# CVE-2021-47103: inet: fully convert sk->sk_rx_dst to RCU rules

> A use-after-free vulnerability in the Linux kernel's TCP early demux path allows attackers to cause a denial of service or potentially execute code.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 4 Mar 2024
- **Updated:** 5 Aug 2026
- **Also known as:** DEBIAN-CVE-2021-47103, GHSA-rpv2-375w-83vr, UBUNTU-CVE-2021-47103

## What is CVE-2021-47103?

CVE-2021-47103 is a critical use after free vulnerability in Linux kernel, published on 4 Mar 2024. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

inet: fully convert sk->sk_rx_dst to RCU rules

syzbot reported various issues around early demux,
one being included in this changelog [1]

sk->sk_rx_dst is using RCU protection without clearly
documenting it.

And following sequences in tcp_v4_do_rcv()/tcp_v6_do_rcv()
are not following standard RCU rules.

[a]    dst_release(dst);
[b]    sk->sk_rx_dst = NULL;

They look wrong because a delete operation of RCU protected
pointer is supposed to clear the pointer before
the call_rcu()/synchronize_rcu() guarding actual memory freeing.

In some cases indeed, dst could be freed before [b] is done.

We could cheat by clearing sk_rx_dst before calling
dst_release(), but this seems the right time to stick
to standard RCU annotations and debugging facilities.

[1]
BUG: KASAN: use-after-free in dst_check include/net/dst.h:470 [inline]
BUG: KASAN: use-after-free in tcp_v4_early_demux+0x95b/0x960 net/ipv4/tcp_ipv4.c:1792
Read of size 2 at addr ffff88807f1cb73a by task syz-executor.5/9204

CPU: 0 PID: 9204 Comm: syz-executor.5 Not tainted 5.16.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 print_address_description.constprop.0.cold+0x8d/0x320 mm/kasan/report.c:247
 __kasan_report mm/kasan/report.c:433 [inline]
 kasan_report.cold+0x83/0xdf mm/kasan/report.c:450
 dst_check include/net/dst.h:470 [inline]
 tcp_v4_early_demux+0x95b/0x960 net/ipv4/tcp_ipv4.c:1792
 ip_rcv_finish_core.constprop.0+0x15de/0x1e80 net/ipv4/ip_input.c:340
 ip_list_rcv_finish.constprop.0+0x1b2/0x6e0 net/ipv4/ip_input.c:583
 ip_sublist_rcv net/ipv4/ip_input.c:609 [inline]
 ip_list_rcv+0x34e/0x490 net/ipv4/ip_input.c:644
 __netif_receive_skb_list_ptype net/core/dev.c:5508 [inline]
 __netif_receive_skb_list_core+0x549/0x8e0 net/core/dev.c:5556
 __netif_receive_skb_list net/core/dev.c:5608 [inline]
 netif_receive_skb_list_internal+0x75e/0xd80 net/core/dev.c:5699
 gro_normal_list net/core/dev.c:5853 [inline]
 gro_normal_list net/core/dev.c:5849 [inline]
 napi_complete_done+0x1f1/0x880 net/core/dev.c:6590
 virtqueue_napi_complete drivers/net/virtio_net.c:339 [inline]
 virtnet_poll+0xca2/0x11b0 drivers/net/virtio_net.c:1557
 __napi_poll+0xaf/0x440 net/core/dev.c:7023
 napi_poll net/core/dev.c:7090 [inline]
 net_rx_action+0x801/0xb40 net/core/dev.c:7177
 __do_softirq+0x29b/0x9c2 kernel/softirq.c:558
 invoke_softirq kernel/softirq.c:432 [inline]
 __irq_exit_rcu+0x123/0x180 kernel/softirq.c:637
 irq_exit_rcu+0x5/0x20 kernel/softirq.c:649
 common_interrupt+0x52/0xc0 arch/x86/kernel/irq.c:240
 asm_common_interrupt+0x1e/0x40 arch/x86/include/asm/idtentry.h:629
RIP: 0033:0x7f5e972bfd57
Code: 39 d1 73 14 0f 1f 80 00 00 00 00 48 8b 50 f8 48 83 e8 08 48 39 ca 77 f3 48 39 c3 73 3e 48 89 13 48 8b 50 f8 48 89 38 49 8b 0e <48> 8b 3e 48 83 c3 08 48 83 c6 08 eb bc 48 39 d1 72 9e 48 39 d0 73
RSP: 002b:00007fff8a413210 EFLAGS: 00000283
RAX: 00007f5e97108990 RBX: 00007f5e97108338 RCX: ffffffff81d3aa45
RDX: ffffffff81d3aa45 RSI: 00007f5e97108340 RDI: ffffffff81d3aa45
RBP: 00007f5e97107eb8 R08: 00007f5e97108d88 R09: 0000000093c2e8d9
R10: 0000000000000000 R11: 0000000000000000 R12: 00007f5e97107eb0
R13: 00007f5e97108338 R14: 00007f5e97107ea8 R15: 0000000000000019
 </TASK>

Allocated by task 13:
 kasan_save_stack+0x1e/0x50 mm/kasan/common.c:38
 kasan_set_track mm/kasan/common.c:46 [inline]
 set_alloc_info mm/kasan/common.c:434 [inline]
 __kasan_slab_alloc+0x90/0xc0 mm/kasan/common.c:467
 kasan_slab_alloc include/linux/kasan.h:259 [inline]
 slab_post_alloc_hook mm/slab.h:519 [inline]
 slab_alloc_node mm/slub.c:3234 [inline]
 slab_alloc mm/slub.c:3242 [inline]
 kmem_cache_alloc+0x202/0x3a0 mm/slub.c:3247
 dst_alloc+0x146/0x1f0 net/core/dst.c:92
 rt_dst_alloc+0x73/0x430 net/ipv4/route.c:1613
 ip_route_input_slow+0x1817/0x3a20 net/ipv4/route.c:234
---truncated---

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2021-47103?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-47103?

Apply the vendor's patch.

## Is CVE-2021-47103 being exploited?

There is no known exploitation of CVE-2021-47103 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-2021-47103 and fix it if it is affected.

What it is: CVE-2021-47103 is a critical use after free vulnerability in Linux kernel, published on 4 Mar 2024. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-47103 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-2021-47103/ (Markdown: https://insights.sbomify.com/CVE-2021-47103/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47103
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47103
- OSV: https://osv.dev/vulnerability/CVE-2021-47103

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47103/
Summaries are generated and may contain mistakes; the official description above is the record.