Re: [syzbot] [lsm?] general protection fault in hook_inode_free_security

["Mickaël Salaün" <mic@digikod.net>]

On Thu, Jun 27, 2024 at 02:28:03PM -0400, Paul Moore wrote:
> On Thu, Jun 27, 2024 at 9:34 AM Mickaël Salaün <mic@digikod.net> wrote:
> >
> > I didn't find specific issues with Landlock's code except the extra
> > check in hook_inode_free_security().  It looks like inode->i_security is
> > a dangling pointer, leading to UAF.
> >
> > Reading security_inode_free() comments, two things looks weird to me:
> >
> > > /**
> > >  * security_inode_free() - Free an inode's LSM blob
> > >  * @inode: the inode
> > >  *
> > >  * Deallocate the inode security structure and set @inode->i_security to NULL.
> >
> > I don't see where i_security is set to NULL.
> 
> The function header comments are known to be a bit suspect, a side
> effect of being detached from the functions for many years, this may
> be one of those cases.  I tried to fix up the really awful ones when I
> moved the comments back, back I didn't have time to go through each
> one in detail.  Patches to correct the function header comments are
> welcome and encouraged! :)
> 
> > >  */
> > > void security_inode_free(struct inode *inode)
> > > {
> >
> > Shouldn't we add this check here?
> > if (!inode->i_security)
> >         return;
> 
> Unless I'm remembering something wrong, I believe we *should* always
> have a valid i_security pointer each time we are called, if not
> something has gone wrong, e.g. the security_inode_free() hook is no
> longer being called from the right place.  If we add a NULL check, we
> should probably have a WARN_ON(), pr_err(), or something similar to
> put some spew on the console/logs.
> 
> All that said, it would be good to hear some confirmation from the VFS
> folks that the security_inode_free() hook is located in a spot such
> that once it exits it's current RCU critical section it is safe to
> release the associated LSM state.
> 
> It's also worth mentioning that while we always allocate i_security in
> security_inode_alloc() right now, I can see a world where we allocate
> the i_security field based on need using the lsm_blob_size info (maybe
> that works today?  not sure how kmem_cache handled 0 length blobs?).
> The result is that there might be a legitimate case where i_security
> is NULL, yet we still want to call into the LSM using the
> inode_free_security() implementation hook.
> 
> > >       call_void_hook(inode_free_security, inode);
> > >       /*
> > >        * The inode may still be referenced in a path walk and
> > >        * a call to security_inode_permission() can be made
> > >        * after inode_free_security() is called. Ideally, the VFS
> > >        * wouldn't do this, but fixing that is a much harder
> > >        * job. For now, simply free the i_security via RCU, and
> > >        * leave the current inode->i_security pointer intact.
> > >        * The inode will be freed after the RCU grace period too.
> >
> > It's not clear to me why this should be safe if an LSM try to use the
> > partially-freed blob after the hook calls and before the actual blob
> > free.
> 
> I had the same thought while looking at this just now.  At least in
> the SELinux case I think this "works" simply because SELinux doesn't
> do much here, it just drops the inode from a SELinux internal list
> (long story) and doesn't actually release any memory or reset the
> inode's SELinux state (there really isn't anything to "free" in the
> SELinux case).  I haven't checked the other LSMs, but they may behave
> similarly.
> 
> We may want (need?) to consider two LSM implementation hooks called
> from within security_inode_free(): the first where the existing
> inode_free_security() implementation hook is called, the second inside
> the inode_free_by_rcu() callback immediately before the i_security
> data is free'd.

Couldn't we call everything in inode_free_by_rcu()?
I replied here instead:
https://lore.kernel.org/r/20240708.hohNgieja0av@digikod.net

> 
> ... or we find a better placement in the VFS for
> security_inode_free(), is that is possible.  It may not be, our VFS
> friends should be able to help here.

Christian? Al?

> 
> > >        */
> > >       if (inode->i_security)
> > >               call_rcu((struct rcu_head *)inode->i_security,
> > >                        inode_free_by_rcu);
> >
> > And then:
> > inode->i_security = NULL;
> 
> According to the comment we may still need i_security for permission
> checks.  See my comment about decomposing the LSM implementation into
> two hooks to better handle this for LSMs.

That was my though too, but maybe not if the path walk just ends early.

> 
> > But why call_rcu()?  i_security is not protected by RCU barriers.
> 
> I believe the issue is that the inode is protected by RCU and that
> affects the lifetime of the i_security blob.

It seems to be related to commit fa0d7e3de6d6 ("fs: icache RCU free
inodes").