keys: Move the user and user-session keyrings to the user_namespace

Move the user and user-session keyrings to the user_namespace struct rather
than pinning them from the user_struct struct.  This prevents these
keyrings from propagating across user-namespaces boundaries with regard to
the KEY_SPEC_* flags, thereby making them more useful in a containerised
environment.

The issue is that a single user_struct may be represent UIDs in several
different namespaces.

The way the patch does this is by attaching a 'register keyring' in each
user_namespace and then sticking the user and user-session keyrings into
that.  It can then be searched to retrieve them.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jann Horn <jannh@google.com>

9 files changed, 196 insertions, 129 deletions

diff --git a/include/linux/sched/user.h b/include/linux/sched/user.h
index 468d2565a9fe..917d88edb7b9 100644
--- a/include/linux/sched/user.h
+++ b/include/linux/sched/user.h
@@ -7,8 +7,6 @@
 #include <linux/refcount.h>
 #include <linux/ratelimit.h>
 
-struct key;
-
 /*
  * Some day this will be a full-fledged user tracking system..
  */
@@ -30,18 +28,6 @@ struct user_struct {
         unsigned long unix_inflight;    /* How many files in flight in unix sockets */
         atomic_long_t pipe_bufs;  /* how many pages are allocated in pipe buffers */
 
-#ifdef CONFIG_KEYS
-        /*
-         * These pointers can only change from NULL to a non-NULL value once.
-         * Writes are protected by key_user_keyring_mutex.
-         * Unlocked readers should use READ_ONCE() unless they know that
-         * install_user_keyrings() has been called successfully (which sets
-         * these members to non-NULL values, preventing further modifications).
-         */
-        struct key *uid_keyring;        /* UID specific keyring */
-        struct key *session_keyring;    /* UID's default session keyring */
-#endif
-
         /* Hash table maintenance information */
         struct hlist_node uidhash_node;
         kuid_t uid;
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 90457015fa3f..fb9f4f799554 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -65,14 +65,19 @@ struct user_namespace {
         unsigned long           flags;
 
 #ifdef CONFIG_KEYS
-        /* List of joinable keyrings in this namespace */
+        /* List of joinable keyrings in this namespace.  Modification access of
+         * these pointers is controlled by keyring_sem.  Once
+         * user_keyring_register is set, it won't be changed, so it can be
+         * accessed directly with READ_ONCE().
+         */
         struct list_head        keyring_name_list;
+        struct key              *user_keyring_register;
+        struct rw_semaphore     keyring_sem;
 #endif
 
         /* Register of per-UID persistent keyrings for this namespace */
 #ifdef CONFIG_PERSISTENT_KEYRINGS
         struct key              *persistent_keyring_register;
-        struct rw_semaphore     persistent_keyring_register_sem;
 #endif
         struct work_struct      work;
 #ifdef CONFIG_SYSCTL
diff --git a/kernel/user.c b/kernel/user.c
index 50979fd1b7aa..f8519b62cf9a 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -64,10 +64,7 @@ struct user_namespace init_user_ns = {
         .flags = USERNS_INIT_FLAGS,
 #ifdef CONFIG_KEYS
         .keyring_name_list = LIST_HEAD_INIT(init_user_ns.keyring_name_list),
-#endif
-#ifdef CONFIG_PERSISTENT_KEYRINGS
-        .persistent_keyring_register_sem =
-        __RWSEM_INITIALIZER(init_user_ns.persistent_keyring_register_sem),
+        .keyring_sem = __RWSEM_INITIALIZER(init_user_ns.keyring_sem),
 #endif
 };
 EXPORT_SYMBOL_GPL(init_user_ns);
@@ -143,8 +140,6 @@ static void free_user(struct user_struct *up, unsigned long flags)
 {
         uid_hash_remove(up);
         spin_unlock_irqrestore(&uidhash_lock, flags);
-        key_put(up->uid_keyring);
-        key_put(up->session_keyring);
         kmem_cache_free(uid_cachep, up);
 }
 
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index bda6e890ad88..c87c2ecc7085 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -135,9 +135,7 @@ int create_user_ns(struct cred *new)
 
