linux-kernel - Re: [PATCH] vfs: Fix possible NULL pointer dereference in inode

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Date:	Fri, 10 Jan 2014 00:06:42 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Eric Paris <eparis@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Dave Chinner <david@...morbit.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	James Morris <james.l.morris@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	Theodore Ts'o <tytso@....edu>, stable <stable@...r.kernel.org>,
	Paul Moore <paul@...l-moore.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Matthew Wilcox <matthew@....cx>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] vfs: Fix possible NULL pointer dereference in
 inode_permission()

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@...hat.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?
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