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

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Date:	Thu, 9 Jan 2014 14:42:39 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	Christoph Hellwig <hch@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Eric Paris <eparis@...isplace.org>,
	Theodore Ts'o <tytso@....edu>,
	Dave Chinner <david@...morbit.com>,
	James Morris <james.l.morris@...cle.com>,
	Paul Moore <paul@...l-moore.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH] vfs: Fix possible NULL pointer dereference in
	inode_permission()

On Thu, Jan 09, 2014 at 04:27:31PM -0500, Steven Rostedt wrote:
> Note, the crash came from stressing the deletion and reading of debugfs
> files. I was not able to recreate this via normal files. But I'm not
> sure they are safe. It may just be that the race window is much harder
> to hit.

But "normal" files have a 'destroy_inode' method.  So you've basically
only fixed it for debugfs (and maybe a few other unusual filesystems).
Why doesn't the code look like this:

static void i_callback(struct rcu_head *head)
{
	struct inode *inode = container_of(head, struct inode, i_rcu);
	__destroy_inode(inode);
	if (inode->i_sb->s_op->destroy_inode)
		inode->i_sb->s_op->destroy_inode(inode);
	else
		kmem_cache_free(inode_cachep, inode);
}

static void destroy_inode(struct inode *inode)
{
	BUG_ON(!list_empty(&inode->i_lru));
	call_rcu(&inode->i_rcu, i_callback);
}

We'd then have to get rid of all the call_rcu() invocations in individual
filesystems' destroy_inode methods, but that doesn't sound like a bad
thing to me.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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