Re: [PATCH v9 3/4] evm: Align evm_inode_init_security() definition with LSM infrastructure

[Paul Moore]

On Wed, Mar 29, 2023 at 9:05 AM Roberto Sassu
<roberto.sassu@xxxxxxxxxxxxxxx> wrote:
>
> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
>
> Change the evm_inode_init_security() definition to align with the LSM
> infrastructure. Keep the existing behavior of including in the HMAC
> calculation only the first xattr provided by LSMs.
>
> Changing the evm_inode_init_security() definition requires passing the
> xattr array allocated by security_inode_init_security(), and the number of
> xattrs filled by previously invoked LSMs.
>
> Use the newly introduced lsm_find_xattr_slot() to position EVM correctly in
> the xattrs array, like a regular LSM, and to increment the number of filled
> slots. For now, the LSM infrastructure allocates enough xattrs slots to
> store the EVM xattr, without using the reservation mechanism.
>
> Finally, make evm_inode_init_security() return value compatible with the
> inode_init_security hook conventions, i.e. return -EOPNOTSUPP if it is not
> setting an xattr.
>
> EVM is a bit tricky, because xattrs is both an input and an output. If it
> was just output, EVM should have returned zero if xattrs is NULL. But,
> since xattrs is also input, EVM is unable to do its calculations, so return
> -EOPNOTSUPP and handle this error in security_inode_init_security().

I don't quite understand why EVM would return EOPNOTSUPP if it is
enabled but there are not xattrs to measure.  It seems like EVM should
return success/0 in the no-xattr case; there were no xattrs to
measure, so it succeeded in measuring nothing.  Am I missing
something?

> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> ---
>  include/linux/evm.h               | 14 ++++++++------
>  security/integrity/evm/evm_main.c | 18 +++++++++++-------
>  security/security.c               |  6 +++---
>  3 files changed, 22 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/evm.h b/include/linux/evm.h
> index 7dc1ee74169..3c0e8591b69 100644
> --- a/include/linux/evm.h
> +++ b/include/linux/evm.h
> @@ -56,9 +56,10 @@ static inline void evm_inode_post_set_acl(struct dentry *dentry,
>  {
>         return evm_inode_post_setxattr(dentry, acl_name, NULL, 0);
>  }
> -extern int evm_inode_init_security(struct inode *inode,
> -                                  const struct xattr *xattr_array,
> -                                  struct xattr *evm);
> +extern int evm_inode_init_security(struct inode *inode, struct inode *dir,
> +                                  const struct qstr *qstr,
> +                                  struct xattr *xattrs,
> +                                  int *num_filled_xattrs);
>  extern bool evm_revalidate_status(const char *xattr_name);
>  extern int evm_protected_xattr_if_enabled(const char *req_xattr_name);
>  extern int evm_read_protected_xattrs(struct dentry *dentry, u8 *buffer,
> @@ -157,9 +158,10 @@ static inline void evm_inode_post_set_acl(struct dentry *dentry,
>         return;
>  }
>
> -static inline int evm_inode_init_security(struct inode *inode,
> -                                         const struct xattr *xattr_array,
> -                                         struct xattr *evm)
> +static inline int evm_inode_init_security(struct inode *inode, struct inode *dir,
> +                                         const struct qstr *qstr,
> +                                         struct xattr *xattrs,
> +                                         int *num_filled_xattrs)
>  {
>         return 0;
>  }
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index cf24c525558..9e75759150c 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -21,6 +21,7 @@
>  #include <linux/evm.h>
>  #include <linux/magic.h>
>  #include <linux/posix_acl_xattr.h>
> +#include <linux/lsm_hooks.h>
>
>  #include <crypto/hash.h>
>  #include <crypto/hash_info.h>
> @@ -864,23 +865,26 @@ void evm_inode_post_setattr(struct dentry *dentry, int ia_valid)
>  /*
>   * evm_inode_init_security - initializes security.evm HMAC value
>   */
> -int evm_inode_init_security(struct inode *inode,
> -                                const struct xattr *lsm_xattr,
> -                                struct xattr *evm_xattr)
> +int evm_inode_init_security(struct inode *inode, struct inode *dir,
> +                           const struct qstr *qstr, struct xattr *xattrs,
> +                           int *num_filled_xattrs)
>  {
>         struct evm_xattr *xattr_data;
> +       struct xattr *evm_xattr;
>         int rc;
>
> -       if (!(evm_initialized & EVM_INIT_HMAC) ||
> -           !evm_protected_xattr(lsm_xattr->name))
> -               return 0;
> +       if (!(evm_initialized & EVM_INIT_HMAC) || !xattrs ||
> +           !evm_protected_xattr(xattrs->name))
> +               return -EOPNOTSUPP;
> +
> +       evm_xattr = lsm_find_xattr_slot(xattrs, num_filled_xattrs);
>
>         xattr_data = kzalloc(sizeof(*xattr_data), GFP_NOFS);
>         if (!xattr_data)
>                 return -ENOMEM;
>
>         xattr_data->data.type = EVM_XATTR_HMAC;
> -       rc = evm_init_hmac(inode, lsm_xattr, xattr_data->digest);
> +       rc = evm_init_hmac(inode, xattrs, xattr_data->digest);
>         if (rc < 0)
>                 goto out;
>
> diff --git a/security/security.c b/security/security.c
> index be33d643a81..22ab4fb7ebf 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1674,9 +1674,9 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
>         if (!num_filled_xattrs)
>                 goto out;
>
> -       ret = evm_inode_init_security(inode, new_xattrs,
> -                                     new_xattrs + num_filled_xattrs);
> -       if (ret)
> +       ret = evm_inode_init_security(inode, dir, qstr, new_xattrs,
> +                                     &num_filled_xattrs);
> +       if (ret && ret != -EOPNOTSUPP)
>                 goto out;
>         ret = initxattrs(inode, new_xattrs, fs_data);
>  out:
> --
> 2.25.1
>


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