From 9088c5609584684149f3fb5b065aa7f18dcb03ff Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 8 Oct 2008 11:44:17 -0700 Subject: [PATCH] udp: Improve port randomization Current UDP port allocation is suboptimal. We select the shortest chain to chose a port (out of 512) that will hash in this shortest chain. First, it can lead to give not so ramdom ports and ease give attackers more opportunities to break the system. Second, it can consume a lot of CPU to scan all table in order to find the shortest chain. Third, in some pathological cases we can fail to find a free port even if they are plenty of them. This patch zap the search for a short chain and only use one random seed. Problem of getting long chains should be addressed in another way, since we can obtain long chains with non random ports. Based on a report and patch from Vitaly Mayatskikh Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- net/ipv4/udp.c | 56 +++++++++++--------------------------------------- 1 file changed, 12 insertions(+), 44 deletions(-) diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c index 85f8e8e10b1b..67d8430b4a2a 100644 --- a/net/ipv4/udp.c +++ b/net/ipv4/udp.c @@ -155,55 +155,23 @@ int udp_lib_get_port(struct sock *sk, unsigned short snum, write_lock_bh(&udp_hash_lock); if (!snum) { - int i, low, high, remaining; - unsigned rover, best, best_size_so_far; + int low, high, remaining; + unsigned rand; + unsigned short first; inet_get_local_port_range(&low, &high); remaining = (high - low) + 1; - best_size_so_far = UINT_MAX; - best = rover = net_random() % remaining + low; - - /* 1st pass: look for empty (or shortest) hash chain */ - for (i = 0; i < UDP_HTABLE_SIZE; i++) { - int size = 0; - - head = &udptable[udp_hashfn(net, rover)]; - if (hlist_empty(head)) - goto gotit; - - sk_for_each(sk2, node, head) { - if (++size >= best_size_so_far) - goto next; - } - best_size_so_far = size; - best = rover; - next: - /* fold back if end of range */ - if (++rover > high) - rover = low + ((rover - low) - & (UDP_HTABLE_SIZE - 1)); - - - } - - /* 2nd pass: find hole in shortest hash chain */ - rover = best; - for (i = 0; i < (1 << 16) / UDP_HTABLE_SIZE; i++) { - if (! __udp_lib_lport_inuse(net, rover, udptable)) - goto gotit; - rover += UDP_HTABLE_SIZE; - if (rover > high) - rover = low + ((rover - low) - & (UDP_HTABLE_SIZE - 1)); + rand = net_random(); + snum = first = rand % remaining + low; + rand |= 1; + while (__udp_lib_lport_inuse(net, snum, udptable)) { + do { + snum = snum + rand; + } while (snum < low || snum > high); + if (snum == first) + goto fail; } - - - /* All ports in use! */ - goto fail; - -gotit: - snum = rover; } else { head = &udptable[udp_hashfn(net, snum)]; -- GitLab