Re: [PATCH] fs: call security_d_instantiate in d_obtain_alias

[Josef Bacik]

On Wed, Nov 17, 2010 at 05:12:21PM -0500, Eric Paris wrote:
> On Wed, 2010-11-17 at 15:26 -0500, J. Bruce Fields wrote:
> > 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@xxxxxxxxxx>
> > > > > ---
> > > > >  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?
> 
> I only know a bit from the SELinux world and can't speak at all for any
> other LSMs.  SELinux however needs the dentry when an inode first enters
> core for a couple of reasons.  (once the inode is in core it should have
> already been initialized and we skip all this)
> 
> If you have persistent xattr support we need the dentry since the xattr
> code requires a dentry.  I have no idea why but that's what
> inode->i_op->getxattr() requires.
> 
> Then we come to procfs.  In that filesystem we actually label based on
> the pathname.  oh no did I say SELinux uses pathnames?  yes, I did, but
> we only use the part of the pathname relative to the procfs root, so
> really it's a static identifier which is immutable.
> 
> From what I can see SELinux doesn't really care about the dentry, we
> don't care about IS_ROOT() or the name or any of that crap (in the
> non-procfs case).  All we really about is that if something can find an
> inode and use it that security_d_instantiate() was called....
> 
> Calling security_d_instantiate() extra times is very low overhead and
> not harmful.  the dirtiest (but easiest I guess) fix would be to add it
> into the out_iput path as well.  I feel like there has to be an easier
> solution, I'm just not sure what it is...
>

So we're not worried about calling it multiple times, we're more worried about
somebody finding the dentry before we have a chance to run
security_d_instantiate.  Would it be ok to call security_d_instantiate() right
before we do all the actual instantiation work?  That would solve this problem.
Thanks,

Josef 
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