Re: [PATCH 3/5] ima: Reset EVM status upon detecting changes to overlay backing file

[Amir Goldstein]

On Tue, Jan 30, 2024 at 11:46 PM Stefan Berger <stefanb@xxxxxxxxxxxxx> wrote:
>
> To avoid caching effects to take effect reset the EVM status upon
> detecting changes to the overlay backing files. This prevents a not-yet-
> copied-up file on the overlay from executing if for example the
> security.evm xattr on the file on the 'lower' layer has been removed.
>

And what is expected to happen when file is executed after copy up?
Doesn't this change also protect the same threat after copy up?

> Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> ---
>  include/linux/evm.h               | 8 ++++++++
>  security/integrity/evm/evm_main.c | 7 +++++++
>  security/integrity/ima/ima_main.c | 2 ++
>  3 files changed, 17 insertions(+)
>
> diff --git a/include/linux/evm.h b/include/linux/evm.h
> index d8c0343436b8..e7d6742eee9d 100644
> --- a/include/linux/evm.h
> +++ b/include/linux/evm.h
> @@ -66,6 +66,8 @@ extern int evm_protected_xattr_if_enabled(const char *req_xattr_name);
>  extern int evm_read_protected_xattrs(struct dentry *dentry, u8 *buffer,
>                                      int buffer_size, char type,
>                                      bool canonical_fmt);
> +extern void evm_reset_cache_status(struct dentry *dentry,
> +                                  struct integrity_iint_cache *iint);
>  #ifdef CONFIG_FS_POSIX_ACL
>  extern int posix_xattr_acl(const char *xattrname);
>  #else
> @@ -189,5 +191,11 @@ static inline int evm_read_protected_xattrs(struct dentry *dentry, u8 *buffer,
>         return -EOPNOTSUPP;
>  }
>
> +static inline void evm_reset_cache_status(struct dentry *dentry,
> +                                         struct integrity_iint_cache *iint)
> +{
> +       return;
> +}
> +
>  #endif /* CONFIG_EVM */
>  #endif /* LINUX_EVM_H */
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 22a5e26860ea..e96d127b48a2 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -721,6 +721,13 @@ static void evm_reset_status(struct inode *inode)
>                 iint->evm_status = INTEGRITY_UNKNOWN;
>  }
>
> +void evm_reset_cache_status(struct dentry *dentry,
> +                           struct integrity_iint_cache *iint)
> +{
> +       if (d_real_inode(dentry) != d_backing_inode(dentry))
> +               iint->evm_status = INTEGRITY_UNKNOWN;
> +}
> +
>  /**
>   * evm_revalidate_status - report whether EVM status re-validation is necessary
>   * @xattr_name: pointer to the affected extended attribute name
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index cc1217ac2c6f..84bdc6e58329 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -26,6 +26,7 @@
>  #include <linux/ima.h>
>  #include <linux/fs.h>
>  #include <linux/iversion.h>
> +#include <linux/evm.h>
>
>  #include "ima.h"
>
> @@ -295,6 +296,7 @@ static int process_measurement(struct file *file, const struct cred *cred,
>                     !inode_eq_iversion(backing_inode, iint->version)) {
>                         iint->flags &= ~IMA_DONE_MASK;
>                         iint->measured_pcrs = 0;
> +                       evm_reset_cache_status(file_dentry(file), iint);
>                 }
>         }

Make sense.
Unrelated to your change, I now noticed something odd about Mimi's change:

        backing_inode = d_real_inode(file_dentry(file));

I find the choice of variable name to be quite confusing, because ima/evm code
uses  d_backing_inode() all over the place and d_real_inode() !=
d_backing_inode().

First of all, there is never any reason to use d_backing_inode() and its name is
quite confusing in the first place, but it will be a big cleanup to
remove them all.

Suggest to rename the variable to real_inode, same as in
ima_collect_measurement()
to be consistent and reduce confusion factor, which is already high enough ;)

Thanks,
Amir.