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

Mimi Zohar
2017-07-06T21:00:48Z

Hi Jeff,

On Thu, 2017-07-06 at 12:23 -0400, Jeff Layton wrote:
> From: Jeff Layton <jlayton@redhat.com>
>
> 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@redhat.com>

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?

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.

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;