[PATCH v22 12/23] LSM: Specify which LSM to display

Casey Schaufler
2020-11-05T01:03:33Z

Create a new entry "display" in the procfs attr directory for
controlling which LSM security information is displayed for a
process. A process can only read or write its own display value.

The name of an active LSM that supplies hooks for
human readable data may be written to "display" to set the
value. The name of the LSM currently in use can be read from
"display". At this point there can only be one LSM capable
of display active. A helper function lsm_task_display() is
provided to get the display slot for a task_struct.

Setting the "display" requires that all security modules using
setprocattr hooks allow the action. Each security module is
responsible for defining its policy.

AppArmor hook provided by John Johansen <john.johansen@canonical.com>
SELinux hook provided by Stephen Smalley <sds@tycho.nsa.gov>

Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Cc: linux-api@vger.kernel.org
---
fs/proc/base.c | 1 +
include/linux/lsm_hooks.h | 17 +++
security/apparmor/include/apparmor.h | 3 +-
security/apparmor/lsm.c | 32 +++++
security/security.c | 169 ++++++++++++++++++++++++---
security/selinux/hooks.c | 11 ++
security/selinux/include/classmap.h | 2 +-
security/smack/smack_lsm.c | 7 ++
8 files changed, 223 insertions(+), 19 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 0f707003dda5..7432f24f0132 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2806,6 +2806,7 @@ static const struct pid_entry attr_dir_stuff[] = {
ATTR(NULL, "fscreate", 0666),
ATTR(NULL, "keycreate", 0666),
ATTR(NULL, "sockcreate", 0666),
+ ATTR(NULL, "display", 0666),
#ifdef CONFIG_SECURITY_SMACK
DIR("smack", 0555,
proc_smack_attr_dir_inode_ops, proc_smack_attr_dir_ops),
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index fe9203f15993..c2f5adef0ad7 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -1660,4 +1660,21 @@ static inline void security_delete_hooks(struct security_hook_list *hooks,

extern int lsm_inode_alloc(struct inode *inode);

+/**
+ * lsm_task_display - the "display" LSM for this task
+ * @task: The task to report on
+ *
+ * Returns the task's display LSM slot.
+ */
+static inline int lsm_task_display(struct task_struct *task)
+{
+#ifdef CONFIG_SECURITY
+ int *display = task->security;
+
+ if (display)
+ return *display;
+#endif
+ return LSMBLOB_INVALID;
+}
+
#endif /* ! __LINUX_LSM_HOOKS_H */
diff --git a/security/apparmor/include/apparmor.h b/security/apparmor/include/apparmor.h
index 1fbabdb565a8..b1622fcb4394 100644
--- a/security/apparmor/include/apparmor.h
+++ b/security/apparmor/include/apparmor.h
@@ -28,8 +28,9 @@
#define AA_CLASS_SIGNAL 10
#define AA_CLASS_NET 14
#define AA_CLASS_LABEL 16
+#define AA_CLASS_DISPLAY_LSM 17

-#define AA_CLASS_LAST AA_CLASS_LABEL
+#define AA_CLASS_LAST AA_CLASS_DISPLAY_LSM

/* Control parameters settable through module/boot flags */
extern enum audit_mode aa_g_audit;
diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index 432915c1d427..31a6f11890f1 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -612,6 +612,25 @@ static int apparmor_getprocattr(struct task_struct *task, char *name,
return error;
}

+
+static int profile_display_lsm(struct aa_profile *profile,
+ struct common_audit_data *sa)
+{
+ struct aa_perms perms = { };
+ unsigned int state;
+
+ state = PROFILE_MEDIATES(profile, AA_CLASS_DISPLAY_LSM);
+ if (state) {
+ aa_compute_perms(profile->policy.dfa, state, &perms);
+ aa_apply_modes_to_perms(profile, &perms);
+ aad(sa)->label = &profile->label;
+
+ return aa_check_perms(profile, &perms, AA_MAY_WRITE, sa, NULL);
+ }
+
+ return 0;
+}
+
static int apparmor_setprocattr(const char *name, void *value,
size_t size)
{
@@ -623,6 +642,19 @@ static int apparmor_setprocattr(const char *name, void *value,
if (size == 0)
return -EINVAL;

