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CVE Record

CVE-2024-26804: net: ip_tunnel: prevent perpetual headroom growth

In the Linux kernel, the following vulnerability has been resolved: net: ip_tunnel: prevent perpetual headroom growth syzkaller triggered following kasan splat: BUG: KASAN: use-after-free in __skb_flow_dissect+0x19d1/0x7a50 net/core/flow_dissector.c:1170 Read of size 1 at addr ffff88812fb4000e by task syz-executor183/5191 [..] kasan_report+0xda/0x110 mm/kasan/report.c:588 __skb_flow_dissect+0x19d1/0x7a50 net/core/flow_dissector.c:1170 skb_flow_dissect_flow_keys include/linux/skbuff.h:1514 [inline] ___skb_get_hash net/core/flow_dissector.c:1791 [inline] __skb_get_hash+0xc7/0x540 net/core/flow_dissector.c:1856 skb_get_hash include/linux/skbuff.h:1556 [inline] ip_tunnel_xmit+0x1855/0x33c0 net/ipv4/ip_tunnel.c:748 ipip_tunnel_xmit+0x3cc/0x4e0 net/ipv4/ipip.c:308 __netdev_start_xmit include/linux/netdevice.h:4940 [inline] netdev_start_xmit include/linux/netdevice.h:4954 [inline] xmit_one net/core/dev.c:3548 [inline] dev_hard_start_xmit+0x13d/0x6d0 net/core/dev.c:3564 __dev_queue_xmit+0x7c1/0x3d60 net/core/dev.c:4349 dev_queue_xmit include/linux/netdevice.h:3134 [inline] neigh_connected_output+0x42c/0x5d0 net/core/neighbour.c:1592 ... ip_finish_output2+0x833/0x2550 net/ipv4/ip_output.c:235 ip_finish_output+0x31/0x310 net/ipv4/ip_output.c:323 .. iptunnel_xmit+0x5b4/0x9b0 net/ipv4/ip_tunnel_core.c:82 ip_tunnel_xmit+0x1dbc/0x33c0 net/ipv4/ip_tunnel.c:831 ipgre_xmit+0x4a1/0x980 net/ipv4/ip_gre.c:665 __netdev_start_xmit include/linux/netdevice.h:4940 [inline] netdev_start_xmit include/linux/netdevice.h:4954 [inline] xmit_one net/core/dev.c:3548 [inline] dev_hard_start_xmit+0x13d/0x6d0 net/core/dev.c:3564 ... The splat occurs because skb->data points past skb->head allocated area. This is because neigh layer does: __skb_pull(skb, skb_network_offset(skb)); ... but skb_network_offset() returns a negative offset and __skb_pull() arg is unsigned. IOW, we skb->data gets "adjusted" by a huge value. The negative value is returned because skb->head and skb->data distance is more than 64k and skb->network_header (u16) has wrapped around. The bug is in the ip_tunnel infrastructure, which can cause dev->needed_headroom to increment ad infinitum. The syzkaller reproducer consists of packets getting routed via a gre tunnel, and route of gre encapsulated packets pointing at another (ipip) tunnel. The ipip encapsulation finds gre0 as next output device. This results in the following pattern: 1). First packet is to be sent out via gre0. Route lookup found an output device, ipip0. 2). ip_tunnel_xmit for gre0 bumps gre0->needed_headroom based on the future output device, rt.dev->needed_headroom (ipip0). 3). ip output / start_xmit moves skb on to ipip0. which runs the same code path again (xmit recursion). 4). Routing step for the post-gre0-encap packet finds gre0 as output device to use for ipip0 encapsulated packet. tunl0->needed_headroom is then incremented based on the (already bumped) gre0 device headroom. This repeats for every future packet: gre0->needed_headroom gets inflated because previous packets' ipip0 step incremented rt->dev (gre0) headroom, and ipip0 incremented because gre0 needed_headroom was increased. For each subsequent packet, gre/ipip0->needed_headroom grows until post-expand-head reallocations result in a skb->head/data distance of more than 64k. Once that happens, skb->network_header (u16) wraps around when pskb_expand_head tries to make sure that skb_network_offset() is unchanged after the headroom expansion/reallocation. After this skb_network_offset(skb) returns a different (and negative) result post headroom expansion. The next trip to neigh layer (or anything else that would __skb_pull the network header) makes skb->data point to a memory location outside skb->head area. v2: Cap the needed_headroom update to an arbitarily chosen upperlimit to prevent perpetual increase instead of dropping the headroom increment completely.

