Re: LSM hook addition?

[Valdis . Kletnieks]

On Mon, 24 Jan 2005 00:58:08 EST, John Richard Moser said:

> I believe this would be sufficient to finish an LSM module to implement
> linking restrictions from GrSecurity.  I did Symlinks in an LSM module,
> but haven't tested it out; it's purely academic. 
> The hook here would be used (in my academic exploration) to prevent hard
> links from being created to files you don't own, unless you're root.

You don't need a hook there to do that.  The inode_link hook should be sufficient,
using code something like this:

+int vtkit_link (struct dentry *old_entry, struct inode *dir, struct dentry *new_entry)
+{
+       struct inode *i_target = old_entry->d_inode;
+       struct inode *i_new = new_entry->d_inode;
+       int perm_check = 0;
+
+       /* We check as follows - if the target of the link isn't owned by us,
+        * and we're not a privileged user, then we complain if any of the
+        * following are true:
+        * 1) It's not a special file
+        * 2) the target is set-uid or set-gid
+        * 3) user doesn't have permission to the target
+        *
+        * Yes, a "current->uid" check is pig-headed and wrong in the context of a
+        * system that uses priv separation...
+        */
+       perm_check = generic_permission(i_target, MAY_READ | MAY_WRITE, NULL);
+       if (security_safe_hardlink && (current->fsuid != i_target->i_uid) &&
+               !capable(CAP_FOWNER) && current->uid &&
+               (!S_ISREG(i_target->i_mode) || (i_target->i_mode & S_ISUID) ||
+               ((i_target->i_mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) ||
+               perm_check)) {
+                       printk(KERN_NOTICE "vtkit - rejecting hard link (target UID %d) by PID %d (uid=%d, comm=%s)\n",
+                               i_target->i_uid, current->pid, current->uid, current->comm);
+                       return (perm_check ? perm_check:-EPERM);
+       }
+       return 0;
+}

If I'm missing something here, please let me know... ;)

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