corrected a typo in one id in the mailing list.
Vimal
On Thu, Feb 3, 2022 at 10:44 AM Vimal Agrawal <avimalin@xxxxxxxxx> wrote:
>
> commit a504b703bb1da526a01593da0e4be2af9d9f5fa8 ("netfilter: nat:
> limit port clash resolution attempts")
>
> In case almost or all available ports are taken, clash resolution can
> take a very long time, resulting in soft lockup.
>
> This can happen when many to-be-natted hosts connect to same
> destination:port (e.g. a proxy) and all connections pass the same SNAT.
>
> Pick a random offset in the acceptable range, then try ever smaller
> number of adjacent port numbers, until either the limit is reached or a
> useable port was found. This results in at most 248 attempts
> (128 + 64 + 32 + 16 + 8, i.e. 4 restarts with new search offset)
> instead of 64000+,
>
> Signed-off-by: Vimal Agrawal <vimal.agrawal@xxxxxxxxxx>
> ---
> net/netfilter/nf_nat_proto_common.c | 25 +++++++++++++++++++++----
> 1 file changed, 21 insertions(+), 4 deletions(-)
>
> diff --git a/net/netfilter/nf_nat_proto_common.c b/net/netfilter/nf_nat_proto_common.c
> index 7d7466dbf663..d0d9747f68a9 100644
> --- a/net/netfilter/nf_nat_proto_common.c
> +++ b/net/netfilter/nf_nat_proto_common.c
> @@ -41,9 +41,10 @@ void nf_nat_l4proto_unique_tuple(const struct nf_nat_l3proto *l3proto,
> const struct nf_conn *ct,
> u16 *rover)
> {
> - unsigned int range_size, min, max, i;
> + unsigned int range_size, min, max, i, attempts;
> __be16 *portptr;
> - u_int16_t off;
> + u16 off;
> + static const unsigned int max_attempts = 128;
>
> if (maniptype == NF_NAT_MANIP_SRC)
> portptr = &tuple->src.u.all;
> @@ -87,14 +88,30 @@ void nf_nat_l4proto_unique_tuple(const struct nf_nat_l3proto *l3proto,
> off = *rover;
> }
>
> - for (i = 0; ; ++off) {
> + attempts = range_size;
> + if (attempts > max_attempts)
> + attempts = max_attempts;
> +
> + /* We are in softirq; doing a search of the entire range risks
> + * soft lockup when all tuples are already used.
> + *
> + * If we can't find any free port from first offset, pick a new
> + * one and try again, with ever smaller search window.
> + */
> +another_round:
> + for (i = 0; i < attempts; i++, off++) {
> *portptr = htons(min + off % range_size);
> - if (++i != range_size && nf_nat_used_tuple(tuple, ct))
> + if (nf_nat_used_tuple(tuple, ct))
> continue;
> if (!(range->flags & NF_NAT_RANGE_PROTO_RANDOM_ALL))
> *rover = off;
> return;
> }
> + if (attempts >= range_size || attempts < 16)
> + return;
> + attempts /= 2;
> + off = prandom_u32();
> + goto another_round;
> }
> EXPORT_SYMBOL_GPL(nf_nat_l4proto_unique_tuple);
>
> --
> 2.32.0
>
