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From: Roberto Sassu <roberto.sassu@huawei.com>
To: <zohar@linux.ibm.com>, <mjg59@google.com>
Cc: <linux-integrity@vger.kernel.org>,
	<linux-security-module@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>,
	Roberto Sassu <roberto.sassu@huawei.com>
Subject: [PATCH v6 05/11] evm: Introduce evm_hmac_disabled() to safely ignore verification errors
Date: Wed, 5 May 2021 13:33:23 +0200	[thread overview]
Message-ID: <20210505113329.1410943-1-roberto.sassu@huawei.com> (raw)
In-Reply-To: <20210505112935.1410679-1-roberto.sassu@huawei.com>

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.

Unfortunately, EVM will deny any further metadata operation on new files,
as evm_protect_xattr() will always return the INTEGRITY_NOLABEL error, or
INTEGRITY_NOXATTRS if no protected xattrs exist. 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.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Suggested-by: Mimi Zohar <zohar@linux.ibm.com>
---
 security/integrity/evm/evm_main.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 998818283fda..f625196eee8e 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 +376,9 @@ 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)
+		return 0;
 	if (evm_status != INTEGRITY_PASS)
 		integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
 				    dentry->d_name.name, "appraise_metadata",
@@ -515,7 +540,8 @@ 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))
 		return 0;
 	integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
 			    dentry->d_name.name, "appraise_metadata",
-- 
2.25.1


  parent reply	other threads:[~2021-05-05 11:33 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-05 11:29 [PATCH v6 00/11] evm: Improve usability of portable signatures Roberto Sassu
2021-05-05 11:29 ` [PATCH v6 01/11] evm: Execute evm_inode_init_security() only when an HMAC key is loaded Roberto Sassu
2021-05-05 11:29 ` [PATCH v6 02/11] evm: Load EVM key in ima_load_x509() to avoid appraisal Roberto Sassu
2021-05-05 11:29 ` [PATCH v6 03/11] evm: Refuse EVM_ALLOW_METADATA_WRITES only if an HMAC key is loaded Roberto Sassu
2021-05-11 13:41   ` Mimi Zohar
2021-05-11 14:12     ` Roberto Sassu
2021-05-11 19:53       ` Mimi Zohar
2021-05-05 11:29 ` [PATCH v6 04/11] evm: Introduce evm_status_revalidate() Roberto Sassu
2021-05-11 13:42   ` Mimi Zohar
2021-05-05 11:33 ` Roberto Sassu [this message]
2021-05-07 13:31   ` [RESEND][PATCH v6 05/11] evm: Introduce evm_hmac_disabled() to safely ignore verification errors Roberto Sassu
2021-05-11 13:42     ` Mimi Zohar
2021-05-05 11:33 ` [PATCH v6 06/11] evm: Allow xattr/attr operations for portable signatures Roberto Sassu
2021-05-05 11:33 ` [PATCH v6 07/11] evm: Pass user namespace to set/remove xattr hooks Roberto Sassu
2021-05-05 11:33 ` [PATCH v6 08/11] evm: Allow setxattr() and setattr() for unmodified metadata Roberto Sassu
2021-05-11 14:11   ` Mimi Zohar
2021-05-11 14:21     ` Roberto Sassu
2021-05-11 14:40       ` Mimi Zohar
2021-05-11 14:54         ` Roberto Sassu
2021-05-11 20:00           ` Mimi Zohar
2021-05-05 11:33 ` [PATCH v6 09/11] ima: Allow imasig requirement to be satisfied by EVM portable signatures Roberto Sassu
2021-05-05 11:33 ` [PATCH v6 10/11] ima: Introduce template field evmsig and write to field sig as fallback Roberto Sassu
2021-05-11 22:12   ` Mimi Zohar
2021-05-12 10:55     ` Roberto Sassu
2021-05-05 11:33 ` [PATCH v6 11/11] ima: Don't remove security.ima if file must not be appraised Roberto Sassu

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