[PATCH net] ipv6/gro: fix an out of bounds memory bug in ipv6_gro_receive()

[Ziyang Xuan]

IPv6 packets without NEXTHDR_NONE extension header can make continuous
__skb_pull() until pskb_may_pull() failed in ipv6_gso_pull_exthdrs().
That results in a big value of skb_gro_offset(), and after __skb_push()
in ipv6_gro_receive(), skb->data will less than skb->head, an out of
bounds memory bug occurs. That will trigger the problem as following:

==================================================================
BUG: KASAN: use-after-free in eth_type_trans+0x100/0x260
...
Call trace:
 dump_backtrace+0xd8/0x130
 show_stack+0x1c/0x50
 dump_stack_lvl+0x64/0x7c
 print_address_description.constprop.0+0xbc/0x2e8
 print_report+0x100/0x1e4
 kasan_report+0x80/0x120
 __asan_load8+0x78/0xa0
 eth_type_trans+0x100/0x260
 napi_gro_frags+0x164/0x550
 tun_get_user+0xda4/0x1270
 tun_chr_write_iter+0x74/0x130
 do_iter_readv_writev+0x130/0x1ec
 do_iter_write+0xbc/0x1e0
 vfs_writev+0x13c/0x26c

Add comparison between skb->data - skb_gro_offset() and skb->head
and exception handler before __skb_push() to fix the bug.

Fixes: 86911732d399 ("gro: Avoid copying headers of unmerged packets")
Signed-off-by: Ziyang Xuan <william.xuanziyang@xxxxxxxxxx>
---
 net/ipv6/ip6_offload.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv6/ip6_offload.c b/net/ipv6/ip6_offload.c
index 3ee345672849..6659ccf25387 100644
--- a/net/ipv6/ip6_offload.c
+++ b/net/ipv6/ip6_offload.c
@@ -237,6 +237,10 @@ INDIRECT_CALLABLE_SCOPE struct sk_buff *ipv6_gro_receive(struct list_head *head,
 		proto = ipv6_gso_pull_exthdrs(skb, proto);
 		skb_gro_pull(skb, -skb_transport_offset(skb));
 		skb_reset_transport_header(skb);
+		if (unlikely(skb_headroom(skb) < skb_gro_offset(skb))) {
+			kfree_skb(skb);
+			return ERR_PTR(-EINPROGRESS);
+		}
 		__skb_push(skb, skb_gro_offset(skb));
 
 		ops = rcu_dereference(inet6_offloads[proto]);
-- 
2.25.1