Re: [PATCH net] route: Avoid crash from dereferencing NULL rt->from

[Eric Dumazet <eric.dumazet@gmail.com>]

On 04/14/2019 02:21 PM, Jonathan Lemon wrote:
> When __ip6_rt_update_pmtu() is called, rt->from is RCU dereferenced, but is
> never checked for null - rt6_flush_exceptions() may have removed the entry.
> 
> [ 1913.989004] RIP: 0010:ip6_rt_cache_alloc+0x13/0x170
> [ 1914.209410] Call Trace:
> [ 1914.214798]  <IRQ>
> [ 1914.219226]  __ip6_rt_update_pmtu+0xb0/0x190
> [ 1914.228649]  ip6_tnl_xmit+0x2c2/0x970 [ip6_tunnel]
> [ 1914.239223]  ? ip6_tnl_parse_tlv_enc_lim+0x32/0x1a0 [ip6_tunnel]
> [ 1914.252489]  ? __gre6_xmit+0x148/0x530 [ip6_gre]
> [ 1914.262678]  ip6gre_tunnel_xmit+0x17e/0x3c7 [ip6_gre]
> [ 1914.273831]  dev_hard_start_xmit+0x8d/0x1f0
> [ 1914.283061]  sch_direct_xmit+0xfa/0x230
> [ 1914.291521]  __qdisc_run+0x154/0x4b0
> [ 1914.299407]  net_tx_action+0x10e/0x1f0
> [ 1914.307678]  __do_softirq+0xca/0x297
> [ 1914.315567]  irq_exit+0x96/0xa0
> [ 1914.322494]  smp_apic_timer_interrupt+0x68/0x130
> [ 1914.332683]  apic_timer_interrupt+0xf/0x20
> [ 1914.341721]  </IRQ>
> 
> Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
> ---
>  net/ipv6/route.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index d555edaaff13..b9cbe6815f2c 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -2331,6 +2331,10 @@ static void __ip6_rt_update_pmtu(struct dst_entry *dst, const struct sock *sk,
>  
>  		rcu_read_lock();
>  		from = rcu_dereference(rt6->from);
> +		if (!from) {
> +			rcu_read_unlock();
> +			return;
> +		}
>  		nrt6 = ip6_rt_cache_alloc(from, daddr, saddr);
>  		if (nrt6) {
>  			rt6_do_update_pmtu(nrt6, mtu);
> 

Interesting...

Please take the time to track the bug origin for your patches, thanks !

Fixes: a68886a69180 ("net/ipv6: Make from in rt6_info rcu protected")
Reviewed-by: Eric Dumazet <edumazet@google.com>