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

[Jeff Layton]

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 at linux.ibm.com>
Signed-off-by: Jeff Layton <jlayton at 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