netdev - Re: [PATCH ipsec-next v7 07/16] xfrm: iptfs: add new iptfs xfrm mode impl

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Message-ID: <m2y15g75rl.fsf@ja-home.int.chopps.org>
Date: Thu, 01 Aug 2024 08:36:03 -0400
From: Christian Hopps <chopps@...pps.org>
To: Florian Westphal <fw@...len.de>
Cc: Christian Hopps <chopps@...pps.org>, devel@...ux-ipsec.org, Steffen
 Klassert <steffen.klassert@...unet.com>, netdev@...r.kernel.org, Christian
 Hopps <chopps@...n.net>
Subject: Re: [PATCH ipsec-next v7 07/16] xfrm: iptfs: add new iptfs xfrm
 mode impl


Florian Westphal <fw@...len.de> writes:

> Christian Hopps <chopps@...pps.org> wrote:
>> +static int __iptfs_init_state(struct xfrm_state *x,
>> +			      struct xfrm_iptfs_data *xtfs)
>> +{
>> +	/* Modify type (esp) adjustment values */
>> +
>> +	if (x->props.family == AF_INET)
>> +		x->props.header_len += sizeof(struct iphdr) + sizeof(struct ip_iptfs_hdr);
>> +	else if (x->props.family == AF_INET6)
>> +		x->props.header_len += sizeof(struct ipv6hdr) + sizeof(struct ip_iptfs_hdr);
>> +	x->props.enc_hdr_len = sizeof(struct ip_iptfs_hdr);
>> +
>> +	/* Always have a module reference if x->mode_data is set */
>> +	if (!try_module_get(x->mode_cbs->owner))
>> +		return -EINVAL;
>
> If the comment means that we already have a module owner ref taken
> before this try_module_get, then this should use __module_get and
> a mention where the first ref was taken.
>
> If not, then this needs an explanation as to what prevents another cpu to
> rmmod the owning module between the lookup in xfrm_init_state and the
> module reference in __iptfs_init_state.
>
> cpu0					cpu1
>  xfrm_init_state
>    -> xfrm_get_mode_cbs                 rmmod
>      -> __iptfs_init_state		  xfrm_iptfs_fini
>        <interrupt>                         xfrm_unregister_mode_cbs
>                                             release memory
>        <resume>
> 	try_module_get -> UaF

You are correct the code is not sufficiently protective.

I need to use rcu to keep `xfrm_mode_cbs_map[mode]` around long enough to do a `try_module_get(&xfrm_mode_cbs_map[mode]->owner)` and return from `xfrm_get_mode_cbs()` with that ref count held. The caller (xfrm_init_state()) will then need do a `module_put()` after it calls `mode_cbs->create_state()` to give the reference back.

Thanks,
Chris.