserves attribution and is published for security-research purposes only.
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Yeah, yeah, I know. However, this vulnerability is a descendant of "Dirty Pipe", and it is a bug class that "dirties" the `frag` member of `struct sk_buff`, so this name is the most appropriate.
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If the upstream author requests removal or relicensing, please open an issue
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and the repository will be updated accordingly.
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## What is its relationship with the "Copy Fail" vulnerability?
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Copy Fail was the motivation for starting this research. In particular, xfrm-ESP Page-Cache Write in the Dirty Frag vulnerability chain shares the same sink as Copy Fail. However, it is triggered regardless of whether the algif_aead module is available. In other words, even on systems where the publicly known Copy Fail mitigation (algif_aead blacklist) is applied, your Linux is still vulnerable to Dirty Frag.
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## So, how do I fix my Linux?
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Refer to the Mitigation section above.
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