Re: [PATCH] overlayfs: Trigger file re-evaluation by IMA / EVM after writes

Jeff Layton
2023-04-06T22:04:43Z

On Thu, 2023-04-06 at 17:24 -0400, Jeff Layton wrote:
> On Thu, 2023-04-06 at 16:22 -0400, Stefan Berger wrote:
> >
> > On 4/6/23 15:37, Jeff Layton wrote:
> > > On Thu, 2023-04-06 at 15:11 -0400, Stefan Berger wrote:
> > > >
> > > > On 4/6/23 14:46, Jeff Layton wrote:
> > > > > On Thu, 2023-04-06 at 17:01 +0200, Christian Brauner wrote:
> > > > > > On Thu, Apr 06, 2023 at 10:36:41AM -0400, Paul Moore wrote:
> > > >
> > > > >
> > > > > Correct. As long as IMA is also measuring the upper inode then it seems
> > > > > like you shouldn't need to do anything special here.
> > > >
> > > > Unfortunately IMA does not notice the changes. With the patch provided in the other email IMA works as expected.
> > > >
> > >
> > >
> > > It looks like remeasurement is usually done in ima_check_last_writer.
> > > That gets called from __fput which is called when we're releasing the
> > > last reference to the struct file.
> > >
> > > You've hooked into the ->release op, which gets called whenever
> > > filp_close is called, which happens when we're disassociating the file
> > > from the file descriptor table.
> > >
> > > So...I don't get it. Is ima_file_free not getting called on your file
> > > for some reason when you go to close it? It seems like that should be
> > > handling this.
> >
> > I would ditch the original proposal in favor of this 2-line patch shown here:
> >
> > https://lore.kernel.org/linux-integrity/a95f62ed-8b8a-38e5-e468-ecbde3b221af@linux.ibm.com/T/#m3bd047c6e5c8200df1d273c0ad551c645dd43232
> >
> >
>
> Ok, I think I get it. IMA is trying to use the i_version from the
> overlayfs inode.
>
> I suspect that the real problem here is that IMA is just doing a bare
> inode_query_iversion. Really, we ought to make IMA call
> vfs_getattr_nosec (or something like it) to query the getattr routine in
> the upper layer. Then overlayfs could just propagate the results from
> the upper layer in its response.
>
> That sort of design may also eventually help IMA work properly with more
> exotic filesystems, like NFS or Ceph.
>
>
>

Maybe something like this? It builds for me but I haven't tested it. It
looks like overlayfs already should report the upper layer's i_version
in getattr, though I haven't tested that either:

-----------------------8<---------------------------

[PATCH] IMA: use vfs_getattr_nosec to get the i_version

IMA currently accesses the i_version out of the inode directly when it
does a measurement. This is fine for most simple filesystems, but can be
problematic with more complex setups (e.g. overlayfs).

Make IMA instead call vfs_getattr_nosec to get this info. This allows
the filesystem to determine whether and how to report the i_version, and
should allow IMA to work properly with a broader class of filesystems in
the future.

Reported-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
security/integrity/ima/ima_api.c | 9 ++++++---
security/integrity/ima/ima_main.c | 12 ++++++++----
2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/security/integrity/ima/ima_api.c b/security/integrity/ima/ima_api.c
index d3662f4acadc..c45902e72044 100644
--- a/security/integrity/ima/ima_api.c
+++ b/security/integrity/ima/ima_api.c
@@ -13,7 +13,6 @@
#include <linux/fs.h>
#include <linux/xattr.h>
#include <linux/evm.h>
-#include <linux/iversion.h>
#include <linux/fsverity.h>

#include "ima.h"
@@ -246,10 +245,11 @@ int ima_collect_measurement(struct integrity_iint_cache *iint,
struct inode *inode = file_inode(file);
const char *filename = file->f_path.dentry->d_name.name;
struct ima_max_digest_data hash;
+ struct kstat stat;
int result = 0;
int length;
void *tmpbuf;
- u64 i_version;
+ u64 i_version = 0;

/*
* Always collect the modsig, because IMA might have already collected
@@ -268,7 +268,10 @@ int ima_collect_measurement(struct integrity_iint_cache *iint,
* to an initial measurement/appraisal/audit, but was modified to
* assume the file changed.
*/
- i_version = inode_query_iversion(inode);
+ result = vfs_getattr_nosec(&file->f_path, &stat, STATX_CHANGE_COOKIE,
+ AT_STATX_SYNC_AS_STAT);
+ if (!result && (stat.result_mask & STATX_CHANGE_COOKIE))
+ i_version = stat.change_cookie;
hash.hdr.algo = algo;
hash.hdr.length = hash_digest_size[algo];

diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index d66a0a36415e..365db0e43d7c 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -24,7 +24,6 @@
#include <linux/slab.h>
#include <linux/xattr.h>
#include <linux/ima.h>
-#include <linux/iversion.h>
#include <linux/fs.h>

#include "ima.h"
@@ -164,11 +163,16 @@ static void ima_check_last_writer(struct integrity_iint_cache *iint,

mutex_lock(&iint->mutex);
if (atomic_read(&inode->i_writecount) == 1) {
+ struct kstat stat;
+
update = test_and_clear_bit(IMA_UPDATE_XATTR,
&iint->atomic_flags);
- if (!IS_I_VERSION(inode) ||
- !inode_eq_iversion(inode, iint->version) ||
- (iint->flags & IMA_NEW_FILE)) {
+ if ((iint->flags & IMA_NEW_FILE) ||
+ vfs_getattr_nosec(&file->f_path, &stat,
+ STATX_CHANGE_COOKIE,
+ AT_STATX_SYNC_AS_STAT) ||
+ !(stat.result_mask & STATX_CHANGE_COOKIE) ||
+ stat.change_cookie != iint->version) {
iint->flags &= ~(IMA_DONE_MASK | IMA_NEW_FILE);
iint->measured_pcrs = 0;
if (update)
--
2.39.2