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

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Message-ID: <20101121025945.GA4365@fieldses.org>
Date:	Sat, 20 Nov 2010 21:59:45 -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 V2

On Fri, Nov 19, 2010 at 05:35:52PM -0500, J. Bruce Fields wrote:
> On Thu, Nov 18, 2010 at 08:52:55PM -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.
> > 
> > Usually we are protected from getting a hashed dentry that hasn't yet run
> > security_d_instantiate() by the parent's i_mutex, but obviously this isn't an
> > option there, so in order to deal with the case that a second thread comes in
> > and finds our new dentry before we get to run security_d_instantiate(), we go
> > ahead and call it if we find a dentry already.  Eric assures me that this is ok
> > as the code checks to see if the dentry has been initialized already so calling
> > security_d_instantiate() against the same dentry multiple times is ok.  With
> > this patch I'm no longer getting errant -EACCESS values.
> 
> Thanks, I can't see any reason that wouldn't work.

(FWIW, I also ran my usual nfs regression tests with this applied.  They
don't exercise the problem you were seeing, but maybe it's at least some
sort of sanity check.)

--b.

> 
> --b.
> 
> > 
> > Signed-off-by: Josef Bacik <josef@...hat.com>
> > ---
> > V1->V2:
> > -added second security_d_instantiate() call
> > 
> >  fs/dcache.c |    3 +++
> >  1 files changed, 3 insertions(+), 0 deletions(-)
> > 
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index 23702a9..119d489 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -1201,9 +1201,12 @@ struct dentry *d_obtain_alias(struct inode *inode)
> >  	spin_unlock(&tmp->d_lock);
> >  
> >  	spin_unlock(&dcache_lock);
> > +	security_d_instantiate(tmp, inode);
> >  	return tmp;
> >  
> >   out_iput:
> > +	if (res && !IS_ERR(res))
> > +		security_d_instantiate(res, inode);
> >  	iput(inode);
> >  	return res;
> >  }
> > -- 
> > 1.6.6.1
> > 
> --
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