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

[Jeff Layton]

On Thu, 2017-07-06 at 17:00 -0400, Mimi Zohar wrote:
> Hi Jeff,
> 
> On Thu, 2017-07-06 at 12:23 -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 really be gated on IS_I_VERSION. When that returns
> > false, you can't rely on the i_version field changing like you expect.
> > 
> > Have the code also track and check the ctime for the file. If the
> > IS_I_VERSION returns false, then use it to detect whether the file
> > might have changed.
> > 
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> 
> Prior to IMA being upstreamed, we used mtime to determine when a file
> changed.  At that time, Andrew Morton suggested using i_version.  Is
> there a specific filesystem that you are interested in that doesn't
> have i_version support?
> 

No, I just noticed this by inspection. It's just that very few of them
actually do support i_version properly. Only the ones that set
MS_I_VERSION do so. Filesystems that don't set it often don't bump it at
all.

> Assuming there is a valid reason for adding this support, the check
> should be based on mtime, not ctime.  ctime includes file metadata
> changes, not only file data changes, which would result in re-
> calculating the file hash unnecessarily.
> 

i_version is also bumped on metadata changes. It was initially added to
allow for a proper change counter for NFS, which requires that it also
be bumped when the metadata changes.

> Mimi   
> 
> > ---
> >  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(-)
> > 
> > diff --git a/security/integrity/ima/ima_api.c b/security/integrity/ima/ima_api.c
> > index c2edba8de35e..2a01e8f3c613 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_ctime = inode->i_ctime;
> > 
> >  		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->ctime = i_ctime;
> >  				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..3508b13e7181 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->ctime.tv_sec != inode->i_ctime.tv_sec)
> > +			return true;
> > +		if (iint->ctime.tv_nsec != inode->i_ctime.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..10c3f96beb98 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 ctime;	/* track inode changes */
> >  	unsigned long flags;
> >  	unsigned long measured_pcrs;
> >  	enum integrity_status ima_file_status:4;
> 
> 

-- 
Jeff Layton <jlayton@xxxxxxxxxx>