Android su研究

一段su的源码

/*
**
** Copyright 2008, The Android Open Source Project
**
** Licensed under the Apache License, Version 2.0 (the "License");
** you may not use this file except in compliance with the License.
** You may obtain a copy of the License at
**
**     http://www.apache.org/licenses/LICENSE-2.0
**
** Unless required by applicable law or agreed to in writing, software
** distributed under the License is distributed on an "AS IS" BASIS,
** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
** See the License for the specific language governing permissions and
** limitations under the License.
*/

#define LOG_TAG "su"

#include 
#include 
#include 
#include 
#include 
#include 

#include 
#include 

#include 

//#include 

/*
 * SU can be given a specific command to exec. UID _must_ be
 * specified for this (ie argc => 3).
 *
 * Usage:
 * su 1000
 * su 1000 ls -l
 */
int main(int argc, char **argv)
{
    struct passwd *pw;
    int uid, gid, myuid;

    if(argc < 2) {
        uid = gid = 0;
    } else {
        pw = getpwnam(argv[1]);

        if(pw == 0) {
            uid = gid = atoi(argv[1]);
        } else {
            uid = pw->pw_uid;
            gid = pw->pw_gid;
        }
    }

    /* Until we have something better, only root and the shell can use su. */
    /*
    myuid = getuid();
    if (myuid != AID_ROOT && myuid != AID_SHELL) {
        fprintf(stderr,"su: uid %d not allowed to su\n", myuid);
        return 1;
    }
     */

    if(setgid(gid) || setuid(uid)) {
        fprintf(stderr,"lilisu: permission denied\n : %s", strerror(errno));
        return 1;
    }

    /* User specified command for exec. */
    if (argc == 3 ) {
        if (execlp(argv[2], argv[2], NULL) < 0) {
            fprintf(stderr, "lilisu: exec failed for %s Error:%s\n", argv[2],
                    strerror(errno));
            return -errno;
        }
    } else if (argc > 3) {
        /* Copy the rest of the args from main. */
        char *exec_args[argc - 1];
        memset(exec_args, 0, sizeof(exec_args));
        memcpy(exec_args, &argv[2], sizeof(exec_args));
        if (execvp(argv[2], exec_args) < 0) {
            fprintf(stderr, "su: exec failed for %s Error:%s\n", argv[2],
                    strerror(errno));
            return -errno;
        }
    }

    /* Default exec shell. */
    execlp("/system/bin/sh", "sh", NULL);

    fprintf(stderr, "lilisu: exec failed\n");
    return 1;
}

默认至少 UID 为 root 或 shell 才可以提权，我们这里屏蔽掉相应代码

Android 的 su 必须放入 System 分区，建议 /System/xbin

root@lte26007:/ # mount
...
/dev/block/platform/comip-mmc.1/by-name/system /system ext4 rw,seclabel,relatime,data=ordered 0 0
/dev/block/platform/comip-mmc.1/by-name/cache /cache ext4 rw,seclabel,nosuid,nodev,noatime,data=ordered 0 0
/dev/block/platform/comip-mmc.1/by-name/userdata /data ext4 rw,seclabel,nosuid,nodev,noatime,noauto_da_alloc,data=ordered 0 0
/dev/block/vold/179:129 /mnt/media_rw/sdcard1 vfat rw,dirsync,nosuid,nodev,noexec,relatime,uid=1023,gid=1023,fmask=0007,dmask=0007,allow_utime=0020,codepage=437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro 0 0
/dev/fuse /storage/sdcard1 fuse rw,nosuid,nodev,relatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0

请看 /data 下 的 nosuid

要把 su 拷贝入 /system/xbin

一般需要先把此分区只读属性去掉 mount -o remount,rw /system 需要在 root 权限下执行

su 的权限应该是这样子的 -rwsr-sr-x root root su

其中第一个 s 表示可以通过设置 setuid (来提权), 第二个 s 表示可以 setgid, 第三个。。。是 t

su 已经放入 /system/xbin，并设置了正确的权限，还不能执行

4.4有些厂商增加了drop capbnd，甚至有些kernel还增加了no_new_privs，普通应用不能提权的
看看应用的pid，观察一下CapBnd:是否是全F，不是的话就没set*id 这个能力。
cat /proc/pid/status
!!! FUCK MIUI

http://bbs.pediy.com/showthread.php?p=1268886