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

[Stefan Berger]

On 4/6/23 18:04, Jeff Layton wrote:
> 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)

I tested this in the OpenBMC setup with overlayfs acting as rootfs. It works now as expected.

Tested-by: Stefan Berger <stefanb at linux.ibm.com>