# CVE-2021-46954: net/sched: sch_frag: fix stack OOB read while fragmenting IPv4 packets

> A stack out-of-bounds read in the Linux kernel's IPv4 fragmentation code can cause a system crash when specific network actions are triggered.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 27 Feb 2024
- **Updated:** 11 May 2026
- **Also known as:** GHSA-6jf4-4jp9-4wmw, UBUNTU-CVE-2021-46954

## What is CVE-2021-46954?

CVE-2021-46954 is a high-severity memory corruption vulnerability in Linux kernel, published on 27 Feb 2024. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

net/sched: sch_frag: fix stack OOB read while fragmenting IPv4 packets

when 'act_mirred' tries to fragment IPv4 packets that had been previously
re-assembled using 'act_ct', splats like the following can be observed on
kernels built with KASAN:

 BUG: KASAN: stack-out-of-bounds in ip_do_fragment+0x1b03/0x1f60
 Read of size 1 at addr ffff888147009574 by task ping/947

 CPU: 0 PID: 947 Comm: ping Not tainted 5.12.0-rc6+ #418
 Hardware name: Red Hat KVM, BIOS 1.11.1-4.module+el8.1.0+4066+0f1aadab 04/01/2014
 Call Trace:
  <IRQ>
  dump_stack+0x92/0xc1
  print_address_description.constprop.7+0x1a/0x150
  kasan_report.cold.13+0x7f/0x111
  ip_do_fragment+0x1b03/0x1f60
  sch_fragment+0x4bf/0xe40
  tcf_mirred_act+0xc3d/0x11a0 [act_mirred]
  tcf_action_exec+0x104/0x3e0
  fl_classify+0x49a/0x5e0 [cls_flower]
  tcf_classify_ingress+0x18a/0x820
  __netif_receive_skb_core+0xae7/0x3340
  __netif_receive_skb_one_core+0xb6/0x1b0
  process_backlog+0x1ef/0x6c0
  __napi_poll+0xaa/0x500
  net_rx_action+0x702/0xac0
  __do_softirq+0x1e4/0x97f
  do_softirq+0x71/0x90
  </IRQ>
  __local_bh_enable_ip+0xdb/0xf0
  ip_finish_output2+0x760/0x2120
  ip_do_fragment+0x15a5/0x1f60
  __ip_finish_output+0x4c2/0xea0
  ip_output+0x1ca/0x4d0
  ip_send_skb+0x37/0xa0
  raw_sendmsg+0x1c4b/0x2d00
  sock_sendmsg+0xdb/0x110
  __sys_sendto+0x1d7/0x2b0
  __x64_sys_sendto+0xdd/0x1b0
  do_syscall_64+0x33/0x40
  entry_SYSCALL_64_after_hwframe+0x44/0xae
 RIP: 0033:0x7f82e13853eb
 Code: 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 f3 0f 1e fa 48 8d 05 75 42 2c 00 41 89 ca 8b 00 85 c0 75 14 b8 2c 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 75 c3 0f 1f 40 00 41 57 4d 89 c7 41 56 41 89
 RSP: 002b:00007ffe01fad888 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
 RAX: ffffffffffffffda RBX: 00005571aac13700 RCX: 00007f82e13853eb
 RDX: 0000000000002330 RSI: 00005571aac13700 RDI: 0000000000000003
 RBP: 0000000000002330 R08: 00005571aac10500 R09: 0000000000000010
 R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffe01faefb0
 R13: 00007ffe01fad890 R14: 00007ffe01fad980 R15: 00005571aac0f0a0

 The buggy address belongs to the page:
 page:000000001dff2e03 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x147009
 flags: 0x17ffffc0001000(reserved)
 raw: 0017ffffc0001000 ffffea00051c0248 ffffea00051c0248 0000000000000000
 raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000
 page dumped because: kasan: bad access detected

 Memory state around the buggy address:
  ffff888147009400: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  ffff888147009480: f1 f1 f1 f1 04 f2 f2 f2 f2 f2 f2 f2 00 00 00 00
 >ffff888147009500: 00 00 00 00 00 00 00 00 00 00 f2 f2 f2 f2 f2 f2
                                                              ^
  ffff888147009580: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  ffff888147009600: 00 00 00 00 00 00 00 00 00 00 00 00 00 f2 f2 f2

for IPv4 packets, sch_fragment() uses a temporary struct dst_entry. Then,
in the following call graph:

  ip_do_fragment()
    ip_skb_dst_mtu()
      ip_dst_mtu_maybe_forward()
        ip_mtu_locked()

the pointer to struct dst_entry is used as pointer to struct rtable: this
turns the access to struct members like rt_mtu_locked into an OOB read in
the stack. Fix this changing the temporary variable used for IPv4 packets
in sch_fragment(), similarly to what is done for IPv6 few lines below.

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2021-46954?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-46954?

No fix has been published for CVE-2021-46954 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-46954 being exploited?

There is no known exploitation of CVE-2021-46954 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-46954 and fix it if it is affected.

What it is: CVE-2021-46954 is a high-severity memory corruption vulnerability in Linux kernel, published on 27 Feb 2024. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-46954 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-46954/ (Markdown: https://insights.sbomify.com/CVE-2021-46954/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-46954
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-46954
- OSV: https://osv.dev/vulnerability/CVE-2021-46954

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-46954/
Summaries are generated and may contain mistakes; the official description above is the record.