[PATCH] ip6_tunnel: Fix uninit-value in ip6_tnl_xmit

[Mazin Al Haddad <mazin@getstate.dev>]

When taking the branch with skb_realloc_headroom, pskb_expand_head is
called, as such, pointers referencing content within the new skb's header
are invalid. Currently, the assignment of hop_limit accesses the now
invalid pointer in the network header of this "new" skb. Fix this by
moving the logic to assign hop_limit earlier so that the assignment
references the original un-resized skb instead.

uninit-value in ip6table_mangle_hook+0x97d/0x9c0 net/ipv6/netfilter/ip6table_mangle.c:72
 ip6t_mangle_out net/ipv6/netfilter/ip6table_mangle.c:56 [inline]
 ip6table_mangle_hook+0x97d/0x9c0 net/ipv6/netfilter/ip6table_mangle.c:72
 nf_hook_entry_hookfn include/linux/netfilter.h:154 [inline]
 nf_hook_slow+0xf4/0x400 net/netfilter/core.c:626
 nf_hook include/linux/netfilter.h:269 [inline]
 __ip6_local_out+0x5ac/0x640 net/ipv6/output_core.c:143
 ip6_local_out+0x4c/0x210 net/ipv6/output_core.c:153
 ip6tunnel_xmit+0x129/0x460 include/net/ip6_tunnel.h:161
 ip6_tnl_xmit+0x341a/0x3860 net/ipv6/ip6_tunnel.c:1281

Uninit was stored to memory at:
 ip6_tnl_xmit+0x34f7/0x3860 net/ipv6/ip6_tunnel.c:1277
 __gre6_xmit+0x14b9/0x1550 net/ipv6/ip6_gre.c:815
 ip6gre_xmit_ipv4 net/ipv6/ip6_gre.c:839 [inline]
 ip6gre_tunnel_xmit+0x18f7/0x2030 net/ipv6/ip6_gre.c:922

Uninit was created at:
 slab_post_alloc_hook mm/slub.c:4091 [inline]
 slab_alloc_node mm/slub.c:4134 [inline]
 __do_kmalloc_node mm/slub.c:4263 [inline]
 __kmalloc_node_track_caller_noprof+0x6c7/0xf90 mm/slub.c:4283
 kmalloc_reserve+0x23e/0x4a0 net/core/skbuff.c:609
 pskb_expand_head+0x226/0x1a60 net/core/skbuff.c:2275
 skb_realloc_headroom+0x140/0x2b0 net/core/skbuff.c:2355
 ip6_tnl_xmit+0x2106/0x3860 net/ipv6/ip6_tunnel.c:1227
 __gre6_xmit+0x14b9/0x1550 net/ipv6/ip6_gre.c:815
 ip6gre_xmit_ipv4 net/ipv6/ip6_gre.c:839 [inline]
 ip6gre_tunnel_xmit+0x18f7/0x2030 net/ipv6/ip6_gre.c:922

Reported-by: syzbot+6023ea32e206eef7920a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6023ea32e206eef7920a
Signed-off-by: Mazin Al Haddad <mazin@getstate.dev>
---
 net/ipv6/ip6_tunnel.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index 48fd53b98972..62a51f03360d 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -1098,7 +1098,7 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 	unsigned int max_headroom = psh_hlen;
 	__be16 payload_protocol;
 	bool use_cache = false;
-	u8 hop_limit;
+	u8 hop_limit = 0;
 	int err = -1;
 
 	payload_protocol = skb_protocol(skb, true);
@@ -1215,6 +1215,15 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 
 	skb_scrub_packet(skb, !net_eq(t->net, dev_net(dev)));
 
+	if (hop_limit == 0) {
+		if (payload_protocol == htons(ETH_P_IP))
+			hop_limit = ip_hdr(skb)->ttl;
+		else if (payload_protocol == htons(ETH_P_IPV6))
+			hop_limit = ipv6_hdr(skb)->hop_limit;
+		else
+			hop_limit = ip6_dst_hoplimit(dst);
+	}
+
 	/*
 	 * Okay, now see if we can stuff it in the buffer as-is.
 	 */
@@ -1243,15 +1252,6 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 	}
 	skb_dst_set(skb, dst);
 
-	if (hop_limit == 0) {
-		if (payload_protocol == htons(ETH_P_IP))
-			hop_limit = ip_hdr(skb)->ttl;
-		else if (payload_protocol == htons(ETH_P_IPV6))
-			hop_limit = ipv6_hdr(skb)->hop_limit;
-		else
-			hop_limit = ip6_dst_hoplimit(dst);
-	}
-
 	/* Calculate max headroom for all the headers and adjust
 	 * needed_headroom if necessary.
 	 */
-- 
2.46.0