Re: [PATCH] vfs: Fix possible NULL pointer dereference in inode_permission()

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Jan 10, 2014 at 07:53:41AM +0800, Linus Torvalds wrote:
> On Fri, Jan 10, 2014 at 7:37 AM, Eric Paris <eparis@redhat.com> wrote:
> >
> > but at least from an SELinux PoV, I think it's quick and easy, but wrong
> > for maintainability...
> 
> Yeah, it's a hack, and it's wrong, and we should figure out how to do
> it right. Likely we should just tie the lifetime of the i_security
> member directly to the lifetime of the inode itself, and just make the
> rule be that security_inode_free() gets called from whatever frees the
> inode itself, and *not* have an extra rcu callback etc. But that
> sounds like a bigger change than I'm comfy with right now, so the
> hacky one might be the band-aid to do for stable..
> 
> The problem, of course, is that all the different filesystems have
> their own inode allocations/freeing. Of course, they all tend to share
> the same pattern ("call_rcu xyz_i_callback"), so maybe we could try to
> make that a more generic thing? Like have a "free_inode" vfs callback,
> and do the call_rcu delaying at the VFS level..
> 
> And maybe, just maybe, we could just say that that is what
> "destroy_inode()" is, and that we will just call it from rcu context.
> All the IO has hopefully been done earlier  Yes/no?

Check what XFS is doing ;-/  That's where those call_rcu() have come from.
Sure, we can separate the simple "just do call_rcu(...->free_inode)" case
and hit it whenever full ->free_inode is there and ->destroy_inode isn't.
Not too pretty, but removal of tons of boilerplate might be worth doing
that anyway.  But ->destroy_inode() is still needed for cases where fs
has its own idea of inode lifetime rules.  Again, check what XFS is doing
in that area...

There's an extra source of headache, BTW - what about the "LSM stacking"
crowd and their plans?