ipv4: ip_gre: make ipgre_header() robust

edumazet · gregkh · commit 06fe0801396a · 2026-01-30T10:27:24.000+01:00
[ Upstream commit e67c577 ] Analog to commit db5b4e3 ("ip6_gre: make ip6gre_header() robust") Over the years, syzbot found many ways to crash the kernel in ipgre_header() [1]. This involves team or bonding drivers ability to dynamically change their dev->needed_headroom and/or dev->hard_header_len In this particular crash mld_newpack() allocated an skb with a too small reserve/headroom, and by the time mld_sendpack() was called, syzbot managed to attach an ipgre device. [1] skbuff: skb_under_panic: 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 mld_ifc_work RIP: 0010:skb_panic+0x157/0x160 net/core/skbuff.c:213 Call Trace: <TASK> skb_under_panic net/core/skbuff.c:223 [inline] skb_push+0xc3/0xe0 net/core/skbuff.c:2641 ipgre_header+0x67/0x290 net/ipv4/ip_gre.c:897 dev_hard_header include/linux/netdevice.h:3436 [inline] neigh_connected_output+0x286/0x460 net/core/neighbour.c:1618 NF_HOOK_COND include/linux/netfilter.h:307 [inline] ip6_output+0x340/0x550 net/ipv6/ip6_output.c:247 NF_HOOK+0x9e/0x380 include/linux/netfilter.h:318 mld_sendpack+0x8d4/0xe60 net/ipv6/mcast.c:1855 mld_send_cr net/ipv6/mcast.c:2154 [inline] mld_ifc_work+0x83e/0xd60 net/ipv6/mcast.c:2693 process_one_work kernel/workqueue.c:3257 [inline] process_scheduled_works+0xad1/0x1770 kernel/workqueue.c:3340 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3421 kthread+0x711/0x8a0 kernel/kthread.c:463 ret_from_fork+0x510/0xa50 arch/x86/kernel/process.c:158 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246 Fixes: c544193 ("GRE: Refactor GRE tunneling code.") Reported-by: syzbot+7c134e1c3aa3283790b9@syzkaller.appspotmail.com Closes: https://www.spinics.net/lists/netdev/msg1147302.html Signed-off-by: Eric Dumazet <edumazet@google.com> Link: https://patch.msgid.link/20260108190214.1667040-1-edumazet@google.com Signed-off-by: Jakub Kicinski <kuba@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org>
diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
@@ -860,10 +860,17 @@ static int ipgre_header(struct sk_buff *skb, struct net_device *dev,
 			const void *daddr, const void *saddr, unsigned int len)
 {
 	struct ip_tunnel *t = netdev_priv(dev);
-	struct iphdr *iph;
 	struct gre_base_hdr *greh;
+	struct iphdr *iph;
+	int needed;
+
+	needed = t->hlen + sizeof(*iph);
+	if (skb_headroom(skb) < needed &&
+	    pskb_expand_head(skb, HH_DATA_ALIGN(needed - skb_headroom(skb)),
+			     0, GFP_ATOMIC))
+		return -needed;
 
-	iph = skb_push(skb, t->hlen + sizeof(*iph));
+	iph = skb_push(skb, needed);
 	greh = (struct gre_base_hdr *)(iph+1);
 	greh->flags = gre_tnl_flags_to_gre_flags(t->parms.o_flags);
 	greh->protocol = htons(type);
