netdev - Re: [RFC net 1/1] net: sched: act: fix rcu race in dump

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 10 Oct 2017 07:12:25 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Alexander Aring <aring@...atatu.com>
Cc:     jhs@...atatu.com, xiyou.wangcong@...il.com, jiri@...nulli.us,
        netdev@...r.kernel.org, kurup.manish@...il.com, bjb@...atatu.com
Subject: Re: [RFC net 1/1] net: sched: act: fix rcu race in dump

On Tue, 2017-10-10 at 08:32 -0400, Alexander Aring wrote:
> This patch fixes an issue with kfree_rcu which is not protected by RTNL
> lock. It could be that the current assigned rcu pointer will be freed by
> kfree_rcu while dump callback is running.
> 
> To prevent this, we call rcu_synchronize at first. Then we are sure all
> latest rcu functions e.g. rcu_assign_pointer and kfree_rcu in init are
> done. After rcu_synchronize we dereference under RTNL lock which is also
> held in init function, which means no other rcu_assign_pointer or
> kfree_rcu will occur.
> 
> To call rcu_synchronize will also prevent weird behaviours by doing over
> netlink:
> 
>  - set params A
>  - set params B
>  - dump params
>   \--> will dump params A
> 
> This could be a unlikely case that the last rcu_assign_pointer was not
> happened before dump callback.
> 
> Signed-off-by: Alexander Aring <aring@...atatu.com>
> ---
>  net/sched/act_skbmod.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sched/act_skbmod.c b/net/sched/act_skbmod.c
> index b642ad3d39dd..231e07bca384 100644
> --- a/net/sched/act_skbmod.c
> +++ b/net/sched/act_skbmod.c
> @@ -198,7 +198,7 @@ static int tcf_skbmod_dump(struct sk_buff *skb, struct tc_action *a,
>  {
>  	struct tcf_skbmod *d = to_skbmod(a);
>  	unsigned char *b = skb_tail_pointer(skb);
> -	struct tcf_skbmod_params  *p = rtnl_dereference(d->skbmod_p);
> +	struct tcf_skbmod_params  *p;
>  	struct tc_skbmod opt = {
>  		.index   = d->tcf_index,
>  		.refcnt  = d->tcf_refcnt - ref,
> @@ -207,6 +207,11 @@ static int tcf_skbmod_dump(struct sk_buff *skb, struct tc_action *a,
>  	};
>  	struct tcf_t t;
>  
> +	/* wait until last rcu_assign_pointer/kfree_rcu is done */
> +	rcu_synchronize();
> +	/* RTNL lock prevents another rcu_assign_pointer/kfree_rcu call */
> +	p = rtnl_dereference(d->skbmod_p);
> +
>  	opt.flags  = p->flags;
>  	if (nla_put(skb, TCA_SKBMOD_PARMS, sizeof(opt), &opt))
>  		goto nla_put_failure;

Sorry but no. This is plainly wrong.

We need to fix this without adding a _very_ expensive rcu_synchronize()
on a path which does not need such thing.

I am confused by this patch, please tell us more what the problem is.

I suspect rcu_read_lock() is what you need, but isn't a writer supposed
to hold RTNL in net/sched/* ???