Re: [RESEND][RFC][PATCH 2/7] implementation of LSM hooks

[Kyle Moffett]

On Apr 19, 2006, at 02:56:28, Valdis.Kletnieks@xxxxxx wrote:
> On Wed, 19 Apr 2006 02:40:25 EDT, Kyle Moffett said:
> > Perhaps the SELinux model should be extended to handle (dir-inode,  
> > path-entry) pairs.  For example, if I want to protect the /etc/ 
> > shadow file regardless of what tool is used to safely modify it, I  
> > would set
>
> Some of us think that the tools can protect /etc/shadow just fine  
> on their own, and are concerned with rogue software that abuses / 
> etc/shadow without bothering to safely modify it..

Here I'm talking about using SELinux to protect /etc/shadow both  
before _and_ after it's edited with vipw.  A tool like vipw does creat 
() write() rename() to overwrite the /etc/shadow file, so any SELinux  
system relying _only_ on inode is guaranteed to break.  In any case,  
the point here is to provide 2 clean semantics for protecting files:  
inodes and directory entries;  They have entirely different use- 
cases.  For an inode all you need is (<inode>), for a directory entry  
you need (<dir-inode>, <path-component>).  The latter would only be  
used when referencing the file by name, if you already have a  
filehandle it would not be used.

> > o  Protect the "/" and "/etc" directory inodes as usual under SELinux
> > (with attributes on directory inodes).
> > o  Create pairs with (etc_inode,"shadow") and (etc_inode,"gshadow")
> > and apply security attributes to those potentially nonexistent pairs.
>
> *bzzt* wrong.  Why should "gshadow" matter? (Think carefully about  
> what happens when a setUID program gets exploited and used to  
> scribble on /etc/shadow - black hats rarely bother to do locking  
> and other such niceties....)

/etc/gshadow matters because it's the group shadow file, no?  If  
you're going to bother protecting the user shadow file, you should  
protect the group shadow file too.

> > I'm not terribly familiar with the exact internal semantics of  
> > SELinux, but that should provide a 90% solution (it fixes bind  
> > mounts and namespaces).
>
> 90% doesn't give the security guys warm-and-fuzzies....

True, but that's why I address the following 2 items below:

> > The remaining 2 issues are hardlinks and fd-passing.  For  
> > hardlinks you don't care about other links to that data, you're  
> > concerned with protecting a particular filesystem location, not  
> > particular contents, so you just need to prevent _new_ hardlinks  
> > to a protected (dir_inode, path_elem) pair, which doesn't seem  
> > very hard.
>
> It's not. include/linux/security.h:
>
>  * @inode_link:
>  *      Check permission before creating a new hard link to a file.
>  *      @old_dentry contains the dentry structure for an existing  
> link to the file.
>  *      @dir contains the inode structure of the parent directory  
> of the new link.
>  *      @new_dentry contains the dentry structure for the new link.
>  *      Return 0 if permission is granted.

I _think_ SELinux provides a way to hook this, but I'm not entirely  
sure.  If not, it should be added, but I suspect it does.

> > For fd-passing, I don't know what to do.  Perhaps nothing.
>
> include/linux/security.h:
>
>  * @file_receive:
>  *      This hook allows security modules to control the ability of  
> a process
>  *      to receive an open file descriptor via socket IPC.
>  *      @file contains the file structure being received.
>  *      Return 0 if permission is granted.
>
> Already a solved problem.

Likewise, this should be solved with inode-based security if it's a  
problem.  The use-case I'm addressing (legitimate use of /etc/shadow  
with vipw) doesn't care about that.  For example, if I want to only  
allow vipw access to /etc/shadow, then associating that policy with  
the /etc/shadow inode is useless, because the first rogue program  
would just remove and rewrite /etc/shadow, getting a different  
inode.  What I need to do is protect the "shadow" entry in the dentry  
for ("/etc") in the particular namespace I care about, right?

Cheers,
Kyle Moffett



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