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Date:   Tue, 24 Oct 2023 15:53:24 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        Raul Rangel <rrangel@...omium.org>,
        Eric Snowberg <eric.snowberg@...cle.com>
Subject: Re: [PATCH] ima: detect changes to the backing overlay file

On Tue, Oct 24, 2023 at 3:09 PM Mimi Zohar <zohar@...ux.ibm.com> wrote:
>
> Commit 18b44bc5a672 ("ovl: Always reevaluate the file signature for
> IMA") forced signature re-evaulation on every file access.
>
> IMA does not detect changes made to the backing overlay file.  Instead
> of always re-evaluating the file's integrity, detect a change to the
> backing overlay file, by comparing the i_version, as stored in the iint,
> with the backing file's i_version.
>
> Signed-off-by: Mimi Zohar <zohar@...ux.ibm.com>
> ---
>  fs/overlayfs/super.c              |  2 +-
>  security/integrity/ima/ima_api.c  |  1 +
>  security/integrity/ima/ima_main.c | 21 ++++++++++++++++++++-
>  3 files changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index def266b5e2a3..4d9137ba2293 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -1482,7 +1482,7 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
>                 ovl_trusted_xattr_handlers;
>         sb->s_fs_info = ofs;
>         sb->s_flags |= SB_POSIXACL;
> -       sb->s_iflags |= SB_I_SKIP_SYNC | SB_I_IMA_UNVERIFIABLE_SIGNATURE;
> +       sb->s_iflags |= SB_I_SKIP_SYNC;
>
>         err = -ENOMEM;
>         root_dentry = ovl_get_root(sb, ctx->upper.dentry, oe);
> diff --git a/security/integrity/ima/ima_api.c b/security/integrity/ima/ima_api.c
> index 452e80b541e5..d1c718ef9295 100644
> --- a/security/integrity/ima/ima_api.c
> +++ b/security/integrity/ima/ima_api.c
> @@ -272,6 +272,7 @@ int ima_collect_measurement(struct integrity_iint_cache *iint,
>                                    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 365db0e43d7c..7c8aac81d16e 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -25,6 +25,7 @@
>  #include <linux/xattr.h>
>  #include <linux/ima.h>
>  #include <linux/fs.h>
> +#include <linux/iversion.h>
>
>  #include "ima.h"
>
> @@ -207,7 +208,7 @@ static int process_measurement(struct file *file, const struct cred *cred,
>                                u32 secid, char *buf, loff_t size, int mask,
>                                enum ima_hooks func)
>  {
> -       struct inode *inode = file_inode(file);
> +       struct inode *backing_inode, *inode = file_inode(file);
>         struct integrity_iint_cache *iint = NULL;
>         struct ima_template_desc *template_desc = NULL;
>         char *pathbuf = NULL;
> @@ -284,6 +285,24 @@ static int process_measurement(struct file *file, const struct cred *cred,
>                 iint->measured_pcrs = 0;
>         }
>
> +       /*
> +        * IMA does not detect changes made to the backing overlay file.
> +        * If the backing file's i_version is greater than the overlay
> +        * file's i_version or the backing file doesn't support iversion,
> +        * clear the cache to force the file's integrity to be re-evaluated.
> +        */
> +       if (inode->i_sb->s_magic == 0x794c7630 &&

Please don't use magic to detect overlayfs.
Overlayfs is not the only fs that can have a backing file
and this test is not needed.

> +           (action & IMA_DO_MASK) && (iint->flags & IMA_DONE_MASK)) {
> +               backing_inode = d_real_inode(file_dentry(file));
> +               if (backing_inode != inode) {

This test is sufficient, because for regular files of filesystems
without ->d_real(),
backing_inode will always be == inode.

> +                       if (!IS_I_VERSION(backing_inode) ||
> +                           inode_peek_iversion(backing_inode) > iint->version) {


IIUC, this checking > iint->version is not enough.
The reason is that you could have recorded iint->version from i_version
of a lower file, after open for write, lower file gets copied up to upper file,
which is a completely different backing_inode, so there is no guarantee
that the copy up will not result in a smaller or even equal i_version.

For a complete solution, you will need to store the backing_inode
pointer or backing dev/ino in iint to make sure that backing_inode did not
change.

Thanks,
Amir.

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