HighCVSS 7.8Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

A Linux kernel tunneling flaw can repeatedly inflate packet headroom until a header offset wraps, causing use-after-free memory access. Successful exploitation could compromise confidentiality, integrity, and availability, but the supplied CVSS vector requires local access with low privileges. Systems using affected kernels and nested GRE/IPIP routing deserve priority review.

Executive priority

Treat as a high-priority kernel maintenance issue on multi-user or tunneling infrastructure, especially systems combining GRE and IPIP routes. Accelerate patching where low-privileged users can interact with affected networking paths. For other systems, verify exposure and remediate through the normal urgent-update process rather than assuming internet-scale exploitation.

Technical view

Recursive GRE and IPIP tunnel routing can make needed_headroom grow without limit. Once packet headroom exceeds 64 KiB, the 16-bit network-header offset wraps, producing a negative offset that is treated as unsigned. Subsequent packet processing can move skb->data outside its allocation and trigger a use-after-free in flow dissection. The kernel fix caps headroom growth.

Likely exposure

Exposure is most plausible on systems running the listed affected Linux kernel releases where GRE and IPIP tunnels can route encapsulated traffic through each other. The source bundle does not establish that ordinary hosts without this tunnel configuration are practically reachable, nor does it provide complete distribution-specific package ranges.

Exploitation context

The issue was reproduced by syzkaller using recursively routed GRE and IPIP packets. The supplied CVSS 3.1 score is 7.8 with local, low-privileged access required and no user interaction. It is not listed in KEV, and the provided sources contain no evidence of active exploitation or a public weaponized exploit.

Researcher notes

CWE-416 is supported by the reported KASAN use-after-free. The underlying condition is unbounded needed_headroom feedback between tunnel devices, followed by u16 network-header offset wraparound and an invalid skb data adjustment. The supplied affected-version data is unusual and incomplete for distribution packages; validate against vendor backports instead of relying only on upstream version strings.

Mitigation direction

  • Install a vendor kernel update containing the applicable referenced stable fix.
  • Prioritize hosts using GRE, IPIP, or complex recursive tunnel routing.
  • If immediate updating is impossible, consult distribution guidance for supported mitigations.
  • Restrict low-privileged access to exposed tunneling hosts until remediation is complete.

Validation and detection

  • Inventory kernel versions and identify hosts using GRE or IPIP tunnels.
  • Compare installed distribution packages with the vendor's CVE advisory and fixed package information.
  • Confirm the updated kernel incorporates the applicable referenced stable commit.
  • Review kernel logs for memory errors or crashes involving tunnel transmission and flow dissection.
  • Test legitimate tunnel routing after updating to detect operational regressions.
Prepared
Confidence
high
Sources
10

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-416: Exact CWE lookup

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cve · low confidence lookup

CVE-2024-26804 mapping review

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.8 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
3Timeline events
2ADP providers
9Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: total

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.8CVSS 3.1HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H1.85.9Linux

Vulnerability scoring details

Base CVSS 3.1 score

7.8High
CVSS 3.1 vector shape for CVE-2024-26804Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
CVECVE Program Container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux243aad830e8a4cdda261626fbaeddde16b08d04a, 243aad830e8a4cdda261626fbaeddde16b08d04a, 243aad830e8a4cdda261626fbaeddde16b08d04a, 243aad830e8a4cdda261626fbaeddde16b08d04a, 243aad830e8a4cdda261626fbaeddde16b08d04a, 243aad830e8a4cdda261626fbaeddde16b08d04a, 243aad830e8a4cdda261626fbaeddde16b08d04a, 03017375b0122453e6dda833ff7bd4191915def5, 2.6.33.2unaffected
LinuxLinux2.6.34, 0, 5.4.271, 5.10.212, 5.15.151, 6.1.81, 6.6.21, 6.7.9, 6.8affected
Weakness

CWE details

CWE-416

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-416 · source CWE mapping

Use After Free

Use After Free represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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