| Message ID | m3prmtmet6.wl%vmayatsk@redhat.com |
|---|---|
| State | Superseded, archived |
| Delegated to: | David Miller |
| Headers | show |
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c index 57e26fa..e9c8dea 100644 --- a/net/ipv4/udp.c +++ b/net/ipv4/udp.c @@ -167,6 +167,9 @@ int udp_lib_get_port(struct sock *sk, unsigned short snum, best_size_so_far = UINT_MAX; best = rover = net_random() % remaining + low; + if (! __udp_lib_lport_inuse(net, rover, udptable)) + goto gotit; + /* 1st pass: look for empty (or shortest) hash chain */ for (i = 0; i < UDP_HTABLE_SIZE; i++) { int size = 0;
UDP port randomizator doesn't always use port number, provided by net_random(), even when port is free. It tries to search empty or shortest chain and use the first available port in it. In practice this weakens the quality of distribution: there are ports in partially filled chains, which will never be allocated, but their entropy will go to the port from next empty or shorter chain. This simple test outputs entropy for random ports allocation (output may be used then in Octave): #include <stdio.h> #include <errno.h> #include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #define PORTS 65536 int main() { int s, err, i, j; char buf[256]; struct sockaddr_in sa = { 0 }, sa1; int optval = 1, port; unsigned int p[PORTS] = { 0 }; sa.sin_addr.s_addr = htonl(INADDR_ANY); sa.sin_family = AF_INET; sa.sin_port = 0; for (i = 0; i < PORTS * 100; ++i) { s = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP); err = bind(s, (const struct sockaddr*)&sa, sizeof(sa)); getsockname(s, (struct sockaddr*)&sa1, &j); port = ntohs(sa1.sin_port); p[port]++; close(s); } printf("x = 32767:1:65535;\ny = ["); for (i = 32767; i < PORTS; i++) printf("%d%s", p[i], (i + 1 < PORTS ? "; " : "")); printf("];\nplot(x,y,'.');pause;"); } It outputs ... 221 237 0 437 242 264 ... in period. Distances between zero time allocated ports also repeat in period: ... 42, 84, 42, 84, ... or ... 5, 4, 0, 1, 3, 0, 15, 5, 9, 16, 7, 13, 0, 35, 0 ... (depends on kernel version). It's possible to utilize this weakness for compromising systems (like dns servers). The simplest way to fix this issue is to use port number, provided by net_random() (if it's not in use). Signed-off-by: Vitaly Mayatskikh <v.mayatskih@gmail.com> -- wbr, Vitaly -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html