Re: [PATCH 2/3] ima: Ensure lock is held when setting iint pointer in inode security blob

[Mimi Zohar <zohar@linux.ibm.com>]

On Wed, 2024-10-09 at 17:43 +0200, Roberto Sassu wrote:
> On Wed, 2024-10-09 at 11:41 -0400, Paul Moore wrote:
> > On Tue, Oct 8, 2024 at 12:57 PM Roberto Sassu
> > <roberto.sassu@huaweicloud.com> wrote:
> > > 
> > > From: Roberto Sassu <roberto.sassu@huawei.com>
> > > 
> > > IMA stores a pointer of the ima_iint_cache structure, containing integrity
> > > metadata, in the inode security blob. However, check and assignment of this
> > > pointer is not atomic, and it might happen that two tasks both see that the
> > > iint pointer is NULL and try to set it, causing a memory leak.
> > > 
> > > Ensure that the iint check and assignment is guarded, by adding a lockdep
> > > assertion in ima_inode_get().
> > > 
> > > Consequently, guard the remaining ima_inode_get() calls, in
> > > ima_post_create_tmpfile() and ima_post_path_mknod(), to avoid the lockdep
> > > warnings.
> > > 
> > > Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> > > ---
> > >  security/integrity/ima/ima_iint.c |  5 +++++
> > >  security/integrity/ima/ima_main.c | 14 ++++++++++++--
> > >  2 files changed, 17 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/security/integrity/ima/ima_iint.c b/security/integrity/ima/ima_iint.c
> > > index c176fd0faae7..fe676ccec32f 100644
> > > --- a/security/integrity/ima/ima_iint.c
> > > +++ b/security/integrity/ima/ima_iint.c
> > > @@ -87,8 +87,13 @@ static void ima_iint_free(struct ima_iint_cache *iint)
> > >   */
> > >  struct ima_iint_cache *ima_inode_get(struct inode *inode)
> > >  {
> > > +       struct ima_iint_cache_lock *iint_lock;
> > >         struct ima_iint_cache *iint;
> > > 
> > > +       iint_lock = ima_inode_security(inode->i_security);
> > > +       if (iint_lock)
> > > +               lockdep_assert_held(&iint_lock->mutex);
> > > +
> > >         iint = ima_iint_find(inode);
> > >         if (iint)
> > >                 return iint;
> > 
> > Can you avoid the ima_iint_find() call here and just do the following?
> > 
> >   /* not sure if you need to check !iint_lock or not? */
> >   if (!iint_lock)
> >     return NULL;
> >   iint = iint_lock->iint;
> >   if (!iint)
> >     return NULL;
> 
> Yes, I also like it much more.

Yes, testing iint_lock and then iint_lock->iint should be fine, but the logic
needs to be inverted.  ima_inode_get() should return the existing iint, if it
exists, or allocate the memory.

Mimi