Re: [patch 136/147] nilfs2: use refcount_dec_and_lock() to fix potential UAF

[Ryusuke Konishi]

Hi,

On Fri, Sep 24, 2021 at 7:35 PM Pavel Machek <pavel@xxxxxxx> wrote:
>
> Hi!
>
> > From: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> > Subject: nilfs2: use refcount_dec_and_lock() to fix potential UAF
> >
> > When the refcount is decreased to 0, the resource reclamation branch is
> > entered.  Before CPU0 reaches the race point (1), CPU1 may obtain the
> > spinlock and traverse the rbtree to find 'root', see nilfs_lookup_root().
> > Although CPU1 will call refcount_inc() to increase the refcount, it is
> > obviously too late.  CPU0 will release 'root' directly, CPU1 then accesses
> > 'root' and triggers UAF.
> >
> > Use refcount_dec_and_lock() to ensure that both the operations of decrease
> > refcount to 0 and link deletion are lock protected eliminates this risk.
> >
> >      CPU0                      CPU1
> > nilfs_put_root():
> >                           <-------- (1)
> > spin_lock(&nilfs->ns_cptree_lock);
> > rb_erase(&root->rb_node, &nilfs->ns_cptree);
> > spin_unlock(&nilfs->ns_cptree_lock);
> >
> > kfree(root);
> >                           <-------- use-after-free
>
> > There is no reproduction program, and the above is only theoretical
> > analysis.
>
> Ok, so we have a theoretical bug, and fix already on its way to
> stable. But ... is it correct?
>
> > +++ a/fs/nilfs2/the_nilfs.c
> > @@ -792,14 +792,13 @@ nilfs_find_or_create_root(struct the_nil
> >
> >  void nilfs_put_root(struct nilfs_root *root)
> >  {
> > -     if (refcount_dec_and_test(&root->count)) {
> > -             struct the_nilfs *nilfs = root->nilfs;
> > +     struct the_nilfs *nilfs = root->nilfs;
> >
> > -             nilfs_sysfs_delete_snapshot_group(root);
> > -
> > -             spin_lock(&nilfs->ns_cptree_lock);
> > +     if (refcount_dec_and_lock(&root->count, &nilfs->ns_cptree_lock)) {
> >               rb_erase(&root->rb_node, &nilfs->ns_cptree);
> >               spin_unlock(&nilfs->ns_cptree_lock);
> > +
> > +             nilfs_sysfs_delete_snapshot_group(root);
> >               iput(root->ifile);
> >
> >               kfree(root);
>
> spin_lock() is deleted, but spin_unlock() is not affected. This means
> unbalanced locking, right?

It's okay.   spin_lock() is integrated into refcount_dec_and_lock(), which was
originally refcount_dec_and_test().

Thanks,
Ryusuke Konishi

>
> Best regards,
>                                                                 Pavel
> --
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>