ipv6: ensure sane device mtu in tunnels

Another syzbot report [1] with no reproducer hints
at a bug in ip6_gre tunnel (dev:ip6gretap0)

Since ipv6 mcast code makes sure to read dev->mtu once
and applies a sanity check on it (see commit b9b312a7
"ipv6: mcast: better catch silly mtu values"), a remaining
possibility is that a layer is able to set dev->mtu to
an underflowed value (high order bit set).

This could happen indeed in ip6gre_tnl_link_config_route(),
ip6_tnl_link_config() and ipip6_tunnel_bind_dev()

Make sure to sanitize mtu value in a local variable before
it is written once on dev->mtu, as lockless readers could
catch wrong temporary value.

[1]
skbuff: skb_over_panic: text:ffff80000b7a2f38 len:40 put:40 head:ffff000149dcf200 data:ffff000149dcf2b0 tail:0xd8 end:0xc0 dev:ip6gretap0
------------[ cut here ]------------
kernel BUG at net/core/skbuff.c:120
Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
Modules linked in:
CPU: 1 PID: 10241 Comm: kworker/1:1 Not tainted 6.0.0-rc7-syzkaller-18095-gbbed346d5a96 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/30/2022
Workqueue: mld mld_ifc_work
pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : skb_panic+0x4c/0x50 net/core/skbuff.c:116
lr : skb_panic+0x4c/0x50 net/core/skbuff.c:116
sp : ffff800020dd3b60
x29: ffff800020dd3b70 x28: 0000000000000000 x27: ffff00010df2a800
x26: 00000000000000c0 x25: 00000000000000b0 x24: ffff000149dcf200
x23: 00000000000000c0 x22: 00000000000000d8 x21: ffff80000b7a2f38
x20: ffff00014c2f7800 x19: 0000000000000028 x18: 00000000000001a9
x17: 0000000000000000 x16: ffff80000db49158 x15: ffff000113bf1a80
x14: 0000000000000000 x13: 00000000ffffffff x12: ffff000113bf1a80
x11: ff808000081c0d5c x10: 0000000000000000 x9 : 73f125dc5c63ba00
x8 : 73f125dc5c63ba00 x7 : ffff800008161d1c x6 : 0000000000000000
x5 : 0000000000000080 x4 : 0000000000000001 x3 : 0000000000000000
x2 : ffff0001fefddcd0 x1 : 0000000100000000 x0 : 0000000000000089
Call trace:
skb_panic+0x4c/0x50 net/core/skbuff.c:116
skb_over_panic net/core/skbuff.c:125 [inline]
skb_put+0xd4/0xdc net/core/skbuff.c:2049
ip6_mc_hdr net/ipv6/mcast.c:1714 [inline]
mld_newpack+0x14c/0x270 net/ipv6/mcast.c:1765
add_grhead net/ipv6/mcast.c:1851 [inline]
add_grec+0xa20/0xae0 net/ipv6/mcast.c:1989
mld_send_cr+0x438/0x5a8 net/ipv6/mcast.c:2115
mld_ifc_work+0x38/0x290 net/ipv6/mcast.c:2653
process_one_work+0x2d8/0x504 kernel/workqueue.c:2289
worker_thread+0x340/0x610 kernel/workqueue.c:2436
kthread+0x12c/0x158 kernel/kthread.c:376
ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:860
Code: 91011400 aa0803e1 a90027ea 94373093 (d4210000)

Fixes: c12b395a ("gre: Support GRE over IPv6")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20221024020124.3756833-1-eric.dumazet@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/ip6_gre.c

@@ -1175,14 +1175,16 @@ static void ip6gre_tnl_link_config_route(struct ip6_tnl *t, int set_mtu,
 				dev->needed_headroom = dst_len;
 
 			if (set_mtu) {
-				dev->mtu = rt->dst.dev->mtu - t_hlen;
+				int mtu = rt->dst.dev->mtu - t_hlen;
+
 				if (!(t->parms.flags & IP6_TNL_F_IGN_ENCAP_LIMIT))
-					dev->mtu -= 8;
+					mtu -= 8;
 				if (dev->type == ARPHRD_ETHER)
-					dev->mtu -= ETH_HLEN;
+					mtu -= ETH_HLEN;
 
-				if (dev->mtu < IPV6_MIN_MTU)
-					dev->mtu = IPV6_MIN_MTU;
+				if (mtu < IPV6_MIN_MTU)
+					mtu = IPV6_MIN_MTU;
+				WRITE_ONCE(dev->mtu, mtu);
 			}
 		}
 		ip6_rt_put(rt);

net/ipv6/ip6_tunnel.c

@@ -1450,8 +1450,8 @@ static void ip6_tnl_link_config(struct ip6_tnl *t)
 	struct net_device *tdev = NULL;
 	struct __ip6_tnl_parm *p = &t->parms;
 	struct flowi6 *fl6 = &t->fl.u.ip6;
-	unsigned int mtu;
 	int t_hlen;
+	int mtu;
 
 	__dev_addr_set(dev, &p->laddr, sizeof(struct in6_addr));
 	memcpy(dev->broadcast, &p->raddr, sizeof(struct in6_addr));
@@ -1498,12 +1498,13 @@ static void ip6_tnl_link_config(struct ip6_tnl *t)
 			dev->hard_header_len = tdev->hard_header_len + t_hlen;
 			mtu = min_t(unsigned int, tdev->mtu, IP6_MAX_MTU);
 
-			dev->mtu = mtu - t_hlen;
+			mtu = mtu - t_hlen;
 			if (!(t->parms.flags & IP6_TNL_F_IGN_ENCAP_LIMIT))
-				dev->mtu -= 8;
+				mtu -= 8;
 
-			if (dev->mtu < IPV6_MIN_MTU)
-				dev->mtu = IPV6_MIN_MTU;
+			if (mtu < IPV6_MIN_MTU)
+				mtu = IPV6_MIN_MTU;
+			WRITE_ONCE(dev->mtu, mtu);
 		}
 	}
 }

net/ipv6/sit.c

@@ -1124,10 +1124,12 @@ static void ipip6_tunnel_bind_dev(struct net_device *dev)
 
 	if (tdev && !netif_is_l3_master(tdev)) {
 		int t_hlen = tunnel->hlen + sizeof(struct iphdr);
+		int mtu;
 
-		dev->mtu = tdev->mtu - t_hlen;
-		if (dev->mtu < IPV6_MIN_MTU)
-			dev->mtu = IPV6_MIN_MTU;
+		mtu = tdev->mtu - t_hlen;
+		if (mtu < IPV6_MIN_MTU)
+			mtu = IPV6_MIN_MTU;
+		WRITE_ONCE(dev->mtu, mtu);
 	}
 }