On Aug 19, 2024, at 11:29 PM, Al Viro [off-list ref] wrote:

On Mon, Aug 19, 2024 at 08:25:38PM +0000, Song Liu wrote:

quoted

int bpf_get_parent_path(struct path *p) {
again:
   if (p->dentry == p->mnt.mnt_root) {
       follow_up(p);
       goto again;
   }
   if (unlikely(IS_ROOT(p->dentry))) {
       return PARENT_WALK_DONE;  
   }
   parent_dentry = dget_parent(p->dentry);
   dput(p->dentry);
   p->dentry = parent_dentry;
   return PARENT_WALK_NEXT; 
}

This will handle the mount. However, we cannot guarantee deny-by-default
policies like LandLock does, because this is just a building block of 
some security policies.

You do realize that above is racy as hell, right?

Filesystem objects do get moved around.  You can, theoretically, play with
rename_lock, but that is highly antisocial.

I do understand filesystem objects may get moved around. However, I am not
sure whether we have to avoid all the race conditions (and whether it is
really possible to avoid all race conditions). 

What's more, _mounts_ can get moved around.  That is to say, there is no
such thing as stable canonical pathname of a file.

Maybe I should really step back and ask for high level suggestions. 

We are hoping to tag all files in a directory with xattr (or something
else) on the directory. For example, a xattr "Do_not_rename" on /usr 
should block rename of all files inside /usr. 

Our original idea is to start from security_file_open() hook, and walk 
up the tree (/usr/bin/gcc => /usr/bin => /usr). However, this appears
to be wasteful and unreliable, and Christian suggested we should use a 
combination of security_inode_permission and security_file_open. I 
tried to build something on this direction, and hits a few issues:

1. Getting xattr from security_inode_permission() is not easy. Some
   FS requires dentry to get xattr. 
2. Finding parent from security_inode_permission() is also tricky.
   (maybe as trick as doing dget_parent() from security_file_open?)
   We need tag on /usr to work on /usr/bin. But how do we know /usr
   is /usr/bin's parent?

For the original goal… tag all files in a directory with xattr on 
the directory, is it possible at all? If not, we will go back and
implement something to tag all the files one at a time. If it is
indeed possible, what's the right way to do it, and what are the
race conditions we need to avoid and to accept?

Comments and suggestions are highly appreciated. 

Thanks,
Song