gaurav gaurav - 3 months ago 9
C Question

Prevent unauthorised write access to a part of filesystem or partition

Hello all I have some very important system files which I want to protect from accidental deletion even by root user. I can create a new partition for that and mount it with readonly access but the problem is that I want my application which handles those system files to have write access to that part and be able to modify them. Is that possible using VFS? As VFS handles access to the files I could have a module inserted in the VFS layer which can see if there is a write access to that part then see the authorization and allow it or otherwise reject it.
If not please provide me suggestions regarding how can such a system be implemented what would I need in that case.
If there exists a system like this please suggest about them also.
I am using linux and want to implement this in C, I think it would be possible in C only.

Edit: There are such kind of programs implemented in windows which can restrict access to administrator even, to some important folders, would that be possible in linux?
My application is a system backup and restore program which needs to keep its backup information safe and secure. So I would like to have a secured part of a partition which could not be accidently deleted in any way. There are methods of locking a flashdrive can we use some of those methods for locking a partition in linux also ? so that mount is password protected ? I am not writing a virus application, my application would give user option to delete the backups but I don't wanna allow them to be deleted by any other application.
Edit: I am writing a system restore and backup program for ubuntu, I am a computer engineering student.

Edit: As I have got opinion from Basile Starynkevitch that I would be committing worst sin of programming if I do anything like this, but you could provide me suggestions considering this as a experimental project, I could make some changes in the VFS layer so that this could work.

Answer

You could use chattr, e.g.

chattr +i yourfile

But I don't think it is a good thing to do that. People using root access are expected to be careful. Those having root access can still issue the command undoing the above.

There is no way to forbid people having root access, or people having physical access to the computer, to access, remove, change your file, if they really want to (they could update & hack the kernel, for instance). Read more about trusted compute base

And I believe it is even unethical (and perhaps illegal, in some countries) to want to do that. I own my PC, and I don't understand why you should disallow me to change some data on it, because I happened to install some software.

By definition of root on Linux, it can do anything... You won't be able to prohibit him to erase or alter data... People with root access can write arbitrary bytes at arbitrary places on the disk.

And on a machine that I own (or perhaps just have physical access to), I will, thanks God, always be able to remove a file (even under Windows: I could for example boot a Linux CDROM and remove the file from Linux accessing an NTFS, and then reboot the Windows...).

So I think you should not bother and take even a minute to find out how to make root altering your precious files more difficult. Leave them as other root files...

PHILOSOPHICAL RANT

The unix philosophy has always been to trust the system administrator (while protecting newbie users from mistakes), that is the root user. The root is able to do anything (this is why people avoid being root, even on a personal machine). There have never been strong features to prohibit root doing mistakes, because the system administrator is expected to know well the system, and is trusted.

And Unix sysadmins understand this fact: it is part of their culture. (This is probably in contrast with Windows administration culture). They know when to be careful, they don't expect software to prevent mistakes as root.