[PATCH] evm: Correct inode_init_security hooks behaviors

[Nicolas Bouchinet <nicolas.bouchinet@clip-os.org>]

From: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>

Fixes a NULL pointer dereference occuring in the
`evm_protected_xattr_common` function of the EVM LSM. The bug is
triggered if a `inode_init_security` hook returns 0 without initializing
the given `struct xattr` fields (which is the case of BPF) and if no
other LSM overrides thoses fields after. This also leads to memory
leaks.

Adds a `call_int_hook_xattr` macro that fetches and feed the
`new_xattrs` array with every called hook xattr values.

Adds a `evm_init_hmacs` function which init the EVM hmac using every
entry of the array contrary to `evm_init_hmac`.

Fixes the `evm_inode_init_security` function to use `evm_init_hmacs`.

The `MAX_LSM_EVM_XATTR` value has been raised to 5 which gives room for
SMACK, SELinux, Apparmor, BPF and IMA/EVM security attributes.

Changes the default return value of the `inode_init_security` hook
definition to `-EOPNOTSUPP`.

Changes the hook documentation to match the behavior of the LSMs using
it (only xattr->value is initialised with kmalloc and thus is the only
one that should be kfreed by the caller).

Cc: stable@vger.kernel.org
Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
---
 include/linux/lsm_hook_defs.h       |  2 +-
 include/linux/lsm_hooks.h           |  4 ++--
 security/integrity/evm/evm.h        |  2 ++
 security/integrity/evm/evm_crypto.c | 23 ++++++++++++++++++++++-
 security/integrity/evm/evm_main.c   | 11 ++++++-----
 security/security.c                 | 29 ++++++++++++++++++++++++++---
 6 files changed, 59 insertions(+), 12 deletions(-)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 806448173033..e5dd0c0f6345 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -111,7 +111,7 @@ LSM_HOOK(int, 0, path_notify, const struct path *path, u64 mask,
 	 unsigned int obj_type)
 LSM_HOOK(int, 0, inode_alloc_security, struct inode *inode)
 LSM_HOOK(void, LSM_RET_VOID, inode_free_security, struct inode *inode)
-LSM_HOOK(int, 0, inode_init_security, struct inode *inode,
+LSM_HOOK(int, -EOPNOTSUPP, inode_init_security, struct inode *inode,
 	 struct inode *dir, const struct qstr *qstr, const char **name,
 	 void **value, size_t *len)
 LSM_HOOK(int, 0, inode_init_security_anon, struct inode *inode,
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 84a0d7e02176..95aff9383de1 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -229,8 +229,8 @@
  *	This hook is called by the fs code as part of the inode creation
  *	transaction and provides for atomic labeling of the inode, unlike
  *	the post_create/mkdir/... hooks called by the VFS.  The hook function
- *	is expected to allocate the name and value via kmalloc, with the caller
- *	being responsible for calling kfree after using them.
+ *	is expected to allocate the value via kmalloc, with the caller
+ *	being responsible for calling kfree after using it.
  *	If the security module does not use security attributes or does
  *	not wish to put a security attribute on this particular inode,
  *	then it should return -EOPNOTSUPP to skip this processing.
diff --git a/security/integrity/evm/evm.h b/security/integrity/evm/evm.h
index f8b8c5004fc7..a2f9886e924d 100644
--- a/security/integrity/evm/evm.h
+++ b/security/integrity/evm/evm.h
@@ -60,6 +60,8 @@ int evm_calc_hash(struct dentry *dentry, const char *req_xattr_name,
 		  struct evm_digest *data);
 int evm_init_hmac(struct inode *inode, const struct xattr *xattr,
 		  char *hmac_val);
+int evm_init_hmacs(struct inode *inode, const struct xattr *xattrs,
+		  char *hmac_val);
 int evm_init_secfs(void);
 
 #endif
diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
index 708de9656bbd..e5a34306cab6 100644
--- a/security/integrity/evm/evm_crypto.c
+++ b/security/integrity/evm/evm_crypto.c
@@ -347,7 +347,6 @@ static int evm_is_immutable(struct dentry *dentry, struct inode *inode)
 	return rc;
 }
 
-
 /*
  * Calculate the hmac and update security.evm xattr
  *
@@ -385,6 +384,28 @@ int evm_update_evmxattr(struct dentry *dentry, const char *xattr_name,
 	return rc;
 }
 
