[PATCH v10 2/4] security: Allow all LSMs to provide xattrs for inode_init_security hook

From: Roberto Sassu
Date: Fri Mar 31 2023 - 08:33:31 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_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.

Dynamically allocate the new_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. Unify the !initxattrs and initxattrs case, simply
don't allocate the new_xattrs array in the former.

Also, pass to the hook the number of xattrs filled by each LSM, so that
there are no gaps when the next LSM fills the array. Gaps might occur
because an LSM can legitimately request xattrs to the LSM infrastructure,
but not fill the reserved slots, if it was not initialized.

Update the documentation of security_inode_init_security() to reflect the
changes, and fix the description of the xattr name, as it is not allocated
anymore.

Finally, adapt both SELinux and Smack to use the new definition of the
inode_init_security hook, and to fill the reserved slots in the xattr
array. Introduce the lsm_get_xattr_slot() helper to retrieve an available
slot to fill, and to increment the number of filled slots.

Move the xattr->name assignment after the xattr->value one, so that it is
done only in case of successful memory allocation. For Smack, also reserve
space for the other defined xattrs although they are not set yet in
smack_inode_init_security().

Reported-by: Nicolas Bouchinet <nicolas.bouchinet@xxxxxxxxxxx> (EVM crash)
Link: https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/
Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
---
include/linux/lsm_hook_defs.h | 6 +--
include/linux/lsm_hooks.h | 20 ++++++++++
security/security.c | 71 +++++++++++++++++++++++------------
security/selinux/hooks.c | 17 +++++----
security/smack/smack_lsm.c | 32 ++++++++++------
5 files changed, 99 insertions(+), 47 deletions(-)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 6bb55e61e8e..a1896f90089 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -111,9 +111,9 @@ 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,
- 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,
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index c2be66c669a..9eb9b686493 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -28,6 +28,7 @@
#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__);
@@ -63,8 +64,27 @@ struct lsm_blob_sizes {
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
* LSM hooks (in include/linux/lsm_hook_defs.h).
diff --git a/security/security.c b/security/security.c
index f4170efcddd..1aeaa8ce449 100644
--- a/security/security.c
+++ b/security/security.c
@@ -31,8 +31,6 @@
#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)

@@ -212,6 +210,8 @@ 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_count,
+ &blob_sizes.lbs_xattr_count);
}

/* Prepare LSM for initialization. */
@@ -378,6 +378,7 @@ 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_count);

/*
* Create any kmem_caches needed for blobs
@@ -1591,11 +1592,15 @@ EXPORT_SYMBOL(security_dentry_create_files_as);
* 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.
@@ -1604,33 +1609,51 @@ 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 = 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(P, &security_hook_heads.inode_init_security,
+ list) {
+ ret = P->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);
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 9a5bdfc2131..882d6383b17 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, 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;
@@ -2899,16 +2901,14 @@ 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 (value && len) {
+ if (xattr) {
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;
+ xattr->name = XATTR_SELINUX_SUFFIX;
}

return 0;
@@ -6918,6 +6918,7 @@ 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_count = SELINUX_INODE_INIT_XATTRS,
};

#ifdef CONFIG_PERF_EVENTS
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index cfcbb748da2..8392983334b 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -52,6 +52,15 @@
#define SMK_RECEIVING 1
#define SMK_SENDING 2

+/*
+ * Smack uses multiple xattrs.
+ * SMACK64 - for access control, SMACK64EXEC - label for the program,
+ * SMACK64MMAP - controls library loading,
+ * SMACK64TRANSMUTE - label initialization,
+ * Not saved on files - SMACK64IPIN and SMACK64IPOUT
+ */
+#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 +948,23 @@ 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 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 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 = lsm_get_xattr_slot(xattrs, xattr_count);
int may;

- if (name)
- *name = XATTR_SMACK_SUFFIX;
-
- if (value && len) {
+ if (xattr) {
rcu_read_lock();
may = smk_access_entry(skp->smk_known, dsp->smk_known,
&skp->smk_rules);
@@ -976,11 +982,12 @@ 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);
+ xattr->name = XATTR_SMACK_SUFFIX;
}

return 0;
@@ -4854,6 +4861,7 @@ 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_count = SMACK_INODE_INIT_XATTRS,
};

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