Re: [PATCH v2] integrity: track mtime in addition to i_version for assessment

[Jeff Layton]

On Fri, 2017-07-07 at 15:59 -0400, Mimi Zohar wrote:
> On Fri, 2017-07-07 at 13:49 -0400, Jeff Layton wrote:
> > On Fri, 2017-07-07 at 13:24 -0400, Mimi Zohar wrote:
> > > On Fri, 2017-07-07 at 12:57 -0400, Jeff Layton wrote:
> > > > On Fri, 2017-07-07 at 10:05 -0400, Jeff Layton wrote:
> > > > > From: Jeff Layton <jlayton@xxxxxxxxxx>
> > > > > 
> > > > > The IMA assessment code tries to use the i_version counter to detect
> > > > > when changes to a file have occurred. Many filesystems don't increment
> > > > > it properly (or at all) so detecting changes with that is not always
> > > > > reliable.
> > > > > 
> > > > > That check should be gated on IS_I_VERSION, as you can't rely on the
> > > > > i_version field changing unless that returns true.
> > > > > 
> > > > > Have the code also track and check the mtime for the file. If the
> > > > > IS_I_VERSION returns false, then use it to detect whether the file's
> > > > > contents might have changed.
> > > > > 
> > > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > > > ---
> > > > >  security/integrity/ima/ima_api.c  |  4 +++-
> > > > >  security/integrity/ima/ima_main.c | 32 ++++++++++++++++++++++++--------
> > > > >  security/integrity/integrity.h    |  1 +
> > > > >  3 files changed, 28 insertions(+), 9 deletions(-)
> > > > > 
> > > > > v2: switch to storing/checking mtime instead of ctime
> > > > > 
> > > > 
> > > > To be clear here, I don't have a large interest in IMA, but I am looking
> > > > at making changes to how the i_version counter is handled. IMA's use of
> > > > it is problematic for some of those changes (and somewhat sketchy).
> > > > 
> > > > I think you either want something like the patch below, or you need to
> > > > somehow ensure that you're not doing any of this on a superblock that
> > > > doesn't have MS_I_VERSION set on it.
> > > > 
> > > > I'm not that familiar with IMA in general though, so it's possible I'm
> > > > missing something. Is that already being done somehow?
> > > 
> > > Before reverting to using mtime, which wasn't fine grained enough at
> > > the time, it would be helpful to first understand the type of changes
> > > and the reasons for the changes you're looking to make to i_version.
> > 
> > Sure, I posted a patchset back in December, actually:
> > 
> >     [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization
> 
> Thanks you.
> 
> > The basic idea is to improve performance on filesystems that implement
> > the i_version counter by allowing them to optimize away metadata updates
> > that are due solely to i_version counter change when no one is actually
> > using it. This will also pave the way to allow us to more reasonably
> > provide an i_version counter on network filesystems and the like that
> > might have weaker consistency guarantees than a local fs.
> > Your point about mtime not being granular enough is valid however. It's
> > certainly possible for extra writes to race in during the jiffy or so
> > window that represents the mtime resolution. It's just that that is
> > still more granular than you'll get on a filesystem that never actually
> > increments the i_version counter on a write.
> > >  After all, i_version has been working all this time (~2009).
> > > 
> > 
> > The i_version counter works just fine on filesystems that implement it
> > properly. That's a very short list: xfs, btrfs, and ext4 for local
> > filesystems. NFS and AFS would also likely be fine here, though they
> > don't set MS_I_VERSION.
> > The rest though do not support it consistently and IMA should not be
> > relying on it on them. This is why the kernel nfs server only relies on
> > the i_version field when IS_I_VERSION returns true.
> > 
> > I'll ask again -- is IMA somehow limited only being used only on that
> > subset of filesystems? My guess from a glance at the integrity_read
> > patchset is that it is not.
> 
> Our main use case scenario is verifying the integrity of files,
> updating the file hash for mutable files, and maintaining a
> measurement list, including re-measurement of files that have changed,
> on local file systems.  Without being in the file write path (or more
> precisely __fput), there are no guarantees of file change
> notification.
> 
> For file systems which do not support i_version, we are limited to an
> initial file integrity verification and measurement.

How is your typical user to know whether to expect this guarantee from
the filesystem?

