我正在尝试将bash安装为在嵌入式设备(Synology DS212 NAS)上运行的ARM
Linux上的默认shell.但是有些事情确实存在问题,我无法弄清楚它是什么.
症状：
1)Root有/ bin / bash作为默认shell,可以通过SSH正常登录：
$grep root /etc/passwd root:x:0:0:root:/root:/bin/bash $ssh root@NAS root@NAS's password: Last login: Sun Dec 16 14:06:56 2012 from desktop #
2)joeuser将/ bin / bash作为默认shell,并在尝试通过SSH登录时收到“权限被拒绝”：
$grep joeuser /etc/passwd joeuser:x:1029:100:Joe User:/home/joeuser:/bin/bash $ssh joeuser@localhost joeuser@NAS's password: Last login: Sun Dec 16 14:07:22 2012 from desktop Permission denied,please try again. Connection to localhost closed.
3)将joeuser的shell更改回/ bin / sh：
$grep joeuser /etc/passwd joeuser:x:1029:100:Joe User:/home/joeuser:/bin/sh $ssh joeuser@localhost Last login: Sun Dec 16 15:50:52 2012 from localhost $
为了让事情变得更奇怪,我可以使用串行控制台(！)使用/ bin / bash以joeuser身份登录.另外一个su – joeuser作为root工作正常,所以bash二进制文件本身工作正常.
在绝望的行为中,我在/ etc / passwd上将joeuser的uid更改为0,但也没有工作,因此它似乎与某些权限无关.
似乎bash正在做一些sshd不喜欢的额外检查,并阻止非root用户的连接.也许某种健全性检查 – 或终端仿真 – 触发SIGCHLD,但仅在通过ssh调用时.
我已经浏览了sshd_config上的每个项目,并且还将SSHD置于调试模式,但没有发现任何奇怪的内容.这是我的/ etc / ssh / sshd_config：
LogLevel DEBUG LoginGraceTime 2m PermitRootLogin yes RSAAuthentication yes PubkeyAuthentication yes AuthorizedKeysFile %h/.ssh/authorized_keys ChallengeResponseAuthentication no UsePAM yes AllowTcpForwarding no ChrootDirectory none Subsystem sftp internal-sftp -f DAEMON -u 000
这是来自/usr/syno / sbin / sshd -d的输出,显示了joeuser尝试登录失败的尝试,使用/ bin / bash作为shell：
debug1: Config token is loglevel debug1: Config token is logingracetime debug1: Config token is permitrootlogin debug1: Config token is rsaauthentication debug1: Config token is pubkeyauthentication debug1: Config token is authorizedkeysfile debug1: Config token is challengeresponseauthentication debug1: Config token is usepam debug1: Config token is allowtcpforwarding debug1: Config token is chrootdirectory debug1: Config token is subsystem debug1: HPN Buffer Size: 87380 debug1: sshd version OpenSSH_5.8p1-hpn13v11 debug1: read PEM private key done: type RSA debug1: private host key: #0 type 1 RSA debug1: read PEM private key done: type DSA debug1: private host key: #1 type 2 DSA debug1: read PEM private key done: type ECDSA debug1: private host key: #2 type 3 ECDSA debug1: rexec_argv[0]='/usr/syno/sbin/sshd' debug1: rexec_argv[1]='-d' Set /proc/self/oom_adj from 0 to -17 debug1: Bind to port 22 on ::. debug1: Server TCP RWIN socket size: 87380 debug1: HPN Buffer Size: 87380 Server listening on :: port 22. debug1: Bind to port 22 on 0.0.0.0. debug1: Server TCP RWIN socket size: 87380 debug1: HPN Buffer Size: 87380 Server listening on 0.0.0.0 port 22. debug1: Server will not fork when running in debugging mode. debug1: rexec start in 6 out 6 newsock 6 pipe -1 sock 9 debug1: inetd sockets after dupping: 4,4 Connection from 127.0.0.1 port 52212 debug1: HPN Disabled: 0,HPN Buffer Size: 87380 debug1: Client protocol version 2.0; client software version OpenSSH_5.8p1-hpn13v11 SSH: Server;Ltype: Version;Remote: 127.0.0.1-52212;Protocol: 2.0;Client: OpenSSH_5.8p1-hpn13v11 debug1: match: OpenSSH_5.8p1-hpn13v11 pat OpenSSH* debug1: Enabling compatibility mode for protocol 2.0 debug1: Local version string SSH-2.0-OpenSSH_5.8p1-hpn13v11 debug1: permanently_set_uid: 1024/100 debug1: MYFLAG IS 1 debug1: list_hostkey_types: ssh-rsa,ssh-dss,ecdsa-sha2-nistp256 debug1: