Re: [PATCH 03/11] evm: Refuse EVM_ALLOW_METADATA_WRITES only if the HMAC key is loaded
From: Mimi Zohar <zohar@linux.ibm.com>
Date: 2020-08-21 20:14:54
Also in:
linux-integrity, lkml, stable
Hi Roberto, On Thu, 2020-06-18 at 18:01 +0200, Roberto Sassu wrote:
Granting metadata write is safe if the HMAC key is not loaded, as it won't let an attacker obtain a valid HMAC from corrupted xattrs. evm_write_key() however does not allow it if any key is loaded, including a public key, which should not be a problem.
Why is the existing hebavior a problem? What is the problem being solved?
quoted hunk ↗ jump to hunk
This patch allows setting EVM_ALLOW_METADATA_WRITES if the EVM_INIT_HMAC flag is not set. 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> --- security/integrity/evm/evm_secfs.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)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;
Documentation/ABI/testing/evm needs to be updated as well. thanks, Mimi