> Mimi
> 
> > > > > diff --git a/security/integrity/ima/ima_api.c b/security/integrity/ima/ima_api.c
> > > > > index c2edba8de35e..b8d746bbc43d 100644
> > > > > --- a/security/integrity/ima/ima_api.c
> > > > > +++ b/security/integrity/ima/ima_api.c
> > > > > @@ -205,7 +205,8 @@ int ima_collect_measurement(struct integrity_iint_cache *iint,
> > > > >  	} hash;
> > > > >  
> > > > >  	if (!(iint->flags & IMA_COLLECTED)) {
> > > > > -		u64 i_version = file_inode(file)->i_version;
> > > > > +		u64 i_version = inode->i_version;
> > > > > +		struct timespec i_mtime = inode->i_mtime;
> > > > >  
> > > > >  		if (file->f_flags & O_DIRECT) {
> > > > >  			audit_cause = "failed(directio)";
> > > > > @@ -225,6 +226,7 @@ int ima_collect_measurement(struct integrity_iint_cache *iint,
> > > > >  				iint->ima_hash = tmpbuf;
> > > > >  				memcpy(iint->ima_hash, &hash, length);
> > > > >  				iint->version = i_version;
> > > > > +				iint->mtime = i_mtime;
> > > > >  				iint->flags |= IMA_COLLECTED;
> > > > >  			} else
> > > > >  				result = -ENOMEM;
> > > > > diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> > > > > index 2aebb7984437..8d12ef2d3ba2 100644
> > > > > --- a/security/integrity/ima/ima_main.c
> > > > > +++ b/security/integrity/ima/ima_main.c
> > > > > @@ -113,6 +113,25 @@ static void ima_rdwr_violation_check(struct file *file,
> > > > >  				  "invalid_pcr", "open_writers");
> > > > >  }
> > > > >  
> > > > > +static bool ima_should_update_iint(struct integrity_iint_cache *iint,
> > > > > +				struct inode *inode)
> > > > > +{
> > > > > +	if (atomic_read(&inode->i_writecount) != 1)
> > > > > +		return false;
> > > > > +	if (iint->flags & IMA_NEW_FILE)
> > > > > +		return true;
> > > > > +	if (IS_I_VERSION(inode)) {
> > > > > +		if (iint->version != inode->i_version)
> > > > > +			return true;
> > > > > +	} else {
> > > > > +		if (iint->mtime.tv_sec != inode->i_mtime.tv_sec)
> > > > > +			return true;
> > > > > +		if (iint->mtime.tv_nsec != inode->i_mtime.tv_nsec)
> > > > > +			return true;
> > > > > +	}
> > > > > +	return false;
> > > > > +}
> > > > > +
> > > > >  static void ima_check_last_writer(struct integrity_iint_cache *iint,
> > > > >  				  struct inode *inode, struct file *file)
> > > > >  {
> > > > > @@ -122,14 +141,11 @@ static void ima_check_last_writer(struct integrity_iint_cache *iint,
> > > > >  		return;
> > > > >  
> > > > >  	inode_lock(inode);
> > > > > -	if (atomic_read(&inode->i_writecount) == 1) {
> > > > > -		if ((iint->version != inode->i_version) ||
> > > > > -		    (iint->flags & IMA_NEW_FILE)) {
> > > > > -			iint->flags &= ~(IMA_DONE_MASK | IMA_NEW_FILE);
> > > > > -			iint->measured_pcrs = 0;
> > > > > -			if (iint->flags & IMA_APPRAISE)
> > > > > -				ima_update_xattr(iint, file);
> > > > > -		}
> > > > > +	if (ima_should_update_iint(iint, inode)) {
> > > > > +		iint->flags &= ~(IMA_DONE_MASK | IMA_NEW_FILE);
> > > > > +		iint->measured_pcrs = 0;
> > > > > +		if (iint->flags & IMA_APPRAISE)
> > > > > +			ima_update_xattr(iint, file);
> > > > >  	}
> > > > >  	inode_unlock(inode);
> > > > >  }
> > > > > diff --git a/security/integrity/integrity.h b/security/integrity/integrity.h
> > > > > index a53e7e4ab06c..61fffa7583bf 100644
> > > > > --- a/security/integrity/integrity.h
> > > > > +++ b/security/integrity/integrity.h
> > > > > @@ -102,6 +102,7 @@ struct integrity_iint_cache {
> > > > >  	struct rb_node rb_node;	/* rooted in integrity_iint_tree */
> > > > >  	struct inode *inode;	/* back pointer to inode in question */
> > > > >  	u64 version;		/* track inode changes */
> > > > > +	struct timespec mtime;	/* track inode changes */
> > > > >  	unsigned long flags;
> > > > >  	unsigned long measured_pcrs;
> > > > >  	enum integrity_status ima_file_status:4;
> > > 
> > > 
> 
> 

-- 
Jeff Layton <jlayton@xxxxxxxxxx>