[PATCH v6 4/6] security: Allow all LSMs to provide xattrs for inode_init_security hook

From: Roberto Sassu
Date: Wed Nov 23 2022 - 10:49:15 EST

From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

Currently, security_inode_init_security() 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 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.

Dynamically allocate the xattrs array to be populated by LSMs with the
inode_init_security hook, and pass it to the latter instead of the
name/value/len triple. Update the documentation accordingly, and fix the
description of the xattr name, as it is not allocated anymore.

Since the LSM infrastructure, at initialization time, updates the number of
the requested xattrs provided by each LSM with a corresponding offset in
the security blob (in this case the xattr array), it makes straightforward
for an LSM to access the right position in the xattr array.

There is still the issue that an LSM might not fill the xattr, even if it
requests it (legitimate case, for example it might have been loaded but not
initialized with a policy). Since users of the xattr array (e.g. the
initxattrs() callbacks) detect the end of the xattr array by checking if
the xattr name is NULL, not filling an xattr would cause those users to
stop scanning xattrs prematurely.

Solve that issue by introducing security_check_compact_filled_xattrs(),
which does a basic check of the xattr array (if the xattr name is filled,
the xattr value should be too, and viceversa), and compacts the xattr array
by removing the holes.

An alternative solution would be to let users of the xattr array know the
number of elements of that array, so that they don't have to check the
termination. However, this seems more invasive, compared to a simple move
of few array elements.

security_check_compact_filled_xattrs() also determines how many xattrs in
the xattr array have been filled. If there is none, skip
evm_inode_init_security() and initxattrs(). Skipping the former also avoids
EVM to crash the kernel, as it is expecting a filled xattr.

Finally, adapt both SELinux and Smack to use the new definition of the
inode_init_security hook, and to correctly fill the designated slots in the
xattr array. For Smack, reserve space for the other defined xattrs although
they are not set yet in smack_inode_init_security().

Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
---
include/linux/lsm_hook_defs.h | 3 +-
include/linux/lsm_hooks.h | 17 ++++--
security/security.c | 103 +++++++++++++++++++++++++++++-----
security/selinux/hooks.c | 19 ++++---
security/smack/smack_lsm.c | 26 +++++----
5 files changed, 127 insertions(+), 41 deletions(-)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index ec119da1d89b..be344d0211f8 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -112,8 +112,7 @@ LSM_HOOK(int, 0, path_notify, const struct path *path, u64 mask,
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)
+ struct inode *dir, const struct qstr *qstr, struct xattr *xattrs)
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,
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 4ec80b96c22e..ba1655370643 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -229,18 +229,22 @@
* 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.
* @inode contains the inode structure of the newly created inode.
* @dir contains the inode structure of the parent directory.
* @qstr contains the last path component of the new object
- * @name will be set to the allocated name suffix (e.g. selinux).
- * @value will be set to the allocated attribute value.
- * @len will be set to the length of the value.
- * Returns 0 if @name and @value have been successfully set,
+ * @xattrs contains the full array of xattrs provided by LSMs where
+ * ->name will be set to the name suffix (e.g. selinux).
+ * ->value will be set to the allocated attribute value.
+ * ->value_len will be set to the length of the value.
+ * Slots in @xattrs need to be reserved by LSMs by providing the number of
+ * the desired xattrs in the lbs_xattr field of the lsm_blob_sizes
+ * structure.
+ * Returns 0 if the requested slots in @xattrs have been successfully set,
* -EOPNOTSUPP if no security attribute is needed, or
* -ENOMEM on memory allocation failure.
* @inode_init_security_anon:
@@ -1624,6 +1628,7 @@ struct lsm_blob_sizes {
int lbs_ipc;
int lbs_msg_msg;
int lbs_task;
+ int lbs_xattr;
};

/*
diff --git a/security/security.c b/security/security.c
index e2857446fd32..26aaa5850867 100644
--- a/security/security.c
+++ b/security/security.c
@@ -30,8 +30,6 @@
#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)

