Hi Eric, On Sun, May 30, 2010 at 11:54:23PM -0400, Eric Paris wrote: > We need to call this function in the SELinux case. So you'll need a > patch like the one attached (not even compiled but I think it is right) > [..] > static int selinux_inode_follow_link(struct dentry *dentry, struct nameidata *nameidata) > [..] > + rc = cap_inode_follow_link(dentry, nameidata); Yeah, when I quickly checked SELinux and AppArmor, it seemed that they were always calling down to all commoncaps functions, but it looks like not in all cases. I think that Eric Biederman's observations here makes the most sense: this check needs to happen without involving the LSMs at all. > > +int cap_inode_follow_link(struct dentry *dentry, > > + struct nameidata *nameidata) > > +{ > > + const struct inode *parent = dentry->d_parent->d_inode; > > + const struct inode *inode = dentry->d_inode; > > + const struct cred *cred = current_cred(); > > + > > + if (weak_sticky_symlinks) > > + return 0; > > + > > + if ((parent->i_mode & (S_ISVTX|S_IWOTH)) == (S_ISVTX|S_IWOTH) && > > + parent->i_uid != inode->i_uid && > > + cred->fsuid != inode->i_uid) { > > + printk_ratelimited(KERN_NOTICE "non-matching-uid symlink " > > + "following attempted in sticky-directory by " > > + "%s (fsuid %d)\n", current->comm, cred->fsuid); > > + return -EACCES; > > + } > > + return 0; > > +} > > What stops us from racing between the assignment of parent and it's > first use with a rename on our object and rmdir on the old parent? I'm > wondering if we need to be doing this test holding dentry->d_lock (which > is what protects dentry->d_parent if I recall correctly) > > Certainly doesn't fix all of the raciness, but I think it would close > the opps part. Maybe someone who knows the VFS better can tell me if I > am misguided. The only other use of d_parent I can see there is in may_delete(). With vfs_unlink() calling that, it would seem to be racey too if we needed to hold a lock for that. But it's not clear to me in vfs_follow_link is doing locking somehow. Thanks, -Kees -- Kees Cook Ubuntu Security Team -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html