Here's a C program that should do it. Note a few known issues:
- Should probably use memcpy instead of strict aliasing violations
- Uses its own environment variables instead of the old tracee's environment variables
- If the tracee doesn't make any syscalls, this will never manage to do anything
- Doesn't check when the tracee gets stopped to make sure it's really a syscall stop and not a signal stop or something
- Should set IP back in syscall-exit-stop instead of syscall-enter-stop
- Doesn't do any sanity checking of the execve arguments (doing this would be a good opportunity for execveat)
- Completely unportable (hardcodes
CONFIG_ARM_THUMB among lots of other things)
- Leaves the process in a state where it'll probably crash if any of the syscalls don't work right
Compile it with -fno-strict-aliasing, then run it as ./a.out 1 /lib/systemd/systemd systemd.
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ptrace.h>
#include <linux/ptrace.h>
#include <sys/wait.h>
#include <sys/user.h>
#include <sys/syscall.h>
#include <sys/mman.h>
#define CONFIG_ARM_THUMB
#ifdef CONFIG_ARM_THUMB
#define thumb_mode(regs) \
(((regs)->ARM_cpsr & PSR_T_BIT))
#else
#define thumb_mode(regs) (0)
#endif
extern char **environ;
static pid_t pid;
/* The length of a string, plus the null terminator, rounded up to the nearest sizeof(long). */
size_t str_size(char *str) {
size_t len = strlen(str);
return len + sizeof(long) - len % sizeof(long);
}
void must_poke(long addr, long data) {
if(ptrace(PTRACE_POKEDATA, pid, (void*)addr, (void*)data)) {
perror("ptrace(PTRACE_POKEDATA, ...)");
exit(1);
}
}
void must_poke_multi(long addr, long* data, size_t len) {
size_t i;
for(i = 0; i < len; ++i) {
must_poke(addr + i * sizeof(long), data[i]);
}
}
long must_poke_string(long addr, char* str) {
size_t len = str_size(str);
size_t longs_len = len / sizeof(long);
char *more_nulls_str = malloc(len);
memset(more_nulls_str + len - sizeof(long), 0, sizeof(long)); /* initialize the bit we might not copy over */
strcpy(more_nulls_str, str);
must_poke_multi(addr, (long*)more_nulls_str, longs_len);
free(more_nulls_str);
return addr + len;
}
int main(int argc, char** argv) {
struct user_regs regs;
int i, envc;
unsigned long mmap_base;
size_t mmap_string_offset, mmap_argv_offset, mmap_envp_offset;
size_t mmap_len = 2 * sizeof(char*); /* for the NULLs at the end of argv and envp */
if(argc < 3) {
fprintf(stderr, "Usage: %s <pid> <executable image> <args...>\n", argv[0]);
return 1;
}
pid = strtol(argv[1], NULL, 10);
/* for the image name */
mmap_len += str_size(argv[2]);
for(i = 3; i < argc; ++i) {
/* for the pointer in argv plus the string itself */
mmap_len += sizeof(char*) + str_size(argv[i]);
}
for(i = 0; environ[i]; ++i) {
/* for the pointer in envp plus the string itself */
mmap_len += sizeof(char*) + str_size(environ[i]);
}
envc = i;
if(ptrace(PTRACE_ATTACH, pid, 0, 0)) {
perror("ptrace(PTRACE_ATTACH, ...)");
return 1;
}
if(waitid(P_PID, pid, NULL, WSTOPPED)) {
perror("waitid");
return 1;
}
/* Stop at whatever syscall happens to be next */
if(ptrace(PTRACE_SYSCALL, pid, 0, 0)) {
