Re: [Linux-ima-devel] [PATCH v4 2/5] ima: use fs method to read integrity data [updated]

[Mimi Z. <zo...@li...>]

On Thu, 2017-08-03 at 12:56 +0200, Jan Kara wrote:
> On Wed 02-08-17 13:11:52, Mimi Zohar wrote:
> > On Wed, 2017-08-02 at 10:01 +0200, Jan Kara wrote:
> > > On Tue 01-08-17 16:24:30, Mimi Zohar wrote:
> > > > From: Christoph Hellwig <hc...@ls...>
> > > > 
> > > > Add a new ->integrity_read file operation to read data for integrity
> > > > hash collection.  This is defined to be equivalent to ->read_iter,
> > > > except that it will be called with the i_rwsem held exclusively.
> > > > 
> > > > Signed-off-by: Christoph Hellwig <hc...@ls...>
> > > > Cc: Matthew Garrett <mat...@ne...>
> > > > Cc: Jan Kara <ja...@su...>
> > > > Cc: "Theodore Ts'o" <ty...@mi...>
> > > > Cc: Andreas Dilger <adi...@di...>
> > > > Cc: Jaegeuk Kim <ja...@ke...>
> > > > Cc: Chao Yu <yu...@hu...>
> > > > Cc: Steven Whitehouse <swh...@re...>
> > > > Cc: Bob Peterson <rpe...@re...>
> > > > Cc: David Woodhouse <dw...@in...>
> > > > Cc: Dave Kleikamp <sh...@ke...>
> > > > Cc: Ryusuke Konishi <kon...@la...>
> > > > Cc: Mark Fasheh <mf...@ve...>
> > > > Cc: Joel Becker <jl...@ev...>
> > > > Cc: Richard Weinberger <ri...@no...>
> > > > Cc: "Darrick J. Wong" <dar...@or...>
> > > > Cc: Hugh Dickins <hu...@go...>
> > > > Cc: Chris Mason <cl...@fb...>
> > > > Signed-off-by: Mimi Zohar <zo...@li...>
> > > 
> > > ...
> > > 
> > > > +static ssize_t ext4_file_integrity_read_iter(struct kiocb *iocb,
> > > > +					     struct iov_iter *to)
> > > > +{
> > > > +	struct inode *inode = file_inode(iocb->ki_filp);
> > > > +	int o_direct = iocb->ki_flags & IOCB_DIRECT;
> > > > +
> > > > +	lockdep_assert_held(&inode->i_rwsem);
> > > > +	if (unlikely(ext4_forced_shutdown(EXT4_SB(inode->i_sb))))
> > > > +		return -EIO;
> > > > +
> > > > +	if (!iov_iter_count(to))
> > > > +		return 0; /* skip atime */
> > > > +
> > > > +#ifdef CONFIG_FS_DAX
> > > > +	if (IS_DAX(inode))
> > > > +		return dax_iomap_rw(iocb, to, &ext4_iomap_ops);
> > > > +#endif
> > > > +	if (o_direct)
> > > > +		return -EINVAL;
> > > > +	return generic_file_read_iter(iocb, to);
> > > > +}
> > > 
> > > I have noticed this o_direct check - why is it only in ext4 and shouldn't
> > > rather higher layers make sure IOCB_DIRECT iocbs cannot reach
> > > .integrity_read() methods?
> >  
> > This failure happens when opening a file with O_DIRECT on a block
> > device that does not support dax (eg. loop). xfs makes it to here too,
> > but the call to generic_file_read_iter() fails properly with -EINVAL. 
> > (Only tested on those filesystems included that support dax (eg. ext2,
> > ext4, and xfs).)
> 
> Well, yes, ext2 and ext4 will silently fall back to buffered read (as do
> pretty much all filesystems except for XFS). However I fail to see why IMA
> should care (which is probably due to my lack of knowledge about IMA).
> Is O_DIRECT somehow excepted from IMA? But then why it is not for DAX?

Thank you for the explanation!  (I was confused about the relationship
between O_DIRECT and DAX.)  You're correct.  IMA does not support
O_DIRECT in the buffered case for two reasons, locking and updating
the file hash, which are described in commit f9b2a735bddd "ima: audit
log files opened with O_DIRECT flag".  After reverting this commit,
the O_DIRECT check is needed before calling generic_file_read_iter().
 Most likely the same would need to be done for other filesystems that
support O_DIRECT.  Probably a generic_integrity_file_read_iter()
should be defined.
 
For DAX, unlike do_blockdev_direct_IO() which takes the lock, reading
the file with O_DIRECT is fine, as dax_iomap_rw() only checks that the
lock has been taken.  Assuming the file system is mounted with
i_version, the file hash is updated properly.

Mimi