Merge branch 'misc-evm-v7' into next-integrity

From cover letter:

EVM portable signatures are particularly suitable for the protection of
metadata of immutable files where metadata is signed by a software vendor.
They can be used for example in conjunction with an IMA policy that
appraises only executed and memory mapped files.

However, until now portable signatures can be properly installed only if
the EVM_ALLOW_METADATA_WRITES initialization flag is also set, which
disables metadata verification until an HMAC key is loaded. This will cause
metadata writes to be allowed even in the situations where they shouldn't
(metadata protected by a portable signature is immutable).

The main reason why setting the flag is necessary is that the operations
necessary to install portable signatures and protected metadata would be
otherwise denied, despite being legitimate, due to the fact that the
decision logic has to avoid an unsafe recalculation of the HMAC that would
make the unsuccessfully verified metadata valid. However, the decision
logic is too coarse, and does not fully take into account all the possible
situations where metadata operations could be allowed.

For example, if the HMAC key is not loaded and it cannot be loaded in the
future due the EVM_SETUP_COMPLETE flag being set, it wouldn't be a problem
to allow metadata operations, as they wouldn't result in an HMAC being
recalculated.

This patch set extends the decision logic and adds the necessary exceptions
to use portable signatures without turning off metadata verification and
deprecates the EVM_ALLOW_METADATA_WRITES flag.

Link: https://lore.kernel.org/linux-integrity/20210514152753.982958-1-roberto.sassu@huawei.com/

5 files changed, 69 insertions, 15 deletions

diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index 4e5eb0236278..940695e7b535 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -242,12 +242,16 @@ static int xattr_verify(enum ima_hooks func, struct integrity_iint_cache *iint,
 		hash_start = 1;
 		fallthrough;
 	case IMA_XATTR_DIGEST:
-		if (iint->flags & IMA_DIGSIG_REQUIRED) {
-			*cause = "IMA-signature-required";
-			*status = INTEGRITY_FAIL;
-			break;
+		if (*status != INTEGRITY_PASS_IMMUTABLE) {
+			if (iint->flags & IMA_DIGSIG_REQUIRED) {
+				*cause = "IMA-signature-required";
+				*status = INTEGRITY_FAIL;
+				break;
+			}
+			clear_bit(IMA_DIGSIG, &iint->atomic_flags);
+		} else {
+			set_bit(IMA_DIGSIG, &iint->atomic_flags);
 		}
-		clear_bit(IMA_DIGSIG, &iint->atomic_flags);
 		if (xattr_len - sizeof(xattr_value->type) - hash_start >=
 				iint->ima_hash->length)
 			/*
@@ -416,6 +420,9 @@ int ima_appraise_measurement(enum ima_hooks func,
 	case INTEGRITY_NOLABEL:		/* No security.evm xattr. */
 		cause = "missing-HMAC";
 		goto out;
+	case INTEGRITY_FAIL_IMMUTABLE:
+		set_bit(IMA_DIGSIG, &iint->atomic_flags);
+		fallthrough;
 	case INTEGRITY_FAIL:		/* Invalid HMAC/signature. */
 		cause = "invalid-HMAC";
 		goto out;
@@ -459,9 +466,12 @@ out:
 				status = INTEGRITY_PASS;
 		}
 
-		/* Permit new files with file signatures, but without data. */
+		/*
+		 * Permit new files with file/EVM portable signatures, but
+		 * without data.
+		 */
 		if (inode->i_size == 0 && iint->flags & IMA_NEW_FILE &&
-		    xattr_value && xattr_value->type == EVM_IMA_XATTR_DIGSIG) {
+		    test_bit(IMA_DIGSIG, &iint->atomic_flags)) {
 			status = INTEGRITY_PASS;
 		}
 
@@ -522,8 +532,6 @@ void ima_inode_post_setattr(struct user_namespace *mnt_userns,
 		return;
 
 	action = ima_must_appraise(mnt_userns, inode, MAY_ACCESS, POST_SETATTR);
-	if (!action)
-		__vfs_removexattr(&init_user_ns, dentry, XATTR_NAME_IMA);
 	iint = integrity_iint_find(inode);
 	if (iint) {
 		set_bit(IMA_CHANGE_ATTR, &iint->atomic_flags);
@@ -570,6 +578,7 @@ int ima_inode_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,
@@ -577,9 +586,14 @@ int ima_inode_setxattr(struct dentry *dentry, const char *xattr_name,
 	if (result == 1) {
 		if (!xattr_value_len || (xvalue->type >= IMA_XATTR_LAST))
 			return -EINVAL;
-		ima_reset_appraise_flags(d_backing_inode(dentry),
-			xvalue->type == EVM_IMA_XATTR_DIGSIG);
-		result = 0;
+		digsig = (xvalue->type == EVM_IMA_XATTR_DIGSIG);
+	} else if (!strcmp(xattr_name, XATTR_NAME_EVM) && xattr_value_len > 0) {
+		digsig = (xvalue->type == EVM_XATTR_PORTABLE_DIGSIG);
+	}
+	if (result == 1 || evm_revalidate_status(xattr_name)) {
+		ima_reset_appraise_flags(d_backing_inode(dentry), digsig);
+		if (result == 1)
+			result = 0;
 	}
 	return result;
 }
@@ -589,9 +603,10 @@ int ima_inode_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_revalidate_status(xattr_name)) {
 		ima_reset_appraise_flags(d_backing_inode(dentry), 0);
-		result = 0;
+		if (result == 1)
+			result = 0;
 	}
 	return result;
 }
diff --git a/security/integrity/ima/ima_init.c b/security/integrity/ima/ima_init.c
index 6e8742916d1d..5076a7d9d23e 100644
--- a/security/integrity/ima/ima_init.c
+++ b/security/integrity/ima/ima_init.c
@@ -108,6 +108,10 @@ void __init ima_load_x509(void)
 
