linux-kernel - Re: [PATCH 0/4] IMA: making i_readcount a first class inode citizen

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Date:	Fri, 05 Nov 2010 13:38:05 -0400
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, hch@...radead.org,
	warthog9@...nel.org, david@...morbit.com, jmorris@...ei.org,
	kyle@...artin.ca, hpa@...or.com, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, mingo@...e.hu, eparis@...hat.com,
	viro@...iv.linux.org.uk, Matthew Wilcox <matthew@....cx>
Subject: Re: [PATCH 0/4] IMA: making i_readcount a first class inode citizen

On Fri, 2010-11-05 at 12:28 -0400, J. Bruce Fields wrote: 
> On Fri, Nov 05, 2010 at 07:08:06AM -0400, Mimi Zohar wrote:

> > Right, like the ima_file_check(), which is after the __dentry_open().
> > Al, is it possible to move the break_lease() in may_open() to later?
> 
> That would still leave a race like:
> 
> 			check count
> 	bump count
> 	break lease
> 			set lease
> 
> But we could extend the i_lock to prevent the lease being bumped between
> the two steps on the right-hand side.

The latest i_readcount patchset, i_readcount is atomic and doesn't
require i_lock, at least for IMA.  Have to think about this more ....

> At that point I think we'd be done?  We're assured the count is still
> zero while the lease is added to the inode, so anyone in the process of
> doing an open has yet to reach the break_lease, which will see the newly
> added lease.
> 
> That leaves the problem that leases really should be broken on anything
> that changes the attributes or the dentries pointing to the inode:
> setattr, link, unlink, rename, at least.

For this reason, IMA is now taking i_mutex, preventing file metadata
from changing.

> One approach: add another counter to the inode named disable_leases, and
> have any of those operations do something like:
> 
> 	disable_lease++
> 	break_lease
> 	... do operation ...
> 	disable_lease--
> 
> ?
> 
> --b.

lol, getting i_readcount was was hard enough. 

Mimi

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