security: Allow all LSMs to provide xattrs for inode_init_security hook

Currently, the LSM infrastructure supports only one LSM providing an xattr
and EVM calculating the HMAC on that xattr, plus other inode metadata.

Allow all LSMs to provide one or multiple xattrs, by extending the security
blob reservation mechanism. Introduce the new lbs_xattr_count field of the
lsm_blob_sizes structure, so that each LSM can specify how many xattrs it
needs, and the LSM infrastructure knows how many xattr slots it should
allocate.

Modify the inode_init_security hook definition, by passing the full
xattr array allocated in security_inode_init_security(), and the current
number of xattr slots in that array filled by LSMs. The first parameter
would allow EVM to access and calculate the HMAC on xattrs supplied by
other LSMs, the second to not leave gaps in the xattr array, when an LSM
requested but did not provide xattrs (e.g. if it is not initialized).

Introduce lsm_get_xattr_slot(), which LSMs can call as many times as the
number specified in the lbs_xattr_count field of the lsm_blob_sizes
structure. During each call, lsm_get_xattr_slot() increments the number of
filled xattrs, so that at the next invocation it returns the next xattr
slot to fill.

Cleanup security_inode_init_security(). Unify the !initxattrs and
initxattrs case by simply not allocating the new_xattrs array in the
former. Update the documentation to reflect the changes, and fix the
description of the xattr name, as it is not allocated anymore.

Adapt both SELinux and Smack to use the new definition of the
inode_init_security hook, and to call lsm_get_xattr_slot() to obtain and
fill the reserved slots in the xattr array.

Move the xattr->name assignment after the xattr->value one, so that it is
done only in case of successful memory allocation.

Finally, change the default return value of the inode_init_security hook
from zero to -EOPNOTSUPP, so that BPF LSM correctly follows the hook
conventions.

Reported-by: Nicolas Bouchinet <[e-mail]>
Link: https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/
Signed-off-by: Roberto Sassu <[e-mail]>
Acked-by: Casey Schaufler <[e-mail]>
[PM: minor comment and variable tweaks, approved by RS]
Signed-off-by: Paul Moore <[e-mail]>

include/linux/lsm_hook_defs.h

···
 	 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,
-	 struct inode *dir, const struct qstr *qstr, const char **name,
-	 void **value, size_t *len)
+LSM_HOOK(int, -EOPNOTSUPP, inode_init_security, struct inode *inode,
+	 struct inode *dir, const struct qstr *qstr, struct xattr *xattrs,
+	 int *xattr_count)
 LSM_HOOK(int, 0, inode_init_security_anon, struct inode *inode,
 	 const struct qstr *name, const struct inode *context_inode)
 LSM_HOOK(int, 0, inode_create, struct inode *dir, struct dentry *dentry,

include/linux/lsm_hooks.h

···
 #include <linux/security.h>
 #include <linux/init.h>
 #include <linux/rculist.h>
+#include <linux/xattr.h>
 
 union security_list_options {
 	#define LSM_HOOK(RET, DEFAULT, NAME, ...) RET (*NAME)(__VA_ARGS__);
···
 	int	lbs_ipc;
 	int	lbs_msg_msg;
 	int	lbs_task;
+	int	lbs_xattr_count; /* number of xattr slots in new_xattrs array */
 };
+
+/**
+ * lsm_get_xattr_slot - Return the next available slot and increment the index
+ * @xattrs: array storing LSM-provided xattrs
+ * @xattr_count: number of already stored xattrs (updated)
+ *
+ * Retrieve the first available slot in the @xattrs array to fill with an xattr,
+ * and increment @xattr_count.
+ *
+ * Return: The slot to fill in @xattrs if non-NULL, NULL otherwise.
+ */
+static inline struct xattr *lsm_get_xattr_slot(struct xattr *xattrs,
+					       int *xattr_count)
+{
+	if (unlikely(!xattrs))
+		return NULL;
+	return &xattrs[(*xattr_count)++];
+}
 
 /*
  * LSM_RET_VOID is used as the default value in LSM_HOOK definitions for void

security/security.c

···
 #include <linux/msg.h>
 #include <net/flow.h>
 
-#define MAX_LSM_EVM_XATTR	2
-
 /* How many LSMs were built into the kernel? */
 #define LSM_COUNT (__end_lsm_info - __start_lsm_info)
 
