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Date:   Wed, 25 Oct 2023 16:12:57 +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 v2] ima: detect changes to the backing overlay file

On Wed, Oct 25, 2023 at 2:29 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.
>
> Instead of always re-evaluating the file's integrity, detect a change
> to the backing file, by comparing the cached file metadata with the
> backing file's metadata.  Verifying just the i_version has not changed
> is insufficient.  In addition save and compare the i_ino and i_rdev
> as well.
>
> Signed-off-by: Mimi Zohar <zohar@...ux.ibm.com>
> ---
> Changelog:
> - Changes made based on Amir's review: removal of unnecessary overlay
> magic test, verify i_version, i_ino and i_rdev haven't changed.
>
>  fs/overlayfs/super.c              |  2 +-
>  security/integrity/ima/ima_api.c  |  4 ++++
>  security/integrity/ima/ima_main.c | 16 +++++++++++++++-
>  security/integrity/integrity.h    |  2 ++
>  4 files changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 3fa2416264a4..c71d185980c0 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -1489,7 +1489,7 @@ 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..f191bdcceef8 100644
> --- a/security/integrity/ima/ima_api.c
> +++ b/security/integrity/ima/ima_api.c
> @@ -243,6 +243,7 @@ int ima_collect_measurement(struct integrity_iint_cache *iint,
>  {
>         const char *audit_cause = "failed";
>         struct inode *inode = file_inode(file);
> +       struct inode *real_inode = d_real_inode(file_dentry(file));
>         const char *filename = file->f_path.dentry->d_name.name;
>         struct ima_max_digest_data hash;
>         struct kstat stat;
> @@ -272,6 +273,7 @@ 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];
>
> @@ -302,6 +304,8 @@ int ima_collect_measurement(struct integrity_iint_cache *iint,
>         iint->ima_hash = tmpbuf;
>         memcpy(iint->ima_hash, &hash, length);
>         iint->version = i_version;

You don't have to store them if real_inode == inode.

> +       iint->real_ino = real_inode->i_ino;
> +       iint->real_rdev = real_inode->i_rdev;

iint->real_dev = real_inode->i_sb->s_dev;

i_rdev is something else.
it's the device pointed to by a blockdev/chardev inode.

Thanks,
Amir.


>
>         /* Possibly temporary failure due to type of read (eg. O_DIRECT) */
>         if (!result)
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 365db0e43d7c..4a6a22f8805b 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 +285,19 @@ static int process_measurement(struct file *file, const struct cred *cred,
>                 iint->measured_pcrs = 0;
>         }
>
> +       /* Detect and re-evaluate changes made to the backing file. */
> +       backing_inode = d_real_inode(file_dentry(file));
> +       if (backing_inode != inode &&
> +           (action & IMA_DO_MASK) && (iint->flags & IMA_DONE_MASK)) {
> +               if (!IS_I_VERSION(backing_inode) ||
> +                   backing_inode->i_rdev != iint->real_rdev ||
> +                   backing_inode->i_ino != iint->real_ino ||
> +                   !inode_eq_iversion(backing_inode, iint->version)) {
> +                       iint->flags &= ~IMA_DONE_MASK;
> +                       iint->measured_pcrs = 0;
> +               }
> +       }
> +
>         /* Determine if already appraised/measured based on bitmask
>          * (IMA_MEASURE, IMA_MEASURED, IMA_XXXX_APPRAISE, IMA_XXXX_APPRAISED,
>          *  IMA_AUDIT, IMA_AUDITED)
> diff --git a/security/integrity/integrity.h b/security/integrity/integrity.h
> index d7553c93f5c0..dd2bb2d150f6 100644
> --- a/security/integrity/integrity.h
> +++ b/security/integrity/integrity.h
> @@ -164,6 +164,8 @@ struct integrity_iint_cache {
>         unsigned long flags;
>         unsigned long measured_pcrs;
>         unsigned long atomic_flags;
> +       unsigned long real_ino;
> +       dev_t real_rdev;
>         enum integrity_status ima_file_status:4;
>         enum integrity_status ima_mmap_status:4;
>         enum integrity_status ima_bprm_status:4;
> --
> 2.39.3
>

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