Can't log in via SSH to any accounts using /bin/bash shell on Synology NAS

Solution 1:

For future reference: after way too many hours researching and debugging this issue, I finally discovered the root cause.

The OpenSSH version used by Synology is a highly customized version, that does not behave like the original code. It has lots of hacks and ad-hoc customizations - e.g., additional checking before accepting a login to see if the SSH service is enabled within the web interface, or stripping special chars (;, |, ') from rsync commands, or... wait for it... avoiding regular users to use a shell different than /bin/sh or /bin/ash. Yeah, hard coded within the binary.

Here's the piece of code from OpenSSH 5.8p1, as distributed by Synology on their source code (DSM4.1 - branch 2636), file 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 */
...
}

As you can imagine, the IsAllowShell(pw) was the culprit:

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;
}

No wonder why I was experiencing such an odd behavior. Only shells /bin/sh and /bin/ash would be accepted for users different than root or admin. And this regardless of the uid (I had tested also making joeuser uid=0, and it didn't work. Now it's obvious why).

Once identified the cause, the fix was easy: just remove the call to IsAllowShell(). It took me a while to get the right configuration to cross-compile openssh and all its dependencies, but it worked well in the end.

If anyone is interested in doing the same (or trying to cross-compile other kernel modules or binaries for Synology), here's my version of Makefile. It was tested with OpenSSH-5.8p1 source, and works well with models running Marvell Kirkwood mv6281/mv6282 CPU (like DS212+). I used a host running Ubuntu 12.10 x64.

Bottom line: bad practice, terrible code, and a great example of what not to do. I understand sometimes OEMs need to develop special customizations, but they should think twice before digging too deep. Not only this results in unmaintainable code for them, but also creates all sorts of unforeseen issues down the road. Thankfully GPL exist to keep them honest - and open.

Solution 2:

To circumvent the problem, and since I installed bash via ipkg and I can't be sure /opt will always be available (mounted correctly), I simply put the following in my .profile

[ -x /opt/bin/bash ] && exec /opt/bin/bash

while /etc/passwd contains /bin/ash as shell.