Re: [PATCH net-next] ipv6: use prandom_u32() for ID generation

[Eric Dumazet]

On Sat, May 29, 2021 at 1:08 PM Willy Tarreau <w@xxxxxx> wrote:
>
> This is a complement to commit aa6dd211e4b1 ("inet: use bigger hash
> table for IP ID generation"), but focusing on some specific aspects
> of IPv6.
>
> Contary to IPv4, IPv6 only uses packet IDs with fragments, and with a
> minimum MTU of 1280, it's much less easy to force a remote peer to
> produce many fragments to explore its ID sequence. In addition packet
> IDs are 32-bit in IPv6, which further complicates their analysis. On
> the other hand, it is often easier to choose among plenty of possible
> source addresses and partially work around the bigger hash table the
> commit above permits, which leaves IPv6 partially exposed to some
> possibilities of remote analysis at the risk of weakening some
> protocols like DNS if some IDs can be predicted with a good enough
> probability.
>
> Given the wide range of permitted IDs, the risk of collision is extremely
> low so there's no need to rely on the positive increment algorithm that
> is shared with the IPv4 code via ip_idents_reserve(). We have a fast
> PRNG, so let's simply call prandom_u32() and be done with it.
>
> Performance measurements at 10 Gbps couldn't show any difference with
> the previous code, even when using a single core, because due to the
> large fragments, we're limited to only ~930 kpps at 10 Gbps and the cost
> of the random generation is completely offset by other operations and by
> the network transfer time. In addition, this change removes the need to
> update a shared entry in the idents table so it may even end up being
> slightly faster on large scale systems where this matters.
>
> The risk of at least one collision here is about 1/80 million among
> 10 IDs, 1/850k among 100 IDs, and still only 1/8.5k among 1000 IDs,
> which remains very low compared to IPv4 where all IDs are reused
> every 4 to 80ms on a 10 Gbps flow depending on packet sizes.
>
> Reported-by: Amit Klein <aksecurity@xxxxxxxxx>
> Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> Signed-off-by: Willy Tarreau <w@xxxxxx>

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>

> ---
>  net/ipv6/output_core.c | 28 +++++-----------------------
>  1 file changed, 5 insertions(+), 23 deletions(-)
>
> diff --git a/net/ipv6/output_core.c b/net/ipv6/output_core.c
> index af36acc1a644..2880dc7d9a49 100644
> --- a/net/ipv6/output_core.c
> +++ b/net/ipv6/output_core.c
> @@ -15,29 +15,11 @@ static u32 __ipv6_select_ident(struct net *net,
>                                const struct in6_addr *dst,
>                                const struct in6_addr *src)
>  {
> -       const struct {
> -               struct in6_addr dst;
> -               struct in6_addr src;
> -       } __aligned(SIPHASH_ALIGNMENT) combined = {
> -               .dst = *dst,
> -               .src = *src,
> -       };
> -       u32 hash, id;
> -
> -       /* Note the following code is not safe, but this is okay. */
> -       if (unlikely(siphash_key_is_zero(&net->ipv4.ip_id_key)))
> -               get_random_bytes(&net->ipv4.ip_id_key,
> -                                sizeof(net->ipv4.ip_id_key));
> -
> -       hash = siphash(&combined, sizeof(combined), &net->ipv4.ip_id_key);
> -
> -       /* Treat id of 0 as unset and if we get 0 back from ip_idents_reserve,
> -        * set the hight order instead thus minimizing possible future
> -        * collisions.
> -        */
> -       id = ip_idents_reserve(hash, 1);
> -       if (unlikely(!id))
> -               id = 1 << 31;
> +       u32 id;
> +
> +       do {
> +               id = prandom_u32();
> +       } while (!id);
>
>         return id;
>  }
> --
> 2.17.5
>