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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-fsdevel@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	Roberto Sassu <roberto.sassu@huawei.com>
Subject: [PATCH v5 05/12] evm: Introduce evm_status_revalidate()
Date: Wed, 7 Apr 2021 12:52:45 +0200
Message-ID: <20210407105252.30721-6-roberto.sassu@huawei.com> (raw)
In-Reply-To: <20210407105252.30721-1-roberto.sassu@huawei.com>

When EVM_ALLOW_METADATA_WRITES is set, EVM allows any operation on
metadata. Its main purpose is to allow users to freely set metadata when it
is protected by a portable signature, until an HMAC key is loaded.

However, callers of evm_verifyxattr() are not notified about metadata
changes and continue to rely on the last status returned by the function.
For example IMA, since it caches the appraisal result, will not call again
evm_verifyxattr() until the appraisal flags are cleared, and will grant
access to the file even if there was a metadata operation that made the
portable signature invalid.

This patch introduces evm_status_revalidate(), which callers of
evm_verifyxattr() can use in their xattr post hooks to determine whether
re-validation is necessary and to do the proper actions. IMA calls it in
its xattr post hooks to reset the appraisal flags, so that the EVM status
is re-evaluated after a metadata operation.

Lastly, this patch also adds a call to evm_reset_status() in
evm_inode_post_setattr() to invalidate the cached EVM status after a
setattr operation.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
---
 include/linux/evm.h                   |  6 +++++
 security/integrity/evm/evm_main.c     | 33 +++++++++++++++++++++++----
 security/integrity/ima/ima_appraise.c |  8 ++++---
 3 files changed, 40 insertions(+), 7 deletions(-)

diff --git a/include/linux/evm.h b/include/linux/evm.h
index 8302bc29bb35..e5b7bcb152b9 100644
--- a/include/linux/evm.h
+++ b/include/linux/evm.h
@@ -35,6 +35,7 @@ extern void evm_inode_post_removexattr(struct dentry *dentry,
 extern int evm_inode_init_security(struct inode *inode,
 				   const struct xattr *xattr_array,
 				   struct xattr *evm);
+extern bool evm_status_revalidate(const char *xattr_name);
 #ifdef CONFIG_FS_POSIX_ACL
 extern int posix_xattr_acl(const char *xattrname);
 #else
@@ -104,5 +105,10 @@ static inline int evm_inode_init_security(struct inode *inode,
 	return 0;
 }
 
+static inline bool evm_status_revalidate(const char *xattr_name)
+{
+	return false;
+}
+
 #endif /* CONFIG_EVM */
 #endif /* LINUX_EVM_H */
diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 7ac5204c8d1f..998818283fda 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -425,6 +425,30 @@ static void evm_reset_status(struct inode *inode)
 		iint->evm_status = INTEGRITY_UNKNOWN;
 }
 
+/**
+ * evm_status_revalidate - report whether EVM status re-validation is necessary
+ * @xattr_name: pointer to the affected extended attribute name
+ *
+ * Report whether callers of evm_verifyxattr() should re-validate the
+ * EVM status.
+ *
+ * Return true if re-validation is necessary, false otherwise.
+ */
+bool evm_status_revalidate(const char *xattr_name)
+{
+	if (!evm_key_loaded())
+		return false;
+
+	/* evm_inode_post_setattr() passes NULL */
+	if (!xattr_name)
+		return true;
+
+	if (!evm_protected_xattr(xattr_name) && !posix_xattr_acl(xattr_name))
+		return false;
+
+	return true;
+}
+
 /**
  * evm_inode_post_setxattr - update 'security.evm' to reflect the changes
  * @dentry: pointer to the affected dentry
@@ -441,8 +465,7 @@ static void evm_reset_status(struct inode *inode)
 void evm_inode_post_setxattr(struct dentry *dentry, const char *xattr_name,
 			     const void *xattr_value, size_t xattr_value_len)
 {
-	if (!evm_key_loaded() || (!evm_protected_xattr(xattr_name)
-				  && !posix_xattr_acl(xattr_name)))
+	if (!evm_status_revalidate(xattr_name))
 		return;
 
 	evm_reset_status(dentry->d_inode);
@@ -462,7 +485,7 @@ void evm_inode_post_setxattr(struct dentry *dentry, const char *xattr_name,
  */
 void evm_inode_post_removexattr(struct dentry *dentry, const char *xattr_name)
 {
-	if (!evm_key_loaded() || !evm_protected_xattr(xattr_name))
+	if (!evm_status_revalidate(xattr_name))
 		return;
 
 	evm_reset_status(dentry->d_inode);
@@ -513,9 +536,11 @@ int evm_inode_setattr(struct dentry *dentry, struct iattr *attr)
  */
 void evm_inode_post_setattr(struct dentry *dentry, int ia_valid)
 {
-	if (!evm_key_loaded())
+	if (!evm_status_revalidate(NULL))
 		return;
 
