linux-kernel - Re: [PATCH] SELinux: Fix possible NULL pointer dereference in selinux_inode

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Date:	Thu, 9 Jan 2014 10:51:14 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Eric Paris <eparis@...isplace.org>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	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] SELinux: Fix possible NULL pointer dereference in
 selinux_inode_permission()

On Thu, 9 Jan 2014 10:31:55 -0500
Eric Paris <eparis@...isplace.org> wrote:

> Didn't Al find this/something very similar.  I really hate this

I'm not involved with the vfs, so I'm unaware of other solutions
presented. I just hit this now and solving bugs is where I get a chance
to learn about other aspects of the kernel. ;-)

> solution.  Why should every LSM try to understand the intimate
> lifetime rules of the parent subsystems?  The real problem is that
> inode_free_security() is being called while the inode is still in use.
>  While I agree with the assessment, I disagree with the solution.  Let
> me try to find where Al and Christoph talked about this....
> 

The other obvious solution (but not as trivial to implement) is to call
the security_inode_free() and friends (probably __destroy_inode()
itself) after a synchronize_rcu().

Perhaps something like this?

-- Steve

diff --git a/fs/inode.c b/fs/inode.c
index 4bcdad3..a8f3b88 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -252,16 +252,17 @@ EXPORT_SYMBOL(__destroy_inode);
 static void i_callback(struct rcu_head *head)
 {
 	struct inode *inode = container_of(head, struct inode, i_rcu);
+	__destroy_inode(inode);
 	kmem_cache_free(inode_cachep, inode);
 }
 
 static void destroy_inode(struct inode *inode)
 {
 	BUG_ON(!list_empty(&inode->i_lru));
-	__destroy_inode(inode);
-	if (inode->i_sb->s_op->destroy_inode)
+	if (inode->i_sb->s_op->destroy_inode) {
+		__destroy_inode(inode);
 		inode->i_sb->s_op->destroy_inode(inode);
-	else
+	} else
 		call_rcu(&inode->i_rcu, i_callback);
 }
 
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