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Eric Dumazet 4e45337556 ipv6: avoid overflows in ip6_datagram_send_ctl()
Yiming Qian reported :
<quote>
 I believe I found a locally triggerable kernel bug in the IPv6 sendmsg
 ancillary-data path that can panic the kernel via `skb_under_panic()`
 (local DoS).

 The core issue is a mismatch between:

 - a 16-bit length accumulator (`struct ipv6_txoptions::opt_flen`, type
 `__u16`) and
 - a pointer to the *last* provided destination-options header (`opt->dst1opt`)

 when multiple `IPV6_DSTOPTS` control messages (cmsgs) are provided.

 - `include/net/ipv6.h`:
   - `struct ipv6_txoptions::opt_flen` is `__u16` (wrap possible).
 (lines 291-307, especially 298)
 - `net/ipv6/datagram.c:ip6_datagram_send_ctl()`:
   - Accepts repeated `IPV6_DSTOPTS` and accumulates into `opt_flen`
 without rejecting duplicates. (lines 909-933)
 - `net/ipv6/ip6_output.c:__ip6_append_data()`:
   - Uses `opt->opt_flen + opt->opt_nflen` to compute header
 sizes/headroom decisions. (lines 1448-1466, especially 1463-1465)
 - `net/ipv6/ip6_output.c:__ip6_make_skb()`:
   - Calls `ipv6_push_frag_opts()` if `opt->opt_flen` is non-zero.
 (lines 1930-1934)
 - `net/ipv6/exthdrs.c:ipv6_push_frag_opts()` / `ipv6_push_exthdr()`:
   - Push size comes from `ipv6_optlen(opt->dst1opt)` (based on the
 pointed-to header). (lines 1179-1185 and 1206-1211)

 1. `opt_flen` is a 16-bit accumulator:

 - `include/net/ipv6.h:298` defines `__u16 opt_flen; /* after fragment hdr */`.

 2. `ip6_datagram_send_ctl()` accepts *repeated* `IPV6_DSTOPTS` cmsgs
 and increments `opt_flen` each time:

 - In `net/ipv6/datagram.c:909-933`, for `IPV6_DSTOPTS`:
   - It computes `len = ((hdr->hdrlen + 1) << 3);`
   - It checks `CAP_NET_RAW` using `ns_capable(net->user_ns,
 CAP_NET_RAW)`. (line 922)
   - Then it does:
     - `opt->opt_flen += len;` (line 927)
     - `opt->dst1opt = hdr;` (line 928)

 There is no duplicate rejection here (unlike the legacy
 `IPV6_2292DSTOPTS` path which rejects duplicates at
 `net/ipv6/datagram.c:901-904`).

 If enough large `IPV6_DSTOPTS` cmsgs are provided, `opt_flen` wraps
 while `dst1opt` still points to a large (2048-byte)
 destination-options header.

 In the attached PoC (`poc.c`):

 - 32 cmsgs with `hdrlen=255` => `len = (255+1)*8 = 2048`
 - 1 cmsg with `hdrlen=0` => `len = 8`
 - Total increment: `32*2048 + 8 = 65544`, so `(__u16)opt_flen == 8`
 - The last cmsg is 2048 bytes, so `dst1opt` points to a 2048-byte header.

 3. The transmit path sizes headers using the wrapped `opt_flen`:

- In `net/ipv6/ip6_output.c:1463-1465`:
  - `headersize = sizeof(struct ipv6hdr) + (opt ? opt->opt_flen +
 opt->opt_nflen : 0) + ...;`

 With wrapped `opt_flen`, `headersize`/headroom decisions underestimate
 what will be pushed later.

 4. When building the final skb, the actual push length comes from
 `dst1opt` and is not limited by wrapped `opt_flen`:

 - In `net/ipv6/ip6_output.c:1930-1934`:
   - `if (opt->opt_flen) proto = ipv6_push_frag_opts(skb, opt, proto);`
 - In `net/ipv6/exthdrs.c:1206-1211`, `ipv6_push_frag_opts()` pushes
 `dst1opt` via `ipv6_push_exthdr()`.
 - In `net/ipv6/exthdrs.c:1179-1184`, `ipv6_push_exthdr()` does:
   - `skb_push(skb, ipv6_optlen(opt));`
   - `memcpy(h, opt, ipv6_optlen(opt));`

 With insufficient headroom, `skb_push()` underflows and triggers
 `skb_under_panic()` -> `BUG()`:

 - `net/core/skbuff.c:2669-2675` (`skb_push()` calls `skb_under_panic()`)
 - `net/core/skbuff.c:207-214` (`skb_panic()` ends in `BUG()`)

 - The `IPV6_DSTOPTS` cmsg path requires `CAP_NET_RAW` in the target
 netns user namespace (`ns_capable(net->user_ns, CAP_NET_RAW)`).
 - Root (or any task with `CAP_NET_RAW`) can trigger this without user
 namespaces.
 - An unprivileged `uid=1000` user can trigger this if unprivileged
 user namespaces are enabled and it can create a userns+netns to obtain
 namespaced `CAP_NET_RAW` (the attached PoC does this).

