CVE-2026-23011: ipv4: ip_gre: make ipgre_header() robust

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

ipv4: ipgre: make ipgreheader() robust

Analog to commit db5b4e39c4e6 ("ip6gre: make ip6greheader() robust")

Over the years, syzbot found many ways to crash the kernel in ipgreheader() [1].

This involves team or bonding drivers ability to dynamically change their dev->neededheadroom and/or dev->hardheaderlen

In this particular crash mldnewpack() allocated an skb with a too small reserve/headroom, and by the time mldsendpack() was called, syzbot managed to attach an ipgre device.

[1] skbuff: skbunderpanic: text:ffffffff89ea3cb7 len:2030915468 put:2030915372 head:ffff888058b43000 data:ffff887fdfa6e194 tail:0x120 end:0x6c0 dev:team0 kernel BUG at net/core/skbuff.c:213 ! Oops: invalid opcode: 0000 [#1] SMP KASAN PTI CPU: 1 UID: 0 PID: 1322 Comm: kworker/1:9 Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025 Workqueue: mld mldifcwork RIP: 0010:skbpanic+0x157/0x160 net/core/skbuff.c:213 Call Trace: <TASK> skbunderpanic net/core/skbuff.c:223 [inline] skbpush+0xc3/0xe0 net/core/skbuff.c:2641 ipgreheader+0x67/0x290 net/ipv4/ipgre.c:897 devhardheader include/linux/netdevice.h:3436 [inline] neighconnectedoutput+0x286/0x460 net/core/neighbour.c:1618 NFHOOKCOND include/linux/netfilter.h:307 [inline] ip6output+0x340/0x550 net/ipv6/ip6output.c:247 NFHOOK+0x9e/0x380 include/linux/netfilter.h:318 mldsendpack+0x8d4/0xe60 net/ipv6/mcast.c:1855 mldsendcr net/ipv6/mcast.c:2154 [inline] mldifcwork+0x83e/0xd60 net/ipv6/mcast.c:2693 processonework kernel/workqueue.c:3257 [inline] processscheduledworks+0xad1/0x1770 kernel/workqueue.c:3340 workerthread+0x8a0/0xda0 kernel/workqueue.c:3421 kthread+0x711/0x8a0 kernel/kthread.c:463 retfromfork+0x510/0xa50 arch/x86/kernel/process.c:158 retfromforkasm+0x1a/0x30 arch/x86/entry/entry64.S:246