linux-kernel - Re: [PATCH] fs: call security_d_instantiate in d_obtain

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Message-ID: <20101117202617.GA31009@fieldses.org>
Date:	Wed, 17 Nov 2010 15:26:17 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Josef Bacik <josef@...hat.com>
Cc:	linux-fsdevel@...r.kernel.org, eparis@...hat.com,
	linux-kernel@...r.kernel.org, sds@...ho.nsa.gov,
	selinux@...ho.nsa.gov
Subject: Re: [PATCH] fs: call security_d_instantiate in d_obtain_alias

On Wed, Nov 17, 2010 at 02:28:22PM -0500, Josef Bacik wrote:
> On Wed, Nov 17, 2010 at 02:18:17PM -0500, J. Bruce Fields wrote:
> > On Wed, Nov 17, 2010 at 12:51:03PM -0500, Josef Bacik wrote:
> > > While trying to track down some NFS problems with BTRFS, I kept noticing I was
> > > getting -EACCESS for no apparent reason.  Eric Paris and printk() helped me
> > > figure out that it was SELinux that was giving me grief, with the following
> > > denial
> > > 
> > > type=AVC msg=audit(1290013638.413:95): avc:  denied  { 0x800000 } for  pid=1772
> > > comm="nfsd" name="" dev=sda1 ino=256 scontext=system_u:system_r:kernel_t:s0
> > > tcontext=system_u:object_r:unlabeled_t:s0 tclass=file
> > > 
> > > Turns out this is because in d_obtain_alias if we can't find an alias we create
> > > one and do all the normal instantiation stuff, but we don't do the
> > > security_d_instantiate.  With this patch I'm no longer seeing these errant
> > > -EACCESS return values.  Thanks,
> > 
> > Possibly dumb question: Is there still a small race here?  Is it
> > possible for another nfsd thread to find the new alias on the list while
> > this thread is still:
> > 
> > > 
> > > Signed-off-by: Josef Bacik <josef@...hat.com>
> > > ---
> > >  fs/dcache.c |    1 +
> > >  1 files changed, 1 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/fs/dcache.c b/fs/dcache.c
> > > index 23702a9..890a59e 100644
> > > --- a/fs/dcache.c
> > > +++ b/fs/dcache.c
> > > @@ -1201,6 +1201,7 @@ struct dentry *d_obtain_alias(struct inode *inode)
> > >  	spin_unlock(&tmp->d_lock);
> > >  
> > >  	spin_unlock(&dcache_lock);
> > 
> > ... right here, so that that other nfsd thread still ends up trying to
> > do something with a dentry that hasn't had security_d_instantiate called
> > on it yet?
> > 
> > > +	security_d_instantiate(tmp, inode);
> > >  	return tmp;
> > >  
> > >   out_iput:
> > > -- 
> > 
> > Or does something else prevent that?
> > 
> 
> That's a good question, I have no idea actually.  Every other consumer of
> security_d_instantiate seems to hold the i_mutex of the parent directory inode,
> tho I'm not sure if that is appropriate for d_obtain_alias, maybe somebody else
> has an idea?  Thanks,

Actually, I don't get it:

	- Why is selinux using a *dentry* operation to initialize an
	  *inode*?
	- Are security hooks necessarily prepared to handle a
	  disconnected dentry?  (Which has no real parent, name an empty
	  string, etc.)
	- What use is the dentry to the security module in this case
	  anyway?

--b.
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