SSH2_MSG_KEXINIT sent debug1: SSH2_MSG_KEXINIT received debug1: AUTH STATE IS 0 debug1: REQUESTED ENC.NAME is 'aes128-ctr' debug1: kex: client->server aes128-ctr hmac-md5 none SSH: Server;Ltype: Kex;Remote: 127.0.0.1-52212;Enc: aes128-ctr;MAC: hmac-md5;Comp: none debug1: REQUESTED ENC.NAME is 'aes128-ctr' debug1: kex: server->client aes128-ctr hmac-md5 none debug1: expecting SSH2_MSG_KEX_ECDH_INIT debug1: SSH2_MSG_NEWKEYS sent debug1: expecting SSH2_MSG_NEWKEYS debug1: SSH2_MSG_NEWKEYS received debug1: KEX done debug1: userauth-request for user joeuser service ssh-connection method none SSH: Server;Ltype: Authname;Remote: 127.0.0.1-52212;Name: joeuser debug1: attempt 0 failures 0 debug1: Config token is loglevel debug1: Config token is logingracetime debug1: Config token is permitrootlogin debug1: Config token is rsaauthentication debug1: Config token is pubkeyauthentication debug1: Config token is authorizedkeysfile debug1: Config token is challengeresponseauthentication debug1: Config token is usepam debug1: Config token is allowtcpforwarding debug1: Config token is chrootdirectory debug1: Config token is subsystem debug1: PAM: initializing for "joeuser" debug1: PAM: setting PAM_RHOST to "localhost" debug1: PAM: setting PAM_TTY to "ssh" debug1: userauth-request for user joeuser service ssh-connection method password debug1: attempt 1 failures 0 debug1: do_pam_account: called Accepted password for joeuser from 127.0.0.1 port 52212 ssh2 debug1: monitor_child_preauth: joeuser has been authenticated by privileged process debug1: PAM: establishing credentials User child is on pid 9129 debug1: Entering interactive session for SSH2. debug1: server_init_dispatch_20 debug1: server_input_channel_open: ctype session rchan 0 win 65536 max 16384 debug1: input_session_request debug1: channel 0: new [server-session] debug1: session_new: session 0 debug1: session_open: channel 0 debug1: session_open: session 0: link with channel 0 debug1: server_input_channel_open: confirm session debug1: server_input_global_request: rtype no-more-sessions@openssh.com want_reply 0 debug1: server_input_channel_req: channel 0 request pty-req reply 1 debug1: session_by_channel: session 0 channel 0 debug1: session_input_channel_req: session 0 req pty-req debug1: Allocating pty. debug1: session_new: session 0 debug1: session_pty_req: session 0 alloc /dev/pts/1 debug1: server_input_channel_req: channel 0 request shell reply 1 debug1: session_by_channel: session 0 channel 0 debug1: session_input_channel_req: session 0 req shell debug1: Setting controlling tty using TIOCSCTTY. debug1: Received SIGCHLD. debug1: session_by_pid: pid 9130 debug1: session_exit_message: session 0 channel 0 pid 9130 debug1: session_exit_message: release channel 0 debug1: session_by_tty: session 0 tty /dev/pts/1 debug1: session_pty_cleanup: session 0 release /dev/pts/1 Received disconnect from 127.0.0.1: 11: disconnected by user debug1: do_cleanup debug1: do_cleanup debug1: PAM: cleanup debug1: PAM: closing session debug1: PAM: deleting credentials
在这里你有full output of sshd -dd,together with ssh -vv.
击：
# bash --version GNU bash,version 3.2.49(1)-release (arm-none-linux-gnueabi) Copyright (C) 2007 Free Software Foundation,Inc.
bash二进制文件是从源代码交叉编译的.我也试过使用Optware distribution的预编译二进制文件,但是遇到了完全相同的问题.我使用objdump -x检查了缺少的共享库,但它们都在那里.