 	ima_policy_flag &= ~unset_flags;
 	integrity_load_x509(INTEGRITY_KEYRING_IMA, CONFIG_IMA_X509_PATH);
+
+	/* load also EVM key to avoid appraisal */
+	evm_load_x509();
+
 	ima_policy_flag |= unset_flags;
 }
 #endif
diff --git a/security/integrity/ima/ima_template.c b/security/integrity/ima/ima_template.c
index 4e081e650047..7a60848c04a5 100644
--- a/security/integrity/ima/ima_template.c
+++ b/security/integrity/ima/ima_template.c
@@ -45,6 +45,8 @@ static const struct ima_template_field supported_fields[] = {
 	 .field_show = ima_show_template_digest_ng},
 	{.field_id = "modsig", .field_init = ima_eventmodsig_init,
 	 .field_show = ima_show_template_sig},
+	{.field_id = "evmsig", .field_init = ima_eventevmsig_init,
+	 .field_show = ima_show_template_sig},
 };
 
 /*
diff --git a/security/integrity/ima/ima_template_lib.c b/security/integrity/ima/ima_template_lib.c
index c022ee9e2a4e..4314d9a3514c 100644
--- a/security/integrity/ima/ima_template_lib.c
+++ b/security/integrity/ima/ima_template_lib.c
@@ -10,6 +10,7 @@
  */
 
 #include "ima_template_lib.h"
+#include <linux/xattr.h>
 
 static bool ima_template_hash_algo_allowed(u8 algo)
 {
@@ -438,7 +439,7 @@ int ima_eventsig_init(struct ima_event_data *event_data,
 	struct evm_ima_xattr_data *xattr_value = event_data->xattr_value;
 
 	if ((!xattr_value) || (xattr_value->type != EVM_IMA_XATTR_DIGSIG))
-		return 0;
+		return ima_eventevmsig_init(event_data, field_data);
 
 	return ima_write_template_field_data(xattr_value, event_data->xattr_len,
 					     DATA_FMT_HEX, field_data);
@@ -484,3 +485,33 @@ int ima_eventmodsig_init(struct ima_event_data *event_data,
 	return ima_write_template_field_data(data, data_len, DATA_FMT_HEX,
 					     field_data);
 }
+
+/*
+ *  ima_eventevmsig_init - include the EVM portable signature as part of the
+ *  template data
+ */
+int ima_eventevmsig_init(struct ima_event_data *event_data,
+			 struct ima_field_data *field_data)
+{
+	struct evm_ima_xattr_data *xattr_data = NULL;
+	int rc = 0;
+
+	if (!event_data->file)
+		return 0;
+
+	rc = vfs_getxattr_alloc(&init_user_ns, file_dentry(event_data->file),
+				XATTR_NAME_EVM, (char **)&xattr_data, 0,
+				GFP_NOFS);
+	if (rc <= 0)
+		return 0;
+
+	if (xattr_data->type != EVM_XATTR_PORTABLE_DIGSIG) {
+		kfree(xattr_data);
+		return 0;
+	}
+
+	rc = ima_write_template_field_data((char *)xattr_data, rc, DATA_FMT_HEX,
+					   field_data);
+	kfree(xattr_data);
+	return rc;
+}
diff --git a/security/integrity/ima/ima_template_lib.h b/security/integrity/ima/ima_template_lib.h
index 6b3b880637a0..f4b2a2056d1d 100644
--- a/security/integrity/ima/ima_template_lib.h
+++ b/security/integrity/ima/ima_template_lib.h
@@ -46,4 +46,6 @@ int ima_eventbuf_init(struct ima_event_data *event_data,
 		      struct ima_field_data *field_data);
 int ima_eventmodsig_init(struct ima_event_data *event_data,
 			 struct ima_field_data *field_data);
+int ima_eventevmsig_init(struct ima_event_data *event_data,
+			 struct ima_field_data *field_data);
 #endif /* __LINUX_IMA_TEMPLATE_LIB_H */