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SubjectNew LSM security operation
Hi people,

I've looked around on how to hide inodes in a Linux filesystem but
surprisingly the kernel lacks this functionality. It would be desirable
for me to add an ACL to a file in order not to be seen in the directory
contents but only for some users.

Some Selinux experts point out that the correct way to do this is via
poly-instantiated directories such as /tmp or /var/tmp, so each user
"view" his own version of this directory. But that's not what I want,
for example in /etc, I would want to hide some directories or files for
some users. I don't want a whole /etc instantiation for each user logged in.

Also, there is a patch called LIDS that does the thing, but patching the
whole kernel and adding much extra functionality such as intrussion
detection system as it is. Some rootkits -like adore- also can hide
inodes, but changing the owner to an uid that the kernel module hides
from the system call. I don't thing this is a good approach...

For me, the correct way to achieve this is to add an extra op in the
"security_operations" struct, as an inode operation. With this, a
Mandatory Access Control system that uses LSM such as SELinux can add
some policies on the "list" file access vector.

So, I'd call this hook in the vfs_readdir() syscall just after the "file
<http://lxr.linux.no/ident?i=file>->f_op->readdir()" -the particular
filesystem readdir()- and walk through the list to ask for each inode if
it has permission to be "listed" in that directory. Then, the MAC system
can handle the grant or deny permission per inode, and then return the
"modified" list to userspace.

I'm not sure if this is the correct way, or maybe it adds to much
overhead to the "ls" command, but I'd like to hear some opinions, I can
try to code it and summit a patch...

Thanks for your answer,

Zeus Gómez.
PS. Please, include me in the CC if you reply this message.
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