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

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Message-ID: <1289088219.3268.4.camel@localhost.localdomain>
Date:	Sat, 06 Nov 2010 20:03:39 -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 15:08 -0400, J. Bruce Fields wrote:
> On Fri, Nov 05, 2010 at 01:38:05PM -0400, Mimi Zohar wrote:
> > 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.
> 
> Lease code could do that as well.  (Probably just with a trylock,
> failing the setlease if we can't get the lock.)
> 
> That misses rename, though, which doesn't take the i_mutex on the
> renamed file.  Which makes sense.

fs/namei.c: vfs_rename_other() seems to be taking the i_mutex.  Am I
looking in the wrong place?

> But a lease is used to give file server clients the right to do an open
> locally, and we want them to be able to guarantee to applications that
> the path (well, the last component, at least) still refers to the same
> file at open time.

Mimi

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