···
 	lsm_set_blob_size(&needed->lbs_msg_msg, &blob_sizes.lbs_msg_msg);
 	lsm_set_blob_size(&needed->lbs_superblock, &blob_sizes.lbs_superblock);
 	lsm_set_blob_size(&needed->lbs_task, &blob_sizes.lbs_task);
+	lsm_set_blob_size(&needed->lbs_xattr_count,
+			  &blob_sizes.lbs_xattr_count);
 }
 
 /* Prepare LSM for initialization. */
···
 	init_debug("msg_msg blob size    = %d\n", blob_sizes.lbs_msg_msg);
 	init_debug("superblock blob size = %d\n", blob_sizes.lbs_superblock);
 	init_debug("task blob size       = %d\n", blob_sizes.lbs_task);
+	init_debug("xattr slots          = %d\n", blob_sizes.lbs_xattr_count);
 
 	/*
 	 * Create any kmem_caches needed for blobs
···
  * created inode and set up the incore security field for the new inode.  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.  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.
+ * hooks called by the VFS.
+ *
+ * The hook function is expected to populate the xattrs array, by calling
+ * lsm_get_xattr_slot() to retrieve the slots reserved by the security module
+ * with the lbs_xattr_count field of the lsm_blob_sizes structure.  For each
+ * slot, the hook function should set ->name to the attribute name suffix
+ * (e.g. selinux), to allocate ->value (will be freed by the caller) and set it
+ * to the attribute value, to set ->value_len to the length of the value.  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.
  *
  * Return: Returns 0 on success, -EOPNOTSUPP if no security attribute is
  * needed, or -ENOMEM on memory allocation failure.
···
 				 const struct qstr *qstr,
 				 const initxattrs initxattrs, void *fs_data)
 {
-	struct xattr new_xattrs[MAX_LSM_EVM_XATTR + 1];
-	struct xattr *lsm_xattr, *evm_xattr, *xattr;
-	int ret;
+	struct security_hook_list *hp;
+	struct xattr *new_xattrs = NULL;
+	int ret = -EOPNOTSUPP, xattr_count = 0;
 
 	if (unlikely(IS_PRIVATE(inode)))
 		return 0;
 
-	if (!initxattrs)
-		return call_int_hook(inode_init_security, -EOPNOTSUPP, inode,
-				     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);
-	if (ret)
+	if (!blob_sizes.lbs_xattr_count)
+		return 0;
+
+	if (initxattrs) {
+		/* Allocate +1 for EVM and +1 as terminator. */
+		new_xattrs = kcalloc(blob_sizes.lbs_xattr_count + 2,
+				     sizeof(*new_xattrs), GFP_NOFS);
+		if (!new_xattrs)
+			return -ENOMEM;
+	}
+
+	hlist_for_each_entry(hp, &security_hook_heads.inode_init_security,
+			     list) {
+		ret = hp->hook.inode_init_security(inode, dir, qstr, new_xattrs,
+						  &xattr_count);
+		if (ret && ret != -EOPNOTSUPP)
+			goto out;
+		/*
+		 * As documented in lsm_hooks.h, -EOPNOTSUPP in this context
+		 * means that the LSM is not willing to provide an xattr, not
+		 * that it wants to signal an error. Thus, continue to invoke
+		 * the remaining LSMs.
+		 */
+	}
+
+	/* If initxattrs() is NULL, xattr_count is zero, skip the call. */
+	if (!xattr_count)
 		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,
+				      &new_xattrs[xattr_count]);
 	if (ret)
 		goto out;
 	ret = initxattrs(inode, new_xattrs, fs_data);
 out:
-	for (xattr = new_xattrs; xattr->value != NULL; xattr++)
-		kfree(xattr->value);
+	for (; xattr_count > 0; xattr_count--)
+		kfree(new_xattrs[xattr_count - 1].value);
+	kfree(new_xattrs);
 	return (ret == -EOPNOTSUPP) ? 0 : ret;
 }
 EXPORT_SYMBOL(security_inode_init_security);

security/selinux/hooks.c

···
 #include "audit.h"
 #include "avc_ss.h"
 
