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Subject[PATCH 4.20 076/111] ipv6: make icmp6_send() robust against null skb->dev
Date
4.20-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@google.com>

commit 8d933670452107e41165bea70a30dffbd281bef1 upstream.

syzbot was able to crash one host with the following stack trace :

kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 8625 Comm: syz-executor4 Not tainted 4.20.0+ #8
RIP: 0010:dev_net include/linux/netdevice.h:2169 [inline]
RIP: 0010:icmp6_send+0x116/0x2d30 net/ipv6/icmp.c:426
icmpv6_send
smack_socket_sock_rcv_skb
security_sock_rcv_skb
sk_filter_trim_cap
__sk_receive_skb
dccp_v6_do_rcv
release_sock

This is because a RX packet found socket owned by user and
was stored into socket backlog. Before leaving RCU protected section,
skb->dev was cleared in __sk_receive_skb(). When socket backlog
was finally handled at release_sock() time, skb was fed to
smack_socket_sock_rcv_skb() then icmp6_send()

We could fix the bug in smack_socket_sock_rcv_skb(), or simply
make icmp6_send() more robust against such possibility.

In the future we might provide to icmp6_send() the net pointer
instead of infering it.

Fixes: d66a8acbda92 ("Smack: Inform peer that IPv6 traffic has been blocked")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Piotr Sawicki <p.sawicki2@partner.samsung.com>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
net/ipv6/icmp.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

--- a/net/ipv6/icmp.c
+++ b/net/ipv6/icmp.c
@@ -421,10 +421,10 @@ static int icmp6_iif(const struct sk_buf
static void icmp6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info,
const struct in6_addr *force_saddr)
{
- struct net *net = dev_net(skb->dev);
struct inet6_dev *idev = NULL;
struct ipv6hdr *hdr = ipv6_hdr(skb);
struct sock *sk;
+ struct net *net;
struct ipv6_pinfo *np;
const struct in6_addr *saddr = NULL;
struct dst_entry *dst;
@@ -435,12 +435,16 @@ static void icmp6_send(struct sk_buff *s
int iif = 0;
int addr_type = 0;
int len;
- u32 mark = IP6_REPLY_MARK(net, skb->mark);
+ u32 mark;

if ((u8 *)hdr < skb->head ||
(skb_network_header(skb) + sizeof(*hdr)) > skb_tail_pointer(skb))
return;

+ if (!skb->dev)
+ return;
+ net = dev_net(skb->dev);
+ mark = IP6_REPLY_MARK(net, skb->mark);
/*
* Make sure we respect the rules
* i.e. RFC 1885 2.4(e)

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