+int evm_protected_xattr(const char *req_xattr_name);
+
+int evm_init_hmacs(struct inode *inode, const struct xattr *lsm_xattrs,
+		  char *hmac_val)
+{
+	struct shash_desc *desc;
+
+	desc = init_desc(EVM_XATTR_HMAC, HASH_ALGO_SHA1);
+	if (IS_ERR(desc)) {
+		pr_info("init_desc failed\n");
+		return PTR_ERR(desc);
+	}
+
+	for (int i = 0; lsm_xattrs[i].value != NULL; i++) {
+		if (evm_protected_xattr(lsm_xattrs[i].name))
+			crypto_shash_update(desc, lsm_xattrs[i].value, lsm_xattrs[i].value_len);
+	}
+	hmac_add_misc(desc, inode, EVM_XATTR_HMAC, hmac_val);
+	kfree(desc);
+	return 0;
+}
+
 int evm_init_hmac(struct inode *inode, const struct xattr *lsm_xattr,
 		  char *hmac_val)
 {
diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 2e6fb6e2ffd2..bb071c55d656 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -284,6 +284,8 @@ static int evm_protected_xattr_common(const char *req_xattr_name,
 	int found = 0;
 	struct xattr_list *xattr;
 
+	if (!req_xattr_name)
+		return found;
 	namelen = strlen(req_xattr_name);
 	list_for_each_entry_lockless(xattr, &evm_config_xattrnames, list) {
 		if (!all_xattrs && !xattr->enabled)
@@ -305,7 +307,7 @@ static int evm_protected_xattr_common(const char *req_xattr_name,
 	return found;
 }
 
-static int evm_protected_xattr(const char *req_xattr_name)
+int evm_protected_xattr(const char *req_xattr_name)
 {
 	return evm_protected_xattr_common(req_xattr_name, false);
 }
@@ -835,14 +837,13 @@ void evm_inode_post_setattr(struct dentry *dentry, int ia_valid)
  * evm_inode_init_security - initializes security.evm HMAC value
  */
 int evm_inode_init_security(struct inode *inode,
-				 const struct xattr *lsm_xattr,
+				 const struct xattr *lsm_xattrs,
 				 struct xattr *evm_xattr)
 {
 	struct evm_xattr *xattr_data;
 	int rc;
 
-	if (!(evm_initialized & EVM_INIT_HMAC) ||
-	    !evm_protected_xattr(lsm_xattr->name))
+	if (!(evm_initialized & EVM_INIT_HMAC))
 		return 0;
 
 	xattr_data = kzalloc(sizeof(*xattr_data), GFP_NOFS);
@@ -850,7 +851,7 @@ int evm_inode_init_security(struct inode *inode,
 		return -ENOMEM;
 
 	xattr_data->data.type = EVM_XATTR_HMAC;
-	rc = evm_init_hmac(inode, lsm_xattr, xattr_data->digest);
+	rc = evm_init_hmacs(inode, lsm_xattrs, xattr_data->digest);
 	if (rc < 0)
 		goto out;
 
diff --git a/security/security.c b/security/security.c
index 14d30fec8a00..47012c118536 100644
--- a/security/security.c
+++ b/security/security.c
@@ -30,7 +30,7 @@
 #include <linux/msg.h>
 #include <net/flow.h>
 
-#define MAX_LSM_EVM_XATTR	2
+#define MAX_LSM_EVM_XATTR	5
 
 /* How many LSMs were built into the kernel? */
 #define LSM_COUNT (__end_lsm_info - __start_lsm_info)
@@ -746,6 +746,29 @@ static int lsm_superblock_alloc(struct super_block *sb)
 	RC;							\
 })
 
+#define call_int_hook_xattr(XATTRS, FUNC, IRC, ...) ({		\
+	int RC = IRC;						\
+	int i = 0;						\
+	do {							\
+		struct security_hook_list *P;			\
+								\
+		hlist_for_each_entry(P, &security_hook_heads.FUNC, list) { \
+			RC = P->hook.FUNC(__VA_ARGS__);		\
+			if (RC == -EOPNOTSUPP)			\
+				continue;			\
+			if (RC != 0 && RC != IRC)		\
+				break;				\
+			if (i >= MAX_LSM_EVM_XATTR) {		\
+				RC = -ENOMEM;			\
+				break;				\
+			}					\
+			XATTRS++;				\
+			i++;					\
+		}						\
+	} while (0);						\
+	RC;							\
+})
+
 /* Security operations */
 
 int security_binder_set_context_mgr(const struct cred *mgr)
@@ -1103,7 +1126,7 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
 				     dir, qstr, NULL, NULL, NULL);
 	memset(new_xattrs, 0, sizeof(new_xattrs));
 	lsm_xattr = new_xattrs;
-	ret = call_int_hook(inode_init_security, -EOPNOTSUPP, inode, dir, qstr,
+	ret = call_int_hook_xattr(lsm_xattr, inode_init_security, -EOPNOTSUPP, inode, dir, qstr,
 						&lsm_xattr->name,
 						&lsm_xattr->value,
 						&lsm_xattr->value_len);
@@ -1111,7 +1134,7 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
 		goto out;
 
 	evm_xattr = lsm_xattr + 1;
-	ret = evm_inode_init_security(inode, lsm_xattr, evm_xattr);
+	ret = evm_inode_init_security(inode, new_xattrs, evm_xattr);
 	if (ret)
 		goto out;
 	ret = initxattrs(inode, new_xattrs, fs_data);
-- 
2.38.1