perror("ptrace(PTRACE_SYSCALL, ...)");
return 1;
}
printf("Waiting for the target process to make a syscall...\n");
if(waitid(P_PID, pid, NULL, WSTOPPED)) {
perror("waitid");
return 1;
}
printf("Target made a syscall. Proceeding with injection.\n");
if(ptrace(PTRACE_GETREGS, pid, 0, ®s)) {
perror("ptrace(PTRACE_GETREGS, ...)");
return 1;
}
/* End up back on the syscall instruction so we can use it again */
regs.ARM_pc -= (thumb_mode(®s) ? 2 : 4);
/* mmap some space for the exec parameters */
regs.ARM_r0 = (long)0;
regs.ARM_r1 = (long)mmap_len;
regs.ARM_r2 = (long)(PROT_READ|PROT_WRITE);
regs.ARM_r3 = (long)(MAP_PRIVATE|MAP_ANONYMOUS);
regs.ARM_r4 = (long)-1;
regs.ARM_r5 = (long)0;
if(ptrace(PTRACE_SETREGS, pid, 0, ®s)) {
perror("ptrace(PTRACE_SETREGS, ...)");
return 1;
}
if(ptrace(PTRACE_SET_SYSCALL, pid, 0, SYS_mmap2)) {
perror("ptrace(PTRACE_SET_SYSCALL, ...)");
return 1;
}
/* jump to the end of the syscall */
if(ptrace(PTRACE_SYSCALL, pid, 0, 0)) {
perror("ptrace(PTRACE_SYSCALL, ...)");
return 1;
}
if(waitid(P_PID, pid, NULL, WSTOPPED)) {
perror("waitid");
return 1;
}
/* make sure it worked and get the memory address */
if(ptrace(PTRACE_GETREGS, pid, 0, ®s)) {
perror("ptrace(PTRACE_GETREGS, ...)");
return 1;
}
if(regs.ARM_r0 > -4096UL) {
errno = -regs.ARM_r0;
perror("traced process: mmap");
return 1;
}
mmap_base = regs.ARM_r0;
/* set up the execve args in memory */
mmap_argv_offset = must_poke_string(mmap_base, argv[2]);
mmap_string_offset = mmap_argv_offset + (argc - 2) * sizeof(char*); /* don't forget the null pointer */
for(i = 0; i < argc - 3; ++i) {
must_poke(mmap_argv_offset + i * sizeof(char*), mmap_string_offset);
mmap_string_offset = must_poke_string(mmap_string_offset, argv[i + 3]);
}
must_poke(mmap_argv_offset + (argc - 3) * sizeof(char*), 0);
mmap_envp_offset = mmap_string_offset;
mmap_string_offset = mmap_envp_offset + (envc + 1) * sizeof(char*); /* don't forget the null pointer */
for(i = 0; i < envc; ++i) {
must_poke(mmap_envp_offset + i * sizeof(char*), mmap_string_offset);
mmap_string_offset = must_poke_string(mmap_string_offset, environ[i]);
}
must_poke(mmap_envp_offset + envc * sizeof(char*), 0);
/* jump to the start of the next syscall (same PC since we reset it) */
if(ptrace(PTRACE_SYSCALL, pid, 0, 0)) {
perror("ptrace(PTRACE_SYSCALL, ...)");
return 1;
}
if(waitid(P_PID, pid, NULL, WSTOPPED)) {
perror("waitid");
return 1;
}
if(ptrace(PTRACE_GETREGS, pid, 0, ®s)) {
perror("ptrace(PTRACE_GETREGS, ...)");
return 1;
}
/* call execve */
regs.ARM_r0 = (long)mmap_base;
regs.ARM_r1 = (long)mmap_argv_offset;
regs.ARM_r2 = (long)mmap_envp_offset;
if(ptrace(PTRACE_SETREGS, pid, 0, ®s)) {
perror("ptrace(PTRACE_SETREGS, ...)");
return 1;
}
if(ptrace(PTRACE_SET_SYSCALL, pid, 0, SYS_execve)) {
perror("ptrace(PTRACE_SET_SYSCALL, ...)");
return 1;
}
/* and done. */
if(ptrace(PTRACE_DETACH, pid, 0, 0)) {
perror("ptrace(PTRACE_DETACH, ...)");
return 1;
}
return 0;
}
I developed and tested this via qemu-system-arm with the 3.2.0-4 kernel and wheezy userland from https://people.debian.org/~aurel32/qemu/armel/
