[PATCH v2] 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 occurring 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>
---

Changes since v1:
https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/
* The `call_int_hook_xattr` macro has been inlined into the
  `security_inode_init_security` hook (as suggested by Paul Moor,
  Mickaël Salaün and Casey Schaufler).
* The MAX_LSM_EVM_XATTR value is processed based on compiled LSMs (as
  proposed by Casey Schaufler).
* Various typos and cosmetic changes has been fixed (as suggested by
  Mickaël Salaün)
* A MAX_LSM_EVM_XATTR test has been wrapped in a WAN_ON_ONCE macro (as
  suggested by Mickaël Salaün).
---
 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 | 22 ++++++++++++++++++++++
 security/integrity/evm/evm_main.c   |  9 +++++----
 security/security.c                 | 28 +++++++++++++++++++++-------
 6 files changed, 53 insertions(+), 14 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..db6868875e09 100644
--- a/security/integrity/evm/evm_crypto.c
+++ b/security/integrity/evm/evm_crypto.c
@@ -385,6 +385,28 @@ 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..4f89b9eaf028 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);
 }
@@ -841,8 +843,7 @@ int evm_inode_init_security(struct inode *inode,
 	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_xattr, xattr_data->digest);
 	if (rc < 0)
 		goto out;
 
diff --git a/security/security.c b/security/security.c
index 14d30fec8a00..cf11bd7d6a1b 100644
--- a/security/security.c
+++ b/security/security.c
@@ -30,7 +30,11 @@
 #include <linux/msg.h>
 #include <net/flow.h>
 
-#define MAX_LSM_EVM_XATTR	2
+#define MAX_LSM_EVM_XATTR                                \
+	((IS_ENABLED(CONFIG_EVM) ? 1 : 0) +              \
+	 (IS_ENABLED(CONFIG_SECURITY_SELINUX) ? 1 : 0) + \
+	 (IS_ENABLED(CONFIG_SECURITY_SMACK) ? 1 : 0) +   \
+	 (IS_ENABLED(CONFIG_BPF_LSM) ? 1 : 0))
 
 /* How many LSMs were built into the kernel? */
 #define LSM_COUNT (__end_lsm_info - __start_lsm_info)
@@ -1091,9 +1095,11 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
 				 const struct qstr *qstr,
 				 const initxattrs initxattrs, void *fs_data)
 {
+	int i = 0;
+	int ret = -EOPNOTSUPP;
 	struct xattr new_xattrs[MAX_LSM_EVM_XATTR + 1];
 	struct xattr *lsm_xattr, *evm_xattr, *xattr;
-	int ret;
+	struct security_hook_list *hook_ptr;
 
 	if (unlikely(IS_PRIVATE(inode)))
 		return 0;
@@ -1103,15 +1109,23 @@ 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,
-						&lsm_xattr->name,
-						&lsm_xattr->value,
-						&lsm_xattr->value_len);
+	hlist_for_each_entry(hook_ptr, &security_hook_heads.inode_init_security, list) {
+		ret = hook_ptr->hook.inode_init_security(inode, dir, qstr, &lsm_xattr->name,
+				&lsm_xattr->value, &lsm_xattr->value_len);
+		if (ret == -EOPNOTSUPP)
+			continue;
+		if (WARN_ON_ONCE(i >= MAX_LSM_EVM_XATTR))
+			ret = -ENOMEM;
+		if (ret != 0)
+			break;
+		lsm_xattr++;
+		i++;
+	}
 	if (ret)
 		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