Re: [PATCH v3 06/11] evm: Ignore INTEGRITY_NOLABEL if no HMAC key is loaded

Hi Roberto,

On Wed, 2020-11-11 at 10:22 +0100, Roberto Sassu wrote:

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.

Unfortunately, EVM will deny any further metadata operation on new files,
as evm_protect_xattr() will always return the INTEGRITY_NOLABEL error. 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.

Ignoring this error won't be an issue if no HMAC key is loaded, as the
inode is locked until the post hook, and EVM won't calculate the HMAC on
metadata that wasn't previously verified. Thus this patch checks if an
HMAC key is loaded and if not, ignores INTEGRITY_NOLABEL.

I'm not sure what problem this patch is trying to solve. 
evm_protect_xattr() is only called by evm_inode_setxattr() and
evm_inode_removexattr(), which first checks whether
EVM_ALLOW_METADATA_WRITES is enabled.

Mimi