Re: [PATCH] Subject: [PATCH] xfs: fix sgid inheritance for subdirectories inheriting default acls [V3]

[Ben Myers <bpm@sgi.com>]

On Wed, Jul 10, 2013 at 10:22:58AM -0300, Carlos Maiolino wrote:
> On Tue, Jul 09, 2013 at 12:06:08PM -0500, Ben Myers wrote:
> > On Tue, Jul 09, 2013 at 09:42:35AM +1000, Dave Chinner wrote:
> > > On Mon, Jul 08, 2013 at 04:11:21PM -0500, Ben Myers wrote:
> > > > On Fri, Jun 21, 2013 at 02:45:53PM -0300, Carlos Maiolino wrote:
> > > > > XFS removes sgid bits of subdirectories under a directory containing a default
> > > > > acl.
> > > > > 
> > > > > When a default acl is set, it implies xfs to call xfs_setattr_nonsize() in its
> > > > > code path. Such function is shared among mkdir and chmod system calls, and
> > > > > does some checks unneeded by mkdir (calling inode_change_ok()). Such checks
> > > > > remove sgid bit from the inode after it has been granted.
> > > > > 
> > > > > With this patch, we extend the meaning of XFS_ATTR_NOACL flag to avoid these
> > > > > checks when acls are being inherited (thanks hch).
> > > > > 
> > > > > Also, xfs_setattr_mode, doesn't need to re-check for group id and capabilities
> > > > > permissions, this only implies in another try to remove sgid bit from the
> > > > > directories. Such check is already done either on inode_change_ok() or
> > > > > xfs_setattr_nonsize().
> > > > > 
> > > > > Changelog:
> > > > > 
> > > > > V2: Extends the meaning of XFS_ATTR_NOACL instead of wrap the tests into another
> > > > >     function
> > > > > 
> > > > > V3: Remove S_ISDIR check in xfs_setattr_nonsize() from the patch
> > > > > 
> > > > > Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
> > > > >  
> > > > > -	if (mp->m_flags & XFS_MOUNT_RDONLY)
> > > > > -		return XFS_ERROR(EROFS);
> > > > > +	/* If acls are being inherited, we already have this checked */
> > > > > +	if (!(flags & XFS_ATTR_NOACL)) {
> > > > > +		if (mp->m_flags & XFS_MOUNT_RDONLY)
> > > > > +			return XFS_ERROR(EROFS);
> > > > >  
> > > > > -	if (XFS_FORCED_SHUTDOWN(mp))
> > > > > -		return XFS_ERROR(EIO);
> > > > > +		if (XFS_FORCED_SHUTDOWN(mp))
> > > > > +			return XFS_ERROR(EIO);
> > > > >  
> > > > > -	error = -inode_change_ok(inode, iattr);
> > > > > -	if (error)
> > > > > -		return XFS_ERROR(error);
> > > > > +		error = -inode_change_ok(inode, iattr);
> > > > > +		if (error)
> > > > > +			return XFS_ERROR(error);
> > > > > +	}
> > > > 
> > > > I'm not so sure about this change yet.  Looks like the two relevant callers are:
> > > > 
> > > > .set - xattr_handler
> > > >   xfs_xattr_acl_set
> > > >     xfs_set_mode
> > > >       xfs_setattr_nonsize(..., XFS_ATTR_NOACL);
> > > > 
> > > > and
> > > > 
> > > > xfs_vn_mknod
> > > >   xfs_inherit_acl
> > > >     xfs_set_mode
> > > >       xfs_setattr_nonsize(..., XFS_ATTR_NOACL);
> > > > 
> > > > I suggest moving the forced shutdown and readonly checks outside of the
> > > > XFS_ATTR_NOACL conditional.  I'm not seeing those checks in xfs_attr_acl_set or
> > > > xfs_vn_mknod and it won't hurt to be careful.
> > > 
> > > In both cases, the read-only checks are done at much higher layers
> > > and so we don't ever get to xfs_setattr_nonsize() through these
> > > paths with a read-only filesystem. Shutdown doesn't really matter -
> > > the transaction commit will fail if the filesystem is shut down...
> > > 
> > > > It also seems like inode_change_ok might have some other checks that are
> > > > necessary to determine whether it is ok to update the mode and ctime here.  A
> > > > call to inode_owner_or_capable as is done in inode_change_ok would cover this
> > > > possibility.
> > > 
> > > The inode permission checks are already done by xfs_xattr_acl_set():
> > > 
> > > 	if ((current_fsuid() != inode->i_uid) && !capable(CAP_FOWNER))
> > > 		return -EPERM;
> > > 
> > > and in the case of xfs_inherit_acl() the user has just created the
> > > file so they - by definition - have permission to inherit the ACL
> > > and modify the mode of the inode they just created.
> > > 
> > > So there is no need for changes to inode_change_ok() here.
> > 
> > Carlos, if you agree with Dave's assessment consider this 
> > 
> > Reviewed-by: Ben Myers <bpm@sgi.com>
> > 
> > let me know what you think and I'll pull it in.
> > 
> > Thanks,
> > 	Ben
> > 
> Hi Ben,
> 
> Yes, I agree with Dave's statements, I removed the unneeded checks, because they
> were done at higher layers.

Excellent.

> please, feel free to pull it in, my apologies to have not answered it on IRC, it
> was holiday here and I just saw your message late this morning.

No worries.  ;)

I've pulled this in.

Thanks,
Ben

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