 - Local denial of service: kernel BUG/panic (system crash).
 - Reproducible with a small userspace PoC.
</quote>

This patch does not reject duplicated options, as this might break
some user applications.

Instead, it makes sure to adjust opt_flen and opt_nflen to correctly
reflect the size of the current option headers, preventing the overflows
and the potential for panics.

This applies to IPV6_DSTOPTS, IPV6_HOPOPTS, and IPV6_RTHDR.

Specifically:

When a new IPV6_DSTOPTS is processed, the length of the old opt->dst1opt
is subtracted from opt->opt_flen before adding the new length.

When a new IPV6_HOPOPTS is processed, the length of the old opt->dst0opt
is subtracted from opt->opt_nflen.

When a new Routing Header (IPV6_RTHDR or IPV6_2292RTHDR) is processed,
the length of the old opt->srcrt is subtracted from opt->opt_nflen.

In the special case within IPV6_2292RTHDR handling where dst1opt is moved
to dst0opt, the length of the old opt->dst0opt is subtracted from
opt->opt_nflen before the new one is added.

Fixes: 333fad5364 ("[IPV6]: Support several new sockopt / ancillary data in Advanced API (RFC3542).")
Reported-by: Yiming Qian <yimingqian591@gmail.com>
Closes: https://lore.kernel.org/netdev/CAL_bE8JNzawgr5OX5m+3jnQDHry2XxhQT5=jThW1zDPtUikRYA@mail.gmail.com/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260401154721.3740056-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
2026-04-02 08:25:22 -07:00
Documentation dma-mapping fixes for Linux 7.0 2026-03-26 08:22:07 -07:00
LICENSES LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section 2025-10-22 07:58:19 +02:00
arch KVM/arm64 fixes for 7.0, take #4 2026-03-24 17:32:30 +01:00
block block-7.0-20260305 2026-03-06 08:36:18 -08:00
certs Convert 'alloc_obj' family to use the new default GFP_KERNEL argument 2026-02-21 17:09:51 -08:00
crypto crypto: testmgr - Fix stale references to aes-generic 2026-03-03 11:57:15 -08:00
drivers bnxt_en: Restore default stat ctxs for ULP when resource is available 2026-04-01 20:12:56 -07:00
fs Changes since last update: 2026-03-25 18:41:35 -07:00
include netfilter: ipset: use nla_strcmp for IPSET_ATTR_NAME attr 2026-04-01 11:55:29 +02:00
init hardening fixes for v7.0-rc6 2026-03-25 14:47:18 -07:00
io_uring io_uring-7.0-20260320 2026-03-20 09:58:56 -07:00
ipc Convert 'alloc_obj' family to use the new default GFP_KERNEL argument 2026-02-21 17:09:51 -08:00
kernel dma-mapping fixes for Linux 7.0 2026-03-26 08:22:07 -07:00
lib Bootconfig fixes for v7.0-rc4 2026-03-21 08:46:13 -07:00
mm dma-mapping fixes for Linux 7.0 2026-03-26 08:22:07 -07:00
net ipv6: avoid overflows in ip6_datagram_send_ctl() 2026-04-02 08:25:22 -07:00
rust Rust fixes for v7.0 (2nd) 2026-03-14 12:35:16 -07:00
samples drm-fixes for 7.0-rc4 2026-03-13 15:38:55 -07:00
scripts hardening fixes for v7.0-rc6 2026-03-25 14:47:18 -07:00
security xen/privcmd: add boot control for restricted usage in domU 2026-03-20 12:06:01 +01:00
sound driver core: platform: use generic driver_override infrastructure 2026-03-17 20:30:57 +01:00
tools selftests/tc-testing: add tests for cls_fw and cls_flow on shared blocks 2026-04-02 15:08:42 +02:00
usr kbuild: uapi: drop dependency on CC_CAN_LINK 2026-01-16 15:02:11 -07:00
virt KVM generic changes for 7.0 2026-03-11 18:01:55 +01:00
.clang-format Devicetree updates for v7.0: 2026-02-11 18:27:08 -08:00
.clippy.toml rust: clean Rust 1.88.0's warning about `clippy::disallowed_macros` configuration 2025-05-07 00:11:47 +02:00
.cocciconfig
.editorconfig editorconfig: add rst extension 2026-01-26 19:07:09 -08:00
.get_maintainer.ignore MAINTAINERS: remove Alyssa Rosenzweig 2025-09-18 21:17:31 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore rust: kbuild: add proc macro library support 2025-11-24 17:15:36 +01:00
.mailmap mailmap: update email address for Muhammad Usama Anjum 2026-03-23 09:35:05 -07:00
.pylintrc docs: Move the python libraries to tools/lib/python 2025-11-18 09:22:40 -07:00
.rustfmt.toml rust: add `.rustfmt.toml` 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: remove Thomas Falcon from IBM ibmvnic 2026-03-05 07:35:45 -08:00
Kbuild sched: Make migrate_{en,dis}able() inline 2025-09-25 09:57:16 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS platform-drivers-x86 for v7.0-3 2026-03-25 14:43:06 -07:00
Makefile Third round of Kbuild fixes for 7.0 2026-03-24 16:48:14 -07:00
README docs: add AI Coding Assistants documentation 2026-01-06 14:55:06 -07:00

