On Tue, Oct 24, 2023 at 3:09 PM Mimi Zohar <zohar@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> ---
> 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.
