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

[Jeff Layton]

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?
 
> 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@xxxxxxxxxxxxxxx>