evm: permit only valid security.evm xattrs to be updated

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.

Changelog v7:
- rename boot paramater 'evm_mode' to 'evm'

Reported-by: Roberto Sassu <roberto.sassu@polito.it>
Signed-off-by: Mimi Zohar <zohar@us.ibm.com>

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index fd248a318211a189163f2c42ba911a44284ea0bc..db97ff1da8c00a0c3ff4134a70d61e7db3559127 100644 (file)
--- 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=            [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 eb07f9d13c247f35a2b1bfe6a9cc84f6863924de..94d66af07aa4127cf8ee68536aef7eaef2dd874c 100644 (file)
--- 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=", 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;
 }
 
 /**