+ /* LSM infrastructure does actual setting of display if allowed */
+ if (!strcmp(name, "display")) {
+ struct aa_profile *profile;
+ struct aa_label *label;
+
+ aad(&sa)->info = "set display lsm";
+ label = begin_current_label_crit_section();
+ error = fn_for_each_confined(label, profile,
+ profile_display_lsm(profile, &sa));
+ end_current_label_crit_section(label);
+ return error;
+ }
+
/* AppArmor requires that the buffer must be null terminated atm */
if (args[size - 1] != '\0') {
/* null terminate */
diff --git a/security/security.c b/security/security.c
index 78aeb2ae7010..f9d9f68d40cf 100644
--- a/security/security.c
+++ b/security/security.c
@@ -76,7 +76,16 @@ static struct kmem_cache *lsm_file_cache;
static struct kmem_cache *lsm_inode_cache;

char *lsm_names;
-static struct lsm_blob_sizes blob_sizes __lsm_ro_after_init;
+
+/*
+ * The task blob includes the "display" slot used for
+ * chosing which module presents contexts.
+ * Using a long to avoid potential alignment issues with
+ * module assigned task blobs.
+ */
+static struct lsm_blob_sizes blob_sizes __lsm_ro_after_init = {
+ .lbs_task = sizeof(long),
+};

/* Boot-time LSM user choice */
static __initdata const char *chosen_lsm_order;
@@ -471,8 +480,10 @@ static int lsm_append(const char *new, char **result)

/*
* Current index to use while initializing the lsmblob secid list.
+ * Pointers to the LSM id structures for local use.
*/
static int lsm_slot __lsm_ro_after_init;
+static struct lsm_id *lsm_slotlist[LSMBLOB_ENTRIES];

/**
* security_add_hooks - Add a modules hooks to the hook lists.
@@ -492,6 +503,7 @@ void __init security_add_hooks(struct security_hook_list *hooks, int count,
if (lsmid->slot == LSMBLOB_NEEDED) {
if (lsm_slot >= LSMBLOB_ENTRIES)
panic("%s Too many LSMs registered.\n", __func__);
+ lsm_slotlist[lsm_slot] = lsmid;
lsmid->slot = lsm_slot++;
init_debug("%s assigned lsmblob slot %d\n", lsmid->lsm,
lsmid->slot);
@@ -621,6 +633,8 @@ int lsm_inode_alloc(struct inode *inode)
*/
static int lsm_task_alloc(struct task_struct *task)
{
+ int *display;
+
if (blob_sizes.lbs_task == 0) {
task->security = NULL;
return 0;
@@ -629,6 +643,15 @@ static int lsm_task_alloc(struct task_struct *task)
task->security = kzalloc(blob_sizes.lbs_task, GFP_KERNEL);
if (task->security == NULL)
return -ENOMEM;
+
+ /*
+ * The start of the task blob contains the "display" LSM slot number.
+ * Start with it set to the invalid slot number, indicating that the
+ * default first registered LSM be displayed.
+ */
+ display = task->security;
+ *display = LSMBLOB_INVALID;
+
return 0;
}

@@ -1629,14 +1652,26 @@ int security_file_open(struct file *file)

int security_task_alloc(struct task_struct *task, unsigned long clone_flags)
{
+ int *odisplay = current->security;
+ int *ndisplay;
int rc = lsm_task_alloc(task);

- if (rc)
+ if (unlikely(rc))
return rc;
+
rc = call_int_hook(task_alloc, 0, task, clone_flags);
- if (unlikely(rc))
+ if (unlikely(rc)) {
security_task_free(task);
- return rc;
+ return rc;
+ }
+
+ if (odisplay) {
+ ndisplay = task->security;
+ if (ndisplay)
+ *ndisplay = *odisplay;
+ }
+
+ return 0;
}

