Re: [PATCH 1/2] evm: don't copy up 'security.evm' xattr

[Amir Goldstein]

On Tue, Dec 19, 2023 at 3:49 PM Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
>
> The security.evm HMAC and the original file signatures contain
> filesystem specific data.  As a result, the HMAC and signature
> are not the same on the stacked and backing filesystems.
>
> Don't copy up 'security.evm'.
>
> Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> ---
>  include/linux/evm.h               | 6 ++++++
>  security/integrity/evm/evm_main.c | 7 +++++++
>  security/security.c               | 4 ++++
>  3 files changed, 17 insertions(+)
>
> diff --git a/include/linux/evm.h b/include/linux/evm.h
> index 01fc495a83e2..36ec884320d9 100644
> --- a/include/linux/evm.h
> +++ b/include/linux/evm.h
> @@ -31,6 +31,7 @@ extern void evm_inode_post_setxattr(struct dentry *dentry,
>                                     const char *xattr_name,
>                                     const void *xattr_value,
>                                     size_t xattr_value_len);
> +extern int evm_inode_copy_up_xattr(const char *name);
>  extern int evm_inode_removexattr(struct mnt_idmap *idmap,
>                                  struct dentry *dentry, const char *xattr_name);
>  extern void evm_inode_post_removexattr(struct dentry *dentry,
> @@ -117,6 +118,11 @@ static inline void evm_inode_post_setxattr(struct dentry *dentry,
>         return;
>  }
>
> +static inline int  evm_inode_copy_up_xattr(const char *name)
> +{
> +       return 0;
> +}
> +
>  static inline int evm_inode_removexattr(struct mnt_idmap *idmap,
>                                         struct dentry *dentry,
>                                         const char *xattr_name)
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 894570fe39bc..02adba635b02 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -863,6 +863,13 @@ void evm_inode_post_setattr(struct dentry *dentry, int ia_valid)
>                 evm_update_evmxattr(dentry, NULL, NULL, 0);
>  }
>
> +int evm_inode_copy_up_xattr(const char *name)
> +{
> +       if (strcmp(name, XATTR_NAME_EVM) == 0)
> +               return 1; /* Discard */
> +       return -EOPNOTSUPP;
> +}
> +
>  /*
>   * evm_inode_init_security - initializes security.evm HMAC value
>   */
> diff --git a/security/security.c b/security/security.c
> index dcb3e7014f9b..a02e78c45007 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2539,6 +2539,10 @@ int security_inode_copy_up_xattr(const char *name)
>                         return rc;
>         }
>
> +       rc = evm_inode_copy_up_xattr(name);
> +       if (rc != LSM_RET_DEFAULT(inode_copy_up_xattr))
> +               return rc;
> +
>         return LSM_RET_DEFAULT(inode_copy_up_xattr);

The rest of the hooks call evm last, e.g.:
        return evm_inode_setattr(idmap, dentry, attr);
        return evm_inode_remove_acl(idmap, dentry, acl_name);
        evm_inode_post_setxattr(dentry, name, value, size);
        return evm_inode_removexattr(idmap, dentry, name);

best keep a consistent LSM order.

Other than that, feel free to add:

Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>

Thanks,
Amir.