Messages in this thread | | | From | Eric Dumazet <> | Date | Mon, 31 May 2021 21:27:13 +0200 | Subject | Re: [PATCH net-next] ipv6: use prandom_u32() for ID generation |
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On Sat, May 29, 2021 at 1:08 PM Willy Tarreau <w@1wt.eu> 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@gmail.com> > Cc: Eric Dumazet <edumazet@google.com> > Signed-off-by: Willy Tarreau <w@1wt.eu>
Reviewed-by: Eric Dumazet <edumazet@google.com>
> --- > 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 >
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