netdev - Re: [PATCH net] netlink: use jhash as hashfn for rhashtable

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Message-ID: <CAA93jw5bAqxbpxyOHEHd6XSEn4-HK0a9qw3Cb_Vecb8Nz=xbfg@mail.gmail.com>
Date:	Mon, 8 Dec 2014 09:20:06 -0800
From:	Dave Taht <dave.taht@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Thomas Graf <tgraf@...g.ch>
Subject: Re: [PATCH net] netlink: use jhash as hashfn for rhashtable

On Mon, Dec 8, 2014 at 8:56 AM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On Mo, 2014-12-08 at 17:30 +0100, Daniel Borkmann wrote:
>> For netlink, we shouldn't be using arch_fast_hash() as a hashing
>> discipline, but rather jhash() instead.

I am not particularly happy with the amount of entropy in

static inline u32 ipv6_addr_hash(const struct in6_addr *a)
{
#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && BITS_PER_LONG == 64
        const unsigned long *ul = (const unsigned long *)a;
        unsigned long x = ul[0] ^ ul[1];

        return (u32)(x ^ (x >> 32));
#else
        return (__force u32)(a->s6_addr32[0] ^ a->s6_addr32[1] ^
                             a->s6_addr32[2] ^ a->s6_addr32[3]);
#endif
}

is this worth improving somehow?

>> Since netlink sockets can be opened by any user, a local attacker
>> would be able to easily create collisions with the DPDK-derived
>> arch_fast_hash(), which trades off performance for security by
>> using crc32 CPU instructions on x86_64.
>>
>> While it might have a legimite use case in other places, it should
>> be avoided in netlink context, though. As rhashtable's API is very
>> flexible, we could later on still decide on other hashing disciplines,
>> if legitimate.
>>
>> Reference: http://thread.gmane.org/gmane.linux.kernel/1844123
>> Fixes: e341694e3eb5 ("netlink: Convert netlink_lookup() to use RCU protected hash table")
>> Cc: Herbert Xu <herbert@...dor.apana.org.au>
>> Cc: Thomas Graf <tgraf@...g.ch>
>> Cc: Hannes Frederic Sowa <hannes@...essinduktion.org>
>> Signed-off-by: Daniel Borkmann <dborkman@...hat.com>
>> ---
>>  net/netlink/af_netlink.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
>> index 0007b81..b6bf8e8 100644
>> --- a/net/netlink/af_netlink.c
>> +++ b/net/netlink/af_netlink.c
>> @@ -3130,7 +3130,7 @@ static int __init netlink_proto_init(void)
>>               .head_offset = offsetof(struct netlink_sock, node),
>>               .key_offset = offsetof(struct netlink_sock, portid),
>>               .key_len = sizeof(u32), /* portid */
>> -             .hashfn = arch_fast_hash,
>> +             .hashfn = jhash,
>>               .max_shift = 16, /* 64K */
>>               .grow_decision = rht_grow_above_75,
>>               .shrink_decision = rht_shrink_below_30,
>
> Acked-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
>
> In net-next, some time soon, we should try to let all function pointers
> to jhash() use one non-inline version. The other arch_fast_hash patch
> adds __jhash for x86-only, we can move it over to lib/.
>
> Thanks,
> Hannes
>
>
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-- 
Dave Täht

http://www.bufferbloat.net/projects/bloat/wiki/Upcoming_Talks
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