Question

I’m developing an application for Android and I need to elevate the Dalvik VM to root uid, because in the alternative I need to write a lot of small applications that are started by the Java code using sh scripts (with superuser, they needs to be root) and I really don’t want to do that!

While I’m trying to use, right now, capsetp to set CAP_SETUID to the calling process, I don’t know if this will be widely supported by all Android kernels so I’m looking for possible alternatives.

My fallback solution would be to start a root process that will first use seteuid to switch to the right user, start Dalvik VM with the application (execl) and, using JNI, switch back to the root user …

Any alternative?

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EDIT

I’m trying this code, but it fails to apply new capabilities (capsetp).

I’ve partially compiled libcap on Android (cap_file stuff is missing, but I don’t need it), but this code doesn’t work on a standard distro (Ubuntu).

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>
#include <sys/types.h>
#ifndef __user
#define __user
#endif
#include <linux/capability.h>
#include <sys/capability.h>

int main(int argc, char** argv)
{
    cap_t caps;
    cap_value_t cap_list[1];

    pid_t process_id = atoi(argv[1]);

    caps = cap_get_pid(process_id);
    if (caps == NULL)
    {
        perror("Failed to get capabilities");
        exit(-1);
    }

    printf("%s\n", cap_to_text(caps, NULL));

    cap_list[0] = CAP_SETUID; 

    if (cap_set_flag(caps, CAP_EFFECTIVE, 1, cap_list, CAP_SET) == -1)
    {
        perror("Failed to add new capability");
        exit(-1);
    }

    if (capsetp(process_id, caps) == -1)     
    {
        perror("Failed to apply capabilities");
        exit(-1);
    }

    cap_free(&caps);

    return 0;
}

Answer 1

I don’t think there’s a way to elevate another process’s privileges directly. The canonical way to do things like this is to exec a setuid-root binary which then determines whether to grant your program privileges before re-execing your program. Think su or sudo. Of course your setuid-root program like that could communicate with the original root process to determine if it’s safe to grant root. Keep in mind that there could be lots of dangerous corner cases.