Re: [PATCH v3 03/11] evm: Refuse EVM_ALLOW_METADATA_WRITES only if an HMAC key is loaded

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

EVM_ALLOW_METADATA_WRITES is an EVM initialization flag that can be set to temporarily disable metadata verification until all xattrs/attrs necessary to verify an EVM portable signature are copied to the file. This flag is cleared when EVM is initialized with an HMAC key, to avoid that the HMAC is calculated on unverified xattrs/attrs.

Currently EVM unnecessarily denies setting this flag if EVM is initialized with a public key, which is not a concern as it cannot be used to trust xattrs/attrs updates. This patch removes this limitation.

Cc: stable@vger.kernel.org # 4.16.x Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of EVM-protected metadata") Signed-off-by: Roberto Sassu roberto.sassu@huawei.com

---

Documentation/ABI/testing/evm | 5 +++-- security/integrity/evm/evm_secfs.c | 2 +- 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/Documentation/ABI/testing/evm b/Documentation/ABI/testing/evm index 3c477ba48a31..eb6d70fd6fa2 100644 --- a/Documentation/ABI/testing/evm +++ b/Documentation/ABI/testing/evm @@ -49,8 +49,9 @@ Description: modification of EVM-protected metadata and disable all further modification of policy

• Note that once a key has been loaded, it will no longer be
  

• possible to enable metadata modification.
  

• Note that once an HMAC key has been loaded, it will no longer
  

• be possible to enable metadata modification and, if it is
  

• already enabled, it will be disabled.
  

Until key loading has been signaled EVM can not create or validate the 'security.evm' xattr, but returns diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c index cfc3075769bb..92fe26ace797 100644 --- a/security/integrity/evm/evm_secfs.c +++ b/security/integrity/evm/evm_secfs.c @@ -84,7 +84,7 @@ static ssize_t evm_write_key(struct file *file, const char __user *buf, * keys are loaded. */ if ((i & EVM_ALLOW_METADATA_WRITES) &&

•    ((evm_initialized & EVM_KEY_MASK) != 0) &&
  

•    ((evm_initialized & EVM_INIT_HMAC) != 0) &&
   !(evm_initialized & EVM_ALLOW_METADATA_WRITES))
  

  return -EPERM;

If an HMAC key is loaded EVM_ALLOW_METADATA_WRITES should already be disabled. Testing EVM_ALLOW_METADATA_WRITES shouldn't be needed. Please update the comment: "Don't allow a request to freshly enable metadata writes if keys are loaded."

thanks,

Mimi