任何想法可能导致这个“权限被拒绝,请再试一次.”？我几乎潜入了bash源代码进行调查,但试图避免数小时追逐可能是愚蠢的事情.
编辑：添加有关bash和系统的更多信息
$ls -la /bin/bash -rwxr-xr-x 1 root root 724676 Dec 15 23:57 /bin/bash $ file /bin/bash /bin/bash: ELF 32-bit LSB executable,ARM,version 1 (SYSV),dynamically linked (uses shared libs),for GNU/Linux 2.6.14,stripped $ uname -a Linux NAS 2.6.32.12 #2661 Mon Nov 12 23:10:15 CST 2012 armv5tel GNU/Linux synology_88f6282_212+ $grep bash /etc/shells /bin/bash /bin/bash2
解决方法
供将来参考：经过太多时间研究和调试这个问题,我终于找到了根本原因.
Synology使用的OpenSSH版本是一个高度定制的版本,其行为与原始代码不同.它有很多hacks和ad-hoc自定义 – 例如,在接受登录之前进行额外检查以查看是否在Web界面中启用了SSH服务,或者从rsync命令中剥离特殊字符(;,|,’),或者..等待它…避免普通用户使用不同于/ bin / sh或/ bin / ash的shell.是的,在二进制文件中硬编码.
这是OpenSSH 5.8p1的代码片段,由Synology在其源代码(DSM4.1 – branch 2636)上分发,文件session.c：
void do_child(Session *s,const char *command)
{
...
#ifdef MY_ABC_HERE
char szValue[8];
int RunSSH = 0;
SSH_CMD SSHCmd = REQ_UNKNOWN;
if (1 == GetKeyValue("/etc/synoinfo.conf","runssh",szValue,sizeof(szValue))) {
if (strcasecmp(szValue,"yes") == 0) {
RunSSH = 1;
}
}
if (IsSFTPReq(command)){
SSHCmd = REQ_SFTP;
} else if (IsRsyncReq(command)){
SSHCmd = REQ_RSYNC;
} else if (IsTimebkpRequest(command)){
SSHCmd = REQ_TIMEBKP;
} else if (RunSSH && IsAllowShell(pw)){
SSHCmd = REQ_SHELL;
} else {
goto Err;
}
if (REQ_RSYNC == SSHCmd) {
pw = SYNOChgValForRsync(pw);
}
if (!SSHCanLogin(SSHCmd,pw)) {
goto Err;
}
goto Pass;
Err:
fprintf(stderr,"Permission denied,please try again.\n");
exit(1);
Pass:
#endif /* MY_ABC_HERE */
...
}
可以想象,IsAllowShell(pw)是罪魁祸首：
static int IsAllowShell(const struct passwd *pw)
{
struct passwd *pUnPrivilege = NULL;
char *szUserName = NULL;
if (!pw || !pw->pw_name) {
return 0;
}
szUserName = pw->pw_name;
if(!strcmp(szUserName,"root") || !strcmp(szUserName,"admin")){
return 1;
}
if (NULL != (pUnPrivilege = getpwnam(szUserName))){
if (!strcmp(pUnPrivilege->pw_shell,"/bin/sh") ||
!strcmp(pUnPrivilege->pw_shell,"/bin/ash")) {
return 1;
}
}
return 0;
}
难怪我为什么会遇到这种奇怪的行为.对于不同于root或admin的用户,只接受shells / bin / sh和/ bin / ash.这不管是什么uid(我测试过也使joeuser uid = 0,它没有用.现在很明显为什么).
确定原因后,修复很简单：只需删除对IsAllowShell()的调用即可.我花了一些时间来获得正确的配置来交叉编译openssh及其所有依赖项,但最终它运行良好.
如果有人有兴趣做同样的事情(或尝试交叉编译Synology的其他内核模块或二进制文件),here’s my version of Makefile.它已经过OpenSSH-5.8p1 source测试,适用于运行Marvell Kirkwood mv6281 / mv6282 CPU(如DS212)的模型.我使用了运行Ubuntu 12.10 x64的主机.
底线：糟糕的做法,可怕的代码,以及不做的事情的一个很好的例子.我理解有时OEM需要开发特殊的自定义,但是在挖掘太深之前他们应该三思而后行.这不仅会导致代码无法维护,还会产生各种不可预见的问题.值得庆幸的是,GPL的存在是为了让他们诚实 – 并且开放.
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