netdev - Re: [PATCH net v4 1/8] ipv4/fib_frontend: Rename ip_valid_fib_dump

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Date:   Fri, 14 Jun 2019 21:16:54 -0600
From:   David Ahern <dsahern@...il.com>
To:     Stefano Brivio <sbrivio@...hat.com>
Cc:     David Miller <davem@...emloft.net>,
        Martin KaFai Lau <kafai@...com>,
        Jianlin Shi <jishi@...hat.com>, Wei Wang <weiwan@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        netdev@...r.kernel.org
Subject: Re: [PATCH net v4 1/8] ipv4/fib_frontend: Rename
 ip_valid_fib_dump_req, provide non-strict version

On 6/14/19 9:13 PM, Stefano Brivio wrote:
> On Fri, 14 Jun 2019 20:54:49 -0600
> David Ahern <dsahern@...il.com> wrote:
> 
>> On 6/14/19 7:32 PM, Stefano Brivio wrote:
>>> ip_valid_fib_dump_req() does two things: performs strict checking on
>>> netlink attributes for dump requests, and sets a dump filter if netlink
>>> attributes require it.
>>>
>>> We might want to just set a filter, without performing strict validation.
>>>
>>> Rename it to ip_filter_fib_dump_req(), and add a 'strict' boolean
>>> argument that must be set if strict validation is requested.
>>>
>>> This patch doesn't introduce any functional changes.
>>>
>>> Signed-off-by: Stefano Brivio <sbrivio@...hat.com>
>>> ---
>>> v4: New patch
>>>   
>>
>> Can you explain why this patch is needed? The existing function requires
>> strict mode and is needed to enable any of the kernel side filtering
>> beyond the RTM_F_CLONED setting in rtm_flags.
> 
> It's mostly to have proper NLM_F_MATCH support. Let's pick an iproute2
> version without strict checking support (< 5.0), that sets NLM_F_MATCH
> though. Then we need this check:
> 
> 	if (nlh->nlmsg_len < nlmsg_msg_size(sizeof(*rtm)))

but that check existed long before any of the strict checking and kernel
side filtering was added.

> 
> and to set filter parameters not just based on flags (i.e. RTM_F_CLONED),
> but also on table, protocol, etc.

and to do that you *must* have strict checking. There is no way to trust
userspace without that strict flag set because iproute2 for the longest
time sent the wrong header for almost all dump requests.

> 
> For example one might want to: 'ip route list cache table main', and this
> is then taken into account in fn_trie_dump_leaf() and rt6_dump_route().
> 
> Reusing this function avoids a nice amount of duplicated code and allows
> to have an almost common path with strict checking.
>