On Wed, 2020-11-11 at 10:22 +0100, Roberto Sassu wrote:
> evm_inode_init_security() requires an HMAC key to calculate the HMAC on
> initial xattrs provided by LSMs. However, it checks generically whether a
> key has been loaded, including also public keys, which is not correct as
> public keys are not suitable to calculate the HMAC.
>
> Originally, support for signature verification was introduced to verify a
> possibly immutable initial ram disk, when no new files are created, and to
> switch to HMAC for the root filesystem. By that time, an HMAC key should
> have been loaded and usable to calculate HMACs for new files.
>
> More recently support for requiring an HMAC key was removed from the
> kernel, so that signature verification can be used alone. Since this is a
> legitimate use case, evm_inode_init_security() should not return an error
> when no HMAC key has been loaded.
>
> This patch fixes this problem by replacing the evm_key_loaded() check with
> a check of the EVM_INIT_HMAC flag in evm_initialized.
>
> Cc: stable@xxxxxxxxxxxxxxx # 4.5.x
> Fixes: 26ddabfe96b ("evm: enable EVM when X509 certificate is loaded")
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> Reviewed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> ---
> security/integrity/evm/evm_main.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 76d19146d74b..001e001eae01 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -530,7 +530,8 @@ int evm_inode_init_security(struct inode *inode,
> struct evm_xattr *xattr_data;
> int rc;
>
> - if (!evm_key_loaded() || !evm_protected_xattr(lsm_xattr->name))
> + if (!(evm_initialized & EVM_INIT_HMAC) ||
> + !evm_protected_xattr(lsm_xattr->name))
> return 0;
>
> xattr_data = kzalloc(sizeof(*xattr_data), GFP_NOFS);
Let's update the function description to make it explicit. Something
like: "evm_inode_init_security - initializes security.evm HMAC value"
Mimi
