RE: [RESEND][PATCH 05/12] evm: Introduce evm_hmac_disabled() to safely ignore verification errors

[Roberto Sassu]

> From: Roberto Sassu
> Sent: Thursday, May 20, 2021 10:49 AM
> When a file is being created, LSMs can set the initial label with the
> inode_init_security hook. If no HMAC key is loaded, the new file will have
> LSM xattrs but not the HMAC. It is also possible that the file remains
> without protected xattrs after creation if no active LSM provided it, or
> because the filesystem does not support them.
> 
> Unfortunately, EVM will deny any further metadata operation on new files,
> as evm_protect_xattr() will return the INTEGRITY_NOLABEL error if protected
> xattrs exist without security.evm, INTEGRITY_NOXATTRS if no protected
> xattrs exist or INTEGRITY_UNKNOWN if xattrs are not supported. This would
> limit the usability of EVM when only a public key is loaded, as commands
> such as cp or tar with the option to preserve xattrs won't work.
> 
> This patch introduces the evm_hmac_disabled() function to determine whether
> or not it is safe to ignore verification errors, based on the ability of
> EVM to calculate HMACs. If the HMAC key is not loaded, and it cannot be
> loaded in the future due to the EVM_SETUP_COMPLETE initialization flag,
> allowing an operation despite the attrs/xattrs being found invalid will not
> make them valid.
> 
> Since the post hooks can be executed even when the HMAC key is not loaded,
> this patch also ensures that the EVM_INIT_HMAC initialization flag is set
> before the post hooks call evm_update_evmxattr().

Resending, to ignore INTEGRITY_UNKNOWN when a filesystem does not
support xattrs.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> Suggested-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> Reviewed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> ---
>  security/integrity/evm/evm_main.c | 39 ++++++++++++++++++++++++++++++-
>  1 file changed, 38 insertions(+), 1 deletion(-)
> 
> diff --git a/security/integrity/evm/evm_main.c
> b/security/integrity/evm/evm_main.c
> index 782915117175..4206c7e492ae 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -90,6 +90,24 @@ static bool evm_key_loaded(void)
>  	return (bool)(evm_initialized & EVM_KEY_MASK);
>  }
> 
> +/*
> + * This function determines whether or not it is safe to ignore verification
> + * errors, based on the ability of EVM to calculate HMACs. If the HMAC key
> + * is not loaded, and it cannot be loaded in the future due to the
> + * EVM_SETUP_COMPLETE initialization flag, allowing an operation despite
> the
> + * attrs/xattrs being found invalid will not make them valid.
> + */
> +static bool evm_hmac_disabled(void)
> +{
> +	if (evm_initialized & EVM_INIT_HMAC)
> +		return false;
> +
> +	if (!(evm_initialized & EVM_SETUP_COMPLETE))
> +		return false;
> +
> +	return true;
> +}
> +
>  static int evm_find_protected_xattrs(struct dentry *dentry)
>  {
>  	struct inode *inode = d_backing_inode(dentry);
> @@ -338,6 +356,10 @@ static int evm_protect_xattr(struct dentry *dentry,
> const char *xattr_name,
>  	if (evm_status == INTEGRITY_NOXATTRS) {
>  		struct integrity_iint_cache *iint;
> 
> +		/* Exception if the HMAC is not going to be calculated. */
> +		if (evm_hmac_disabled())
> +			return 0;
> +
>  		iint = integrity_iint_find(d_backing_inode(dentry));
>  		if (iint && (iint->flags & IMA_NEW_FILE))
>  			return 0;
> @@ -354,6 +376,10 @@ static int evm_protect_xattr(struct dentry *dentry,
> const char *xattr_name,
>  				    -EPERM, 0);
>  	}
>  out:
> +	/* Exception if the HMAC is not going to be calculated. */
> +	if (evm_hmac_disabled() && (evm_status == INTEGRITY_NOLABEL ||
> +	    evm_status == INTEGRITY_UNKNOWN))
> +		return 0;
>  	if (evm_status != INTEGRITY_PASS)
>  		integrity_audit_msg(AUDIT_INTEGRITY_METADATA,
> d_backing_inode(dentry),
>  				    dentry->d_name.name,
> "appraise_metadata",
> @@ -474,6 +500,9 @@ void evm_inode_post_setxattr(struct dentry *dentry,
> const char *xattr_name,
>  	if (!strcmp(xattr_name, XATTR_NAME_EVM))
>  		return;
> 
> +	if (!(evm_initialized & EVM_INIT_HMAC))
> +		return;
> +
>  	evm_update_evmxattr(dentry, xattr_name, xattr_value,
> xattr_value_len);
>  }
> 
> @@ -497,6 +526,9 @@ void evm_inode_post_removexattr(struct dentry
> *dentry, const char *xattr_name)
>  	if (!strcmp(xattr_name, XATTR_NAME_EVM))
>  		return;
> 
> +	if (!(evm_initialized & EVM_INIT_HMAC))
> +		return;
> +
>  	evm_update_evmxattr(dentry, xattr_name, NULL, 0);
>  }
> 
> @@ -522,7 +554,9 @@ int evm_inode_setattr(struct dentry *dentry, struct
> iattr *attr)
>  		return 0;
>  	evm_status = evm_verify_current_integrity(dentry);
>  	if ((evm_status == INTEGRITY_PASS) ||
> -	    (evm_status == INTEGRITY_NOXATTRS))
> +	    (evm_status == INTEGRITY_NOXATTRS) ||
> +	    (evm_hmac_disabled() && (evm_status == INTEGRITY_NOLABEL ||
> +	     evm_status == INTEGRITY_UNKNOWN)))
>  		return 0;
>  	integrity_audit_msg(AUDIT_INTEGRITY_METADATA,
> d_backing_inode(dentry),
>  			    dentry->d_name.name, "appraise_metadata",
> @@ -548,6 +582,9 @@ void evm_inode_post_setattr(struct dentry *dentry,
> int ia_valid)
> 
>  	evm_reset_status(dentry->d_inode);
> 
> +	if (!(evm_initialized & EVM_INIT_HMAC))
> +		return;
> +
>  	if (ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
>  		evm_update_evmxattr(dentry, NULL, NULL, 0);
>  }
> --
> 2.25.1