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Subject[PATCH v6 19/20] evm: permit only valid security.evm xattrs to be updated
Date
In addition to requiring CAP_SYS_ADMIN permission to modify/delete
security.evm, prohibit invalid security.evm xattrs from changing,
unless in fixmode. This patch prevents inadvertent 'fixing' of
security.evm to reflect offline modifications.

Reported-by: Roberto Sassu <roberto.sassu@polito.it>
Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
---
Documentation/kernel-parameters.txt | 6 +++
security/integrity/evm/evm_main.c | 77 ++++++++++++++++++++++++++++------
2 files changed, 69 insertions(+), 14 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 5438a2d..b32657b 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -48,6 +48,7 @@ parameter is applicable:
EDD BIOS Enhanced Disk Drive Services (EDD) is enabled
EFI EFI Partitioning (GPT) is enabled
EIDE EIDE/ATAPI support is enabled.
+ EVM Extended Verification Module
FB The frame buffer device is enabled.
GCOV GCOV profiling is enabled.
HW Appropriate hardware is enabled.
@@ -750,6 +751,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
This option is obsoleted by the "netdev=" option, which
has equivalent usage. See its documentation for details.

+ evm_mode= [EVM]
+ Format: { "fix" }
+ Permit 'security.evm' to be updated regardless of
+ current integrity status.
+
failslab=
fail_page_alloc=
fail_make_request=[KNL]
diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 0dbb562..03524e4 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -37,13 +37,25 @@ char *evm_config_xattrnames[] = {
NULL
};

+static int evm_fixmode;
+static int __init evm_set_fixmode(char *str)
+{
+ if (strncmp(str, "fix", 3) == 0)
+ evm_fixmode = 1;
+ return 0;
+}
+__setup("evm_mode=", evm_set_fixmode);
+
/*
* evm_verify_hmac - calculate and compare the HMAC with the EVM xattr
*
* Compute the HMAC on the dentry's protected set of extended attributes
- * and compare it against the stored security.evm xattr. (For performance,
- * use the previoulsy retrieved xattr value and length to calculate the
- * HMAC.)
+ * and compare it against the stored security.evm xattr.
+ *
+ * For performance:
+ * - use the previoulsy retrieved xattr value and length to calculate the
+ * HMAC.)
+ * - cache the verification result in the iint, when available.
*
* Returns integrity status
*/
@@ -54,9 +66,10 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
struct integrity_iint_cache *iint)
{
struct evm_ima_xattr_data xattr_data;
+ enum integrity_status evm_status;
int rc;

- if (iint->evm_status == INTEGRITY_PASS)
+ if (iint && iint->evm_status == INTEGRITY_PASS)
return iint->evm_status;

/* if status is not PASS, try to check again - against -ENOMEM */
@@ -71,18 +84,21 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
sizeof xattr_data, GFP_NOFS);
if (rc < 0)
goto err_out;
- iint->evm_status = INTEGRITY_PASS;
- return iint->evm_status;
+ evm_status = INTEGRITY_PASS;
+ goto out;

err_out:
switch (rc) {
case -ENODATA: /* file not labelled */
- iint->evm_status = INTEGRITY_NOLABEL;
+ evm_status = INTEGRITY_NOLABEL;
break;
default:
- iint->evm_status = INTEGRITY_FAIL;
+ evm_status = INTEGRITY_FAIL;
}
- return iint->evm_status;
+out:
+ if (iint)
+ iint->evm_status = evm_status;
+ return evm_status;
}

static int evm_protected_xattr(const char *req_xattr_name)
@@ -157,6 +173,22 @@ static int evm_protect_xattr(struct dentry *dentry, const char *xattr_name,
return 0;
}

+/*
+ * evm_verify_current_integrity - verify the dentry's metadata integrity
+ * @dentry: pointer to the affected dentry
+ *
+ * Verify and return the dentry's metadata integrity. The exceptions are
+ * before EVM is initialized or in 'fix' mode.
+ */
+static enum integrity_status evm_verify_current_integrity(struct dentry *dentry)
+{
+ struct inode *inode = dentry->d_inode;
+
+ if (!evm_initialized || !S_ISREG(inode->i_mode) || evm_fixmode)
+ return 0;
+ return evm_verify_hmac(dentry, NULL, NULL, 0, NULL);
+}
+
/**
* evm_inode_setxattr - protect the EVM extended attribute
* @dentry: pointer to the affected dentry
@@ -164,13 +196,22 @@ static int evm_protect_xattr(struct dentry *dentry, const char *xattr_name,
* @xattr_value: pointer to the new extended attribute value
* @xattr_value_len: pointer to the new extended attribute value length
*
- * Prevent 'security.evm' from being modified
+ * Updating 'security.evm' requires CAP_SYS_ADMIN privileges and that
+ * the current value is valid.
*/
int evm_inode_setxattr(struct dentry *dentry, const char *xattr_name,
const void *xattr_value, size_t xattr_value_len)
{
- return evm_protect_xattr(dentry, xattr_name, xattr_value,
- xattr_value_len);
+
+ enum integrity_status evm_status;
+ int ret;
+
+ ret = evm_protect_xattr(dentry, xattr_name, xattr_value,
+ xattr_value_len);
+ if (ret)
+ return ret;
+ evm_status = evm_verify_current_integrity(dentry);
+ return evm_status == INTEGRITY_PASS ? 0 : -EPERM;
}

/**
@@ -178,11 +219,19 @@ int evm_inode_setxattr(struct dentry *dentry, const char *xattr_name,
* @dentry: pointer to the affected dentry
* @xattr_name: pointer to the affected extended attribute name
*
- * Prevent 'security.evm' from being removed.
+ * Removing 'security.evm' requires CAP_SYS_ADMIN privileges and that
+ * the current value is valid.
*/
int evm_inode_removexattr(struct dentry *dentry, const char *xattr_name)
{
- return evm_protect_xattr(dentry, xattr_name, NULL, 0);
+ enum integrity_status evm_status;
+ int ret;
+
+ ret = evm_protect_xattr(dentry, xattr_name, NULL, 0);
+ if (ret)
+ return ret;
+ evm_status = evm_verify_current_integrity(dentry);
+ return evm_status == INTEGRITY_PASS ? 0 : -EPERM;
}

/**
--
1.7.3.4


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