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

From: Hannes Frederic Sowa <[email-masked]>
Date: Mon, 08 Dec 2014 17:56:20 +0100

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.
> 
> 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 <[email-masked]>
> Cc: Thomas Graf <[email-masked]>
> Cc: Hannes Frederic Sowa <[email-masked]>
> Signed-off-by: Daniel Borkmann <[email-masked]>
> ---
>  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 <[email-masked]>

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