# CVE-2026-31517

> Vulnerability · severity: **MEDIUM** (CVSS 5.5).

## Description

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

xfrm: iptfs: fix skb_put() panic on non-linear skb during reassembly

In iptfs_reassem_cont(), IP-TFS attempts to append data to the new inner
packet 'newskb' that is being reassembled. First a zero-copy approach is
tried if it succeeds then newskb becomes non-linear.

When a subsequent fragment in the same datagram does not meet the
fast-path conditions, a memory copy is performed. It calls skb_put() to
append the data and as newskb is non-linear it triggers
SKB_LINEAR_ASSERT check.

 Oops: invalid opcode: 0000 [#1] SMP NOPTI
 [...]
 RIP: 0010:skb_put+0x3c/0x40
 [...]
 Call Trace:
  <IRQ>
  iptfs_reassem_cont+0x1ab/0x5e0 [xfrm_iptfs]
  iptfs_input_ordered+0x2af/0x380 [xfrm_iptfs]
  iptfs_input+0x122/0x3e0 [xfrm_iptfs]
  xfrm_input+0x91e/0x1a50
  xfrm4_esp_rcv+0x3a/0x110
  ip_protocol_deliver_rcu+0x1d7/0x1f0
  ip_local_deliver_finish+0xbe/0x1e0
  __netif_receive_skb_core.constprop.0+0xb56/0x1120
  __netif_receive_skb_list_core+0x133/0x2b0
  netif_receive_skb_list_internal+0x1ff/0x3f0
  napi_complete_done+0x81/0x220
  virtnet_poll+0x9d6/0x116e [virtio_net]
  __napi_poll.constprop.0+0x2b/0x270
  net_rx_action+0x162/0x360
  handle_softirqs+0xdc/0x510
  __irq_exit_rcu+0xe7/0x110
  irq_exit_rcu+0xe/0x20
  common_interrupt+0x85/0xa0
  </IRQ>
  <TASK>

Fix this by checking if the skb is non-linear. If it is, linearize it by
calling skb_linearize(). As the initial allocation of newskb originally
reserved enough tailroom for the entire reassembled packet we do not
need to check if we have enough tailroom or extend it.

## Key facts

- **CVE ID:** CVE-2026-31517
- **Published:** 2026-04-22
- **CVSS severity:** MEDIUM
- **CVSS base score:** 5.5
- **CWE codes:** CWE-401

## Affected products

- `linux:linux_kernel`

## Primary sources

- NIST NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31517

## Citation

> AI Analytics. CVE-2026-31517. Retrieved 2026-08-13 from https://api.ai-analytics.org/cve/CVE-2026-31517. Derived from NIST NVD. Licensed CC0.

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