[PATCH] autocache patche -- resend, updated

[Michael E Brown]

On Fri, 2006-05-26 at 13:04 +0200, Enrico Scholz wrote:
> Michael_E_Brown at dell.com (Michael E Brown) writes:
> 
> > 	Here is a resend/consolidation of the autocache patches from last week.
> > I have combined the patches sent last week. It now defaults to gzip
> > format. I have also made the following fixes:
> 
> > +  check_dir_allowed (rootsdir, argv[2]);
> 
> When you do security checks, then please make them correctly. Everything
> else will give a wrong feeling about security:

Enrico,
	I welcome the feedback, thanks much. 

> 
> * a
> 
>   | check_dir(<dir>);
>   | operate_in_dir(<dir>);      // tar ...

Dir always has to be under /var/lib/mock/. It is not possible for
unprivileged users to create symlinks here.

I suppose the symlink attack could be done by somebody in the mock
group.
   1) mock -r CFG some.src.rpm    #to force creation of builddir with
ownership by me.
   2) cd /var/lib/mock/CFG/root/builddir/
   3) ln -s /  blastroot
   4) mock-helper unpack /var/lib/mock/CFG/root/builddir/blastroot
my-bad.tar.gz

I believe that the fix for this would be to add a check to ensure that
the tarball is always sourced from /var/lib/mock/root-cache/ and that
the perms are correct. Feedback?

> 
>   opens always a window for symlink attacks. Better do
> 
>   | chdirSafe(<dir);
>   | operate_in_dir(".");        // tar ...
> 
>   The security of 'tar' operations is another question; extraction can
>   be made secure by extracting into a private dir and doing an atomic
>   rename(2) then. ATM, I do not see a way how to implement tarball
>   creation securely.

Make sure we are always tarring /var/lib/mock/CFG/root and always place
the tar under /var/lib/mock/root-cache.

Users in group 'mock' would not have sufficient perms to do anything
harmful under this dir.

> 
> * you should check permissions of the tar-file; else, user can provide
>   e.g. a public writable /dev/kmem device in the tarball
> 
> Ok, the question remains whether such checks (inclusive check_dir_allowed())
> are needed overall. 'mock' gives practically root rights to everybody in the
> mock.

Personally, I think they should be there. It makes things much more
difficult, though.

Suggested resolution: 
For a patch, I could change both pack and unpack to only take the file
name and config name. It would then always look for or create the tar
under /var/lib/mock/root-cache/, and it would only ever tar or untar
to /var/lib/mock/CFG/root.

--
Michael