@@ -210,6 +208,7 @@ static void __init lsm_set_blob_sizes(struct lsm_blob_sizes *needed)
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, &blob_sizes.lbs_xattr);
}

/* Prepare LSM for initialization. */
@@ -346,6 +345,7 @@ static void __init ordered_lsm_init(void)
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);

/*
* Create any kmem_caches needed for blobs
@@ -1089,37 +1089,110 @@ int security_dentry_create_files_as(struct dentry *dentry, int mode,
}
EXPORT_SYMBOL(security_dentry_create_files_as);

+/**
+ * security_check_compact_filled_xattrs - check xattrs and make array contiguous
+ * @xattrs: xattr array filled by LSMs
+ * @num_xattrs: length of xattr array
+ * @num_filled_xattrs: number of already processed xattrs
+ *
+ * Ensure that each xattr slot is correctly filled and close the gaps in the
+ * xattr array if an LSM didn't provide an xattr for which it asked space
+ * (legitimate case, it might have been loaded but not initialized). An LSM
+ * might request space in the xattr array for one or multiple xattrs. The LSM
+ * infrastructure ensures that all requests by LSMs are satisfied.
+ *
+ * Track the number of filled xattrs in @num_filled_xattrs, so that it is easy
+ * to determine whether the currently processed xattr is fine in its position
+ * (if all previous xattrs were filled) or it should be moved after the last
+ * filled xattr.
+ *
+ * Return: zero if all xattrs are valid, -EINVAL otherwise.
+ */
+static int security_check_compact_filled_xattrs(struct xattr *xattrs,
+ int num_xattrs,
+ int *num_filled_xattrs)
+{
+ int i;
+
+ for (i = *num_filled_xattrs; i < num_xattrs; i++) {
+ if ((!xattrs[i].name && xattrs[i].value) ||
+ (xattrs[i].name && !xattrs[i].value))
+ return -EINVAL;
+
+ if (!xattrs[i].name)
+ continue;
+
+ if (i == *num_filled_xattrs) {
+ (*num_filled_xattrs)++;
+ continue;
+ }
+
+ memcpy(xattrs + (*num_filled_xattrs)++, xattrs + i,
+ sizeof(*xattrs));
+ memset(xattrs + i, 0, sizeof(*xattrs));
+ }
+
+ return 0;
+}
+
int security_inode_init_security(struct inode *inode, struct inode *dir,
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 *P;
+ struct xattr *new_xattrs;
+ struct xattr *xattr;
+ int ret = -EOPNOTSUPP, num_filled_xattrs = 0;

if (unlikely(IS_PRIVATE(inode)))
return 0;

+ if (!blob_sizes.lbs_xattr)
+ 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)
+ dir, qstr, NULL);
+ /* Allocate +1 for EVM and +1 as terminator. */
+ new_xattrs = kcalloc(blob_sizes.lbs_xattr + 2, sizeof(*new_xattrs),
+ GFP_NOFS);
+ if (!new_xattrs)
+ return -ENOMEM;
+
+ hlist_for_each_entry(P, &security_hook_heads.inode_init_security,
+ list) {
+ ret = P->hook.inode_init_security(inode, dir, qstr, new_xattrs);
+ if (ret && ret != -EOPNOTSUPP)
+ goto out;
+ if (ret == -EOPNOTSUPP)
+ continue;
+ /*
+ * As the number of xattrs reserved by LSMs is not directly
+ * available, directly use the total number blob_sizes.lbs_xattr
+ * to keep the code simple, while being not the most efficient
+ * way.
+ */
+ ret = security_check_compact_filled_xattrs(new_xattrs,
+ blob_sizes.lbs_xattr,
+ &num_filled_xattrs);
+ if (ret < 0) {
+ ret = -ENOMEM;
+ goto out;
+ }
+ }
+
+ if (!num_filled_xattrs)
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 + num_filled_xattrs);
if (ret)
goto out;
ret = initxattrs(inode, new_xattrs, fs_data);
out:
for (xattr = new_xattrs; xattr->value != NULL; xattr++)
kfree(xattr->value);
+ kfree(new_xattrs);
return (ret == -EOPNOTSUPP) ? 0 : ret;
}
EXPORT_SYMBOL(security_inode_init_security);
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index f553c370397e..57e5bc7c9ed8 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -104,6 +104,8 @@
#include "audit.h"
#include "avc_ss.h"

