Question

Does Linux Support ImpersonateValidUser?

Asked by: elsamman

I am writing a daemon that receives requests via a named pipe.  It must carry out file related activities on behalf of the requesting user.  The sender is a CGI program that prompts the user for an ID and password and includes both of these in the request it writes to the named pipe.

The question is how to implement this.  The most secure way would be to run the daemon is an underprivileged user and then impersonate the requesting user who will supply credentials (ID and password).  In Windows this is possible.

In Unix the only way I can think of to do this is run the daemon as a super-user, check the credentials by running crypt on the supplied password and comparing it to the shadow password file.  Only then is the file system ID changed.

The Windows method seems to me more secure since I don’t have to run the daemon as root.  Is there any equivalent function in Linux to ImpersonateValidUser that lets a non-super user impersonate another user?

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Asked On
2004-03-03 at 04:51:56ID20905368
Tags

impersonatevaliduser

,

linux

,

user

Topic

Linux Programming

Participating Experts
2
Points
500
Comments
3

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Answers

 

by: jleviePosted on 2004-03-03 at 05:30:53ID: 10504273

Running the daemon as root, checking the user's credential, and then switching to that user would be the Unix/Linux way if you are using a named pipe. If the connection was via a socket you could have the master process, running as root, accept the connection, validate the user, and then fork off a process that runs as the user, which is quite a bit more secure.

 

by: sunnycoderPosted on 2004-03-04 at 05:42:34ID: 10513679

>The Windows method seems to me more secure since I don’t have
>to run the daemon as root.
Daemons are supposed to be trusted processes ...
Only a process which is trusted enough to be allowed access to authentication information would be run by the sys admin as the kind of daemon you are trying to make ....

Allowing access to not so trusted processes is a bigger security hole rather than allowing such access to only a small set of trusted processes .... IMHO ... and this is what makes *nix more secure

 

by: elsammanPosted on 2004-03-04 at 06:52:03ID: 10514322

To be clear it is not a question of not having a trusted process but rather of always running at the least privilege possible to prevent any possible defects from being exploited.  Since I start out as root an exploitation could switch the daemon back to root and do damage.  

I really like the idea of the master process that does the suid and then forks off the child process.  I think that the assumption that I cannot do this with named pipes is due to the fact that I could not just pass off the pipe to the child the way you can a socket.  Actually what I do is a bit more complicated because I have multiple pipe writers and cannot afford to have requests interleaved.  The pipe writers do not put the entire request into the common pipe.  Instead they create a one-time, uniquely named pipe with the request details and then write the name of that pipe into the common pipe as the "request".   The master process can then do what was suggested and first suid to the requesting user, then fork off and pass the name of the one-time pipe.

I don't really want to fork off each time because it is less efficient (I am using perl2exe) but it sounds to me like it is the only secure way to go.

I sure wish Linux had an impersonate user call!

If no one comes up with a better way in the next day or so I will reward the points to jlevie

20120131-EE-VQP-002

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