README

Linux kernel
============

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
* Get the latest kernel: https://kernel.org
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
* Code of Conduct: Documentation/process/code-of-conduct.rst
* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


Who Are You?
============

Find your role below:

* New Kernel Developer - Getting started with kernel development
* Academic Researcher - Studying kernel internals and architecture
* Security Expert - Hardening and vulnerability analysis
* Backport/Maintenance Engineer - Maintaining stable kernels
* System Administrator - Configuring and troubleshooting
* Maintainer - Leading subsystems and reviewing patches
* Hardware Vendor - Writing drivers for new hardware
* Distribution Maintainer - Packaging kernels for distros
* AI Coding Assistant - LLMs and AI-powered development tools


For Specific Users
==================

New Kernel Developer
--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
* Coding Style: Documentation/process/coding-style.rst
* Build System: Documentation/kbuild/index.rst
* Development Tools: Documentation/dev-tools/index.rst
* Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst
* Core APIs: Documentation/core-api/index.rst

Academic Researcher
-------------------

Explore the kernel's architecture and internals:

* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
* Memory Management: Documentation/mm/index.rst
* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
* Filesystems: Documentation/filesystems/index.rst
* RCU (Read-Copy Update): Documentation/RCU/index.rst
* Locking Primitives: Documentation/locking/index.rst
* Power Management: Documentation/power/index.rst

Security Expert
---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
* Self Protection: Documentation/security/self-protection.rst
* Reporting Vulnerabilities: Documentation/process/security-bugs.rst
* CVE Procedures: Documentation/process/cve.rst
* Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst
* Security Features: Documentation/userspace-api/seccomp_filter.rst

Backport/Maintenance Engineer
-----------------------------

Maintain and stabilize kernel versions:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
* Applying Patches: Documentation/process/applying-patches.rst
* Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git for Maintainers: Documentation/maintainer/configure-git.rst

System Administrator
--------------------

Configure, tune, and troubleshoot Linux systems:

* Admin Guide: Documentation/admin-guide/index.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst
* Tracing/Debugging: Documentation/trace/index.rst
* Performance Security: Documentation/admin-guide/perf-security.rst
* Hardware Monitoring: Documentation/hwmon/index.rst

Maintainer
----------

Lead kernel subsystems and manage contributions:

* Maintainer Handbook: Documentation/maintainer/index.rst
* Pull Requests: Documentation/maintainer/pull-requests.rst
* Managing Patches: Documentation/maintainer/modifying-patches.rst
* Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst
* Development Process: Documentation/process/maintainer-handbooks.rst
* Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git Configuration: Documentation/maintainer/configure-git.rst

Hardware Vendor
---------------

Write drivers and support new hardware:

* Driver API Guide: Documentation/driver-api/index.rst
* Driver Model: Documentation/driver-api/driver-model/driver.rst
* Device Drivers: Documentation/driver-api/infrastructure.rst
* Bus Types: Documentation/driver-api/driver-model/bus.rst
* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

Distribution Maintainer
-----------------------

Package and distribute the kernel:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst

AI Coding Assistant
-------------------

CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and
follow the AI coding assistants documentation before contributing to the Linux
kernel:

* Documentation/process/coding-assistants.rst

This documentation contains essential requirements about licensing, attribution,
and the Developer Certificate of Origin that all AI tools must comply with.


Communication and Support
=========================

* Mailing Lists: https://lore.kernel.org/
* IRC: #kernelnewbies on irc.oftc.net
* Bugzilla: https://bugzilla.kernel.org/
* MAINTAINERS file: Lists subsystem maintainers and mailing lists
* Email Clients: Documentation/process/email-clients.rst