void security_task_free(struct task_struct *task)
@@ -2054,23 +2089,110 @@ int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
char **value)
{
struct security_hook_list *hp;
+ int display = lsm_task_display(current);
+ int slot = 0;
+
+ if (!strcmp(name, "display")) {
+ /*
+ * lsm_slot will be 0 if there are no displaying modules.
+ */
+ if (lsm_slot == 0)
+ return -EINVAL;
+
+ /*
+ * Only allow getting the current process' display.
+ * There are too few reasons to get another process'
+ * display and too many LSM policy issues.
+ */
+ if (current != p)
+ return -EINVAL;
+
+ display = lsm_task_display(p);
+ if (display != LSMBLOB_INVALID)
+ slot = display;
+ *value = kstrdup(lsm_slotlist[slot]->lsm, GFP_KERNEL);
+ if (*value)
+ return strlen(*value);
+ return -ENOMEM;
+ }

hlist_for_each_entry(hp, &security_hook_heads.getprocattr, list) {
if (lsm != NULL && strcmp(lsm, hp->lsmid->lsm))
continue;
+ if (lsm == NULL && display != LSMBLOB_INVALID &&
+ display != hp->lsmid->slot)
+ continue;
return hp->hook.getprocattr(p, name, value);
}
return LSM_RET_DEFAULT(getprocattr);
}

+/**
+ * security_setprocattr - Set process attributes via /proc
+ * @lsm: name of module involved, or NULL
+ * @name: name of the attribute
+ * @value: value to set the attribute to
+ * @size: size of the value
+ *
+ * Set the process attribute for the specified security module
+ * to the specified value. Note that this can only be used to set
+ * the process attributes for the current, or "self" process.
+ * The /proc code has already done this check.
+ *
+ * Returns 0 on success, an appropriate code otherwise.
+ */
int security_setprocattr(const char *lsm, const char *name, void *value,
size_t size)
{
struct security_hook_list *hp;
+ char *termed;
+ char *copy;
+ int *display = current->security;
+ int rc = -EINVAL;
+ int slot = 0;
+
+ if (!strcmp(name, "display")) {
+ /*
+ * Change the "display" value only if all the security
+ * modules that support setting a procattr allow it.
+ * It is assumed that all such security modules will be
+ * cooperative.
+ */
+ if (size == 0)
+ return -EINVAL;
+
+ hlist_for_each_entry(hp, &security_hook_heads.setprocattr,
+ list) {
+ rc = hp->hook.setprocattr(name, value, size);
+ if (rc < 0)
+ return rc;
+ }
+
+ rc = -EINVAL;
+
+ copy = kmemdup_nul(value, size, GFP_KERNEL);
+ if (copy == NULL)
+ return -ENOMEM;
+
+ termed = strsep(&copy, " \n");
+
+ for (slot = 0; slot < lsm_slot; slot++)
+ if (!strcmp(termed, lsm_slotlist[slot]->lsm)) {
+ *display = lsm_slotlist[slot]->slot;
+ rc = size;
+ break;
+ }
+
+ kfree(termed);
+ return rc;
+ }

hlist_for_each_entry(hp, &security_hook_heads.setprocattr, list) {
if (lsm != NULL && strcmp(lsm, hp->lsmid->lsm))
continue;
+ if (lsm == NULL && *display != LSMBLOB_INVALID &&
+ *display != hp->lsmid->slot)
+ continue;
return hp->hook.setprocattr(name, value, size);
}
return LSM_RET_DEFAULT(setprocattr);
@@ -2090,15 +2212,15 @@ EXPORT_SYMBOL(security_ismaclabel);
int security_secid_to_secctx(struct lsmblob *blob, char **secdata, u32 *seclen)
{
struct security_hook_list *hp;
- int rc;
+ int display = lsm_task_display(current);

hlist_for_each_entry(hp, &security_hook_heads.secid_to_secctx, list) {
if (WARN_ON(hp->lsmid->slot < 0 || hp->lsmid->slot >= lsm_slot))
continue;
- rc = hp->hook.secid_to_secctx(blob->secid[hp->lsmid->slot],
- secdata, seclen);
- if (rc != LSM_RET_DEFAULT(secid_to_secctx))
- return rc;
+ if (display == LSMBLOB_INVALID || display == hp->lsmid->slot)
+ return hp->hook.secid_to_secctx(
+ blob->secid[hp->lsmid->slot],
+ secdata, seclen);
}