+#define SELINUX_INODE_INIT_XATTRS 1
+
 struct selinux_state selinux_state;
 
 /* SECMARK reference count */
···
 
 static int selinux_inode_init_security(struct inode *inode, struct inode *dir,
 				       const struct qstr *qstr,
-				       const char **name,
-				       void **value, size_t *len)
+				       struct xattr *xattrs, int *xattr_count)
 {
 	const struct task_security_struct *tsec = selinux_cred(current_cred());
 	struct superblock_security_struct *sbsec;
+	struct xattr *xattr = lsm_get_xattr_slot(xattrs, xattr_count);
 	u32 newsid, clen;
 	int rc;
 	char *context;
···
 	    !(sbsec->flags & SBLABEL_MNT))
 		return -EOPNOTSUPP;
 
-	if (name)
-		*name = XATTR_SELINUX_SUFFIX;
-
-	if (value && len) {
+	if (xattr) {
 		rc = security_sid_to_context_force(newsid,
 						   &context, &clen);
 		if (rc)
 			return rc;
-		*value = context;
-		*len = clen;
+		xattr->value = context;
+		xattr->value_len = clen;
+		xattr->name = XATTR_SELINUX_SUFFIX;
 	}
 
 	return 0;
···
 	.lbs_ipc = sizeof(struct ipc_security_struct),
 	.lbs_msg_msg = sizeof(struct msg_security_struct),
 	.lbs_superblock = sizeof(struct superblock_security_struct),
+	.lbs_xattr_count = SELINUX_INODE_INIT_XATTRS,
 };
 
 #ifdef CONFIG_PERF_EVENTS

security/smack/smack_lsm.c

···
 #define SMK_RECEIVING	1
 #define SMK_SENDING	2
 
+#define SMACK_INODE_INIT_XATTRS 1
+
 #ifdef SMACK_IPV6_PORT_LABELING
 static DEFINE_MUTEX(smack_ipv6_lock);
 static LIST_HEAD(smk_ipv6_port_list);
···
  * @inode: the newly created inode
  * @dir: containing directory object
  * @qstr: unused
- * @name: where to put the attribute name
- * @value: where to put the attribute value
- * @len: where to put the length of the attribute
+ * @xattrs: where to put the attributes
+ * @xattr_count: current number of LSM-provided xattrs (updated)
  *
  * Returns 0 if it all works out, -ENOMEM if there's no memory
  */
 static int smack_inode_init_security(struct inode *inode, struct inode *dir,
-				     const struct qstr *qstr, const char **name,
-				     void **value, size_t *len)
+				     const struct qstr *qstr,
+				     struct xattr *xattrs, int *xattr_count)
 {
 	struct task_smack *tsp = smack_cred(current_cred());
 	struct inode_smack *issp = smack_inode(inode);
 	struct smack_known *skp = smk_of_task(tsp);
 	struct smack_known *isp = smk_of_inode(inode);
 	struct smack_known *dsp = smk_of_inode(dir);
+	struct xattr *xattr = lsm_get_xattr_slot(xattrs, xattr_count);
 	int may;
 
-	if (name)
-		*name = XATTR_SMACK_SUFFIX;
-
-	if (value && len) {
+	if (xattr) {
 		/*
 		 * If equal, transmuting already occurred in
 		 * smack_dentry_create_files_as(). No need to check again.
···
 			issp->smk_flags |= SMK_INODE_CHANGED;
 		}
 
-		*value = kstrdup(isp->smk_known, GFP_NOFS);
-		if (*value == NULL)
+		xattr->value = kstrdup(isp->smk_known, GFP_NOFS);
+		if (!xattr->value)
 			return -ENOMEM;
 
-		*len = strlen(isp->smk_known);
+		xattr->value_len = strlen(isp->smk_known);
+		xattr->name = XATTR_SMACK_SUFFIX;
 	}
 
 	return 0;
···
 	.lbs_ipc = sizeof(struct smack_known *),
 	.lbs_msg_msg = sizeof(struct smack_known *),
 	.lbs_superblock = sizeof(struct superblock_smack),
+	.lbs_xattr_count = SMACK_INODE_INIT_XATTRS,
 };
 
 static struct security_hook_list smack_hooks[] __ro_after_init = {