Re: BUG: Securityfs and bind mounts (2.6.34)

[Tvrtko Ursulin <tvrtko.ursulin@sophos.com>]

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On Thursday 08 Jul 2010 16:46:48 Greg KH wrote:
> On Thu, Jul 08, 2010 at 04:32:42PM +0100, Tvrtko Ursulin wrote:
> > On Thursday 08 Jul 2010 16:20:59 Greg KH wrote:
> > > > :) Well I do not know, but, it kind of smelled like a bug in the
> > > > : vfs/mount
> > > >
> > > > handling/securityfs area so I thought to let experts know. I _think_
> > > > I did nothing that much wrong. Just used the exposed API
> > > > (securityfs_remove) and some bind mount shuffling from userspace.
> > >
> > > securitfs just uses libfs underneath it, and really doesn't have any
> > > bindings for module ownerships, so I wouldn't recommend doing what you
> > > just did.
> >
> > Just do double check what you are saying, securityfs is not safe for use
> > from modules? If so I would then recommend removing the exports otherwise
> > it is an invitation to shoot yourself into the foot.
>
> Hm, did you properly set the module owner of the file_operations that
> you passed to securityfs?  That should protect if you have an open file,
> but I doubt anyone thought you would do crazy things like bind mounts on
> top of a ramfs and then think it was safe to unload a lower module :)

Hi again,

Here is the reproducer. Build the attached test module and put test.ko in the
cwd. Then run:

umount /sys/kernel/security
mount none -t securityfs /sys/kernel/security/
insmod test.ko
mount file --bind /sys/kernel/security/test/file
rmmod test
insmod test.ko
umount /sys/kernel/security/test/file
umount /sys/kernel/security # <-------- BANG:

It is a bit different than what I initially said. It looks like reference
counting on a dentry gets messed up once securityfs_create_dir() in second
insmod of test.ko fails with -EEXIST.

HTH,

Tvrtko


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#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/security.h>


static struct dentry *root;
static struct dentry *file;


static int test_open(struct inode *inode, struct file *file)
{
    return -1;
}

static struct file_operations test_fops = {
    .owner =    THIS_MODULE,
    .open =     test_open,
};


static int __init test_init(void)
{
    root = securityfs_create_dir("test", NULL);
    if (IS_ERR(root)) {
        printk("error creating root %d\n", PTR_ERR(root));
        return PTR_ERR(root);
    }

    file = securityfs_create_file("file", S_IRUSR, root, NULL, &test_fops);
    if (IS_ERR(file)) {
        printk("error creating file %d\n", PTR_ERR(file));
        securityfs_remove(root);
        return PTR_ERR(file);
    }

    return 0;
}

static void __exit test_exit(void)
{
    securityfs_remove(file);
    securityfs_remove(root);
}

MODULE_LICENSE("GPL");

module_init(test_init);
module_exit(test_exit);