return LSM_RET_DEFAULT(secid_to_secctx);
@@ -2109,16 +2231,15 @@ int security_secctx_to_secid(const char *secdata, u32 seclen,
struct lsmblob *blob)
{
struct security_hook_list *hp;
- int rc;
+ int display = lsm_task_display(current);

lsmblob_init(blob, 0);
hlist_for_each_entry(hp, &security_hook_heads.secctx_to_secid, list) {
if (WARN_ON(hp->lsmid->slot < 0 || hp->lsmid->slot >= lsm_slot))
continue;
- rc = hp->hook.secctx_to_secid(secdata, seclen,
- &blob->secid[hp->lsmid->slot]);
- if (rc != 0)
- return rc;
+ if (display == LSMBLOB_INVALID || display == hp->lsmid->slot)
+ return hp->hook.secctx_to_secid(secdata, seclen,
+ &blob->secid[hp->lsmid->slot]);
}
return 0;
}
@@ -2126,7 +2247,14 @@ EXPORT_SYMBOL(security_secctx_to_secid);

void security_release_secctx(char *secdata, u32 seclen)
{
- call_void_hook(release_secctx, secdata, seclen);
+ struct security_hook_list *hp;
+ int display = lsm_task_display(current);
+
+ hlist_for_each_entry(hp, &security_hook_heads.release_secctx, list)
+ if (display == LSMBLOB_INVALID || display == hp->lsmid->slot) {
+ hp->hook.release_secctx(secdata, seclen);
+ return;
+ }
}
EXPORT_SYMBOL(security_release_secctx);

@@ -2267,8 +2395,15 @@ EXPORT_SYMBOL(security_sock_rcv_skb);
int security_socket_getpeersec_stream(struct socket *sock, char __user *optval,
int __user *optlen, unsigned len)
{
- return call_int_hook(socket_getpeersec_stream, -ENOPROTOOPT, sock,
- optval, optlen, len);
+ int display = lsm_task_display(current);
+ struct security_hook_list *hp;
+
+ hlist_for_each_entry(hp, &security_hook_heads.socket_getpeersec_stream,
+ list)
+ if (display == LSMBLOB_INVALID || display == hp->lsmid->slot)
+ return hp->hook.socket_getpeersec_stream(sock, optval,
+ optlen, len);
+ return -ENOPROTOOPT;
}

int security_socket_getpeersec_dgram(struct socket *sock, struct sk_buff *skb,
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 52a50d7ca534..9bfc1eea02de 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -6334,6 +6334,17 @@ static int selinux_setprocattr(const char *name, void *value, size_t size)
/*
* Basic control over ability to set these attributes at all.
*/
+
+ /*
+ * For setting display, we only perform a permission check;
+ * the actual update to the display value is handled by the
+ * LSM framework.
+ */
+ if (!strcmp(name, "display"))
+ return avc_has_perm(&selinux_state,
+ mysid, mysid, SECCLASS_PROCESS2,
+ PROCESS2__SETDISPLAY, NULL);
+
if (!strcmp(name, "exec"))
error = avc_has_perm(&selinux_state,
mysid, mysid, SECCLASS_PROCESS,
diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
index 40cebde62856..1858aa47b32a 100644
--- a/security/selinux/include/classmap.h
+++ b/security/selinux/include/classmap.h
@@ -53,7 +53,7 @@ struct security_class_mapping secclass_map[] = {
"execmem", "execstack", "execheap", "setkeycreate",
"setsockcreate", "getrlimit", NULL } },
{ "process2",
- { "nnp_transition", "nosuid_transition", NULL } },
+ { "nnp_transition", "nosuid_transition", "setdisplay", NULL } },
{ "system",
{ "ipc_info", "syslog_read", "syslog_mod",
"syslog_console", "module_request", "module_load", NULL } },
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index f96be93d1a75..bd03020b3cad 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -3509,6 +3509,13 @@ static int smack_setprocattr(const char *name, void *value, size_t size)
struct smack_known_list_elem *sklep;
int rc;

+ /*
+ * Allow the /proc/.../attr/current and SO_PEERSEC "display"
+ * to be reset at will.
+ */
+ if (strcmp(name, "display") == 0)
+ return 0;
+
if (!smack_privileged(CAP_MAC_ADMIN) && list_empty(&tsp->smk_relabel))
return -EPERM;

--
2.24.1