+	evm_reset_status(dentry->d_inode);
+
 	if (ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
 		evm_update_evmxattr(dentry, NULL, NULL, 0);
 }
diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index 1f029e4c8d7f..d4b8db1acadd 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -586,13 +586,15 @@ void ima_inode_post_setxattr(struct dentry *dentry, const char *xattr_name,
 			     const void *xattr_value, size_t xattr_value_len)
 {
 	const struct evm_ima_xattr_data *xvalue = xattr_value;
+	int digsig = 0;
 	int result;
 
 	result = ima_protect_xattr(dentry, xattr_name, xattr_value,
 				   xattr_value_len);
 	if (result == 1)
-		ima_reset_appraise_flags(d_backing_inode(dentry),
-			xvalue->type == EVM_IMA_XATTR_DIGSIG);
+		digsig = (xvalue->type == EVM_IMA_XATTR_DIGSIG);
+	if (result == 1 || evm_status_revalidate(xattr_name))
+		ima_reset_appraise_flags(d_backing_inode(dentry), digsig);
 }
 
 int ima_inode_removexattr(struct dentry *dentry, const char *xattr_name)
@@ -611,6 +613,6 @@ void ima_inode_post_removexattr(struct dentry *dentry, const char *xattr_name)
 	int result;
 
 	result = ima_protect_xattr(dentry, xattr_name, NULL, 0);
-	if (result == 1)
+	if (result == 1 || evm_status_revalidate(xattr_name))
 		ima_reset_appraise_flags(d_backing_inode(dentry), 0);
 }
-- 
2.26.2


  parent reply index

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-07 10:52 [PATCH v5 00/12] evm: Improve usability of portable signatures Roberto Sassu
2021-04-07 10:52 ` [PATCH v5 01/12] evm: Execute evm_inode_init_security() only when an HMAC key is loaded Roberto Sassu
2021-04-07 10:52 ` [PATCH v5 02/12] evm: Load EVM key in ima_load_x509() to avoid appraisal Roberto Sassu
2021-04-07 10:52 ` [PATCH v5 03/12] evm: Refuse EVM_ALLOW_METADATA_WRITES only if an HMAC key is loaded Roberto Sassu
2021-04-30 20:28   ` Mimi Zohar
2021-04-07 10:52 ` [PATCH v5 04/12] ima: Move ima_reset_appraise_flags() call to post hooks Roberto Sassu
2021-04-07 16:17   ` Casey Schaufler
2021-04-07 16:31     ` Roberto Sassu
2021-04-26 19:49     ` Mimi Zohar
2021-04-07 10:52 ` Roberto Sassu [this message]
2021-04-07 10:52 ` [PATCH v5 06/12] evm: Ignore INTEGRITY_NOLABEL/INTEGRITY_NOXATTRS if conditions are safe Roberto Sassu
2021-05-03  0:12   ` Mimi Zohar
2021-05-03  7:55     ` Roberto Sassu
2021-05-03 12:07       ` Mimi Zohar
2021-05-03 14:15       ` Roberto Sassu
2021-05-03 14:34         ` Mimi Zohar
2021-05-04 13:16           ` Roberto Sassu
2021-05-04 13:45             ` Mimi Zohar
2021-04-07 10:52 ` [PATCH v5 07/12] evm: Allow xattr/attr operations for portable signatures Roberto Sassu
2021-05-03  0:12   ` Mimi Zohar
2021-05-04 14:28     ` Roberto Sassu
2021-05-04 14:49       ` Mimi Zohar
2021-04-07 10:52 ` [PATCH v5 08/12] evm: Pass user namespace to set/remove xattr hooks Roberto Sassu
2021-04-07 12:06   ` Christian Brauner
2021-04-07 10:52 ` [PATCH v5 09/12] evm: Allow setxattr() and setattr() for unmodified metadata Roberto Sassu
2021-04-07 12:05   ` Christian Brauner
2021-04-07 15:23   ` kernel test robot
2021-04-07 18:14   ` kernel test robot
2021-04-07 19:28     ` [RESEND][PATCH " Roberto Sassu
2021-05-03 13:00   ` [PATCH " Mimi Zohar
2021-05-03 14:48     ` Roberto Sassu
2021-05-03 15:13       ` Mimi Zohar
2021-05-03 15:30         ` Roberto Sassu
2021-05-03 15:11     ` Roberto Sassu
2021-05-03 15:26       ` Mimi Zohar
2021-05-03 15:32         ` Roberto Sassu
2021-05-03 15:48           ` Mimi Zohar
2021-04-07 10:52 ` [PATCH v5 10/12] ima: Allow imasig requirement to be satisfied by EVM portable signatures Roberto Sassu
2021-04-07 10:52 ` [PATCH v5 11/12] ima: Introduce template field evmsig and write to field sig as fallback Roberto Sassu
2021-04-07 10:52 ` [PATCH v5 12/12] ima: Don't remove security.ima if file must not be appraised Roberto Sassu

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