+#define SELINUX_INODE_INIT_XATTRS 1
+
struct selinux_state selinux_state;

/* SECMARK reference count */
@@ -2868,11 +2870,11 @@ static int selinux_dentry_create_files_as(struct dentry *dentry, int mode,

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)
{
const struct task_security_struct *tsec = selinux_cred(current_cred());
struct superblock_security_struct *sbsec;
+ struct xattr *xattr = NULL;
u32 newsid, clen;
int rc;
char *context;
@@ -2899,16 +2901,18 @@ static int selinux_inode_init_security(struct inode *inode, struct inode *dir,
!(sbsec->flags & SBLABEL_MNT))
return -EOPNOTSUPP;

- if (name)
- *name = XATTR_SELINUX_SUFFIX;
+ if (xattrs)
+ xattr = xattrs + selinux_blob_sizes.lbs_xattr;
+
+ if (xattr) {
+ xattr->name = XATTR_SELINUX_SUFFIX;

- if (value && len) {
rc = security_sid_to_context_force(&selinux_state, newsid,
&context, &clen);
if (rc)
return rc;
- *value = context;
- *len = clen;
+ xattr->value = context;
+ xattr->value_len = clen;
}

return 0;
@@ -6900,6 +6904,7 @@ struct lsm_blob_sizes selinux_blob_sizes __lsm_ro_after_init = {
.lbs_ipc = sizeof(struct ipc_security_struct),
.lbs_msg_msg = sizeof(struct msg_security_struct),
.lbs_superblock = sizeof(struct superblock_security_struct),
+ .lbs_xattr = SELINUX_INODE_INIT_XATTRS,
};

#ifdef CONFIG_PERF_EVENTS
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index b6306d71c908..a7c3e4284754 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -52,6 +52,8 @@
#define SMK_RECEIVING 1
#define SMK_SENDING 2

+#define SMACK_INODE_INIT_XATTRS 4
+
#ifdef SMACK_IPV6_PORT_LABELING
static DEFINE_MUTEX(smack_ipv6_lock);
static LIST_HEAD(smk_ipv6_port_list);
@@ -939,26 +941,27 @@ static int smack_inode_alloc_security(struct inode *inode)
* @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 attribute
*
* 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)
{
struct inode_smack *issp = smack_inode(inode);
struct smack_known *skp = smk_of_current();
struct smack_known *isp = smk_of_inode(inode);
struct smack_known *dsp = smk_of_inode(dir);
+ struct xattr *xattr = NULL;
int may;

- if (name)
- *name = XATTR_SMACK_SUFFIX;
+ if (xattrs)
+ xattr = xattrs + smack_blob_sizes.lbs_xattr;
+
+ if (xattr) {
+ xattr->name = XATTR_SMACK_SUFFIX;

- if (value && len) {
rcu_read_lock();
may = smk_access_entry(skp->smk_known, dsp->smk_known,
&skp->smk_rules);
@@ -976,11 +979,11 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
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 == NULL)
return -ENOMEM;

- *len = strlen(isp->smk_known);
+ xattr->value_len = strlen(isp->smk_known);
}

return 0;
@@ -4785,6 +4788,7 @@ struct lsm_blob_sizes smack_blob_sizes __lsm_ro_after_init = {
.lbs_ipc = sizeof(struct smack_known *),
.lbs_msg_msg = sizeof(struct smack_known *),
.lbs_superblock = sizeof(struct superblock_smack),
+ .lbs_xattr = SMACK_INODE_INIT_XATTRS,
};

static struct security_hook_list smack_hooks[] __lsm_ro_after_init = {
--
2.25.1