[PATCH v8 4/6] security: Allow all LSMs to provide xattrs for inode_init_security hook

From: Roberto Sassu
Date: Tue Mar 14 2023 - 04:20:41 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().

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>
Reviewed-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
Reviewed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
---
include/linux/lsm_hook_defs.h | 3 +-
include/linux/lsm_hooks.h | 1 +
security/security.c | 119 +++++++++++++++++++++++++++++-----
security/selinux/hooks.c | 19 ++++--
security/smack/smack_lsm.c | 33 ++++++----
5 files changed, 137 insertions(+), 38 deletions(-)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 6bb55e61e8e..b814955ae70 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 c2be66c669a..75a2f85b49d 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -63,6 +63,7 @@ struct lsm_blob_sizes {
int lbs_ipc;
int lbs_msg_msg;
int lbs_task;
+ int lbs_xattr; /* number of xattr slots in new_xattrs array */
};

/*
diff --git a/security/security.c b/security/security.c
index f4170efcddd..f1f5f62f7fa 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 +210,7 @@ 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. */
@@ -378,6 +377,7 @@ 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
@@ -1579,6 +1579,52 @@ 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;
+}
+
/**
* security_inode_init_security() - Initialize an inode's LSM context
* @inode: the inode
@@ -1591,9 +1637,13 @@ 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
+ * hooks called by the VFS. The hook function is expected to populate the
+ * @xattrs array, depending on how many xattrs have been specified by the
+ * security module in the lbs_xattr field of the lsm_blob_sizes structure. For
+ * each array element, the hook function is expected to 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.
*
@@ -1604,33 +1654,66 @@ 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;
+ /*
+ * 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 (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 9a5bdfc2131..3e4308dd336 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;
@@ -6918,6 +6922,7 @@ 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 cfcbb748da2..c8cf8df268b 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 +948,27 @@ 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 attributes
*
* 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 +986,11 @@ 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;
@@ -4854,6 +4864,7 @@ 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