 #ifdef CONFIG_KEYS
         INIT_LIST_HEAD(&ns->keyring_name_list);
-#endif
-#ifdef CONFIG_PERSISTENT_KEYRINGS
-        init_rwsem(&ns->persistent_keyring_register_sem);
+        init_rwsem(&ns->keyring_sem);
 #endif
         ret = -ENOMEM;
         if (!setup_userns_sysctls(ns))
diff --git a/security/keys/internal.h b/security/keys/internal.h
index aa361299a3ec..d3a9439e2386 100644
--- a/security/keys/internal.h
+++ b/security/keys/internal.h
@@ -148,7 +148,8 @@ extern key_ref_t search_process_keyrings_rcu(struct keyring_search_context *ctx)
 
 extern struct key *find_keyring_by_name(const char *name, bool uid_keyring);
 
-extern int install_user_keyrings(void);
+extern int look_up_user_keyrings(struct key **, struct key **);
+extern struct key *get_user_session_keyring_rcu(const struct cred *);
 extern int install_thread_keyring_to_cred(struct cred *);
 extern int install_process_keyring_to_cred(struct cred *);
 extern int install_session_keyring_to_cred(struct cred *, struct key *);
diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index fe851292509e..3663e5168583 100644
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -62,6 +62,7 @@ void key_free_user_ns(struct user_namespace *ns)
         list_del_init(&ns->keyring_name_list);
         write_unlock(&keyring_name_lock);
 
+        key_put(ns->user_keyring_register);
 #ifdef CONFIG_PERSISTENT_KEYRINGS
         key_put(ns->persistent_keyring_register);
 #endif
diff --git a/security/keys/persistent.c b/security/keys/persistent.c
index fc29ec59efa7..90303fe4a394 100644
--- a/security/keys/persistent.c
+++ b/security/keys/persistent.c
@@ -91,9 +91,9 @@ static long key_get_persistent(struct user_namespace *ns, kuid_t uid,
 
         if (ns->persistent_keyring_register) {
                 reg_ref = make_key_ref(ns->persistent_keyring_register, true);
-                down_read(&ns->persistent_keyring_register_sem);
+                down_read(&ns->keyring_sem);
                 persistent_ref = find_key_to_update(reg_ref, &index_key);
-                up_read(&ns->persistent_keyring_register_sem);
+                up_read(&ns->keyring_sem);
 
                 if (persistent_ref)
                         goto found;
@@ -102,9 +102,9 @@ static long key_get_persistent(struct user_namespace *ns, kuid_t uid,
         /* It wasn't in the register, so we'll need to create it.  We might
          * also need to create the register.
          */
-        down_write(&ns->persistent_keyring_register_sem);
+        down_write(&ns->keyring_sem);
         persistent_ref = key_create_persistent(ns, uid, &index_key);
-        up_write(&ns->persistent_keyring_register_sem);
+        up_write(&ns->keyring_sem);
         if (!IS_ERR(persistent_ref))
                 goto found;
 
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index b07f768d23dc..f74d64215942 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -19,15 +19,13 @@
 #include <linux/security.h>
 #include <linux/user_namespace.h>
 #include <linux/uaccess.h>
+#include <linux/init_task.h>
 #include <keys/request_key_auth-type.h>
 #include "internal.h"
 
 /* Session keyring create vs join semaphore */
 static DEFINE_MUTEX(key_session_mutex);
 
-/* User keyring creation semaphore */
-static DEFINE_MUTEX(key_user_keyring_mutex);
-
 /* The root user's tracking struct */
 struct key_user root_key_user = {
         .usage          = REFCOUNT_INIT(3),
@@ -39,99 +37,186 @@ struct key_user root_key_user = {
 };
 
 /*
- * Install the user and user session keyrings for the current process's UID.
+ * Get or create a user register keyring.
+ */
+static struct key *get_user_register(struct user_namespace *user_ns)
+{
+        struct key *reg_keyring = READ_ONCE(user_ns->user_keyring_register);
+
+        if (reg_keyring)
+                return reg_keyring;
+
+        down_write(&user_ns->keyring_sem);
+
+        /* Make sure there's a register keyring.  It gets owned by the
+         * user_namespace's owner.
+         */
+        reg_keyring = user_ns->user_keyring_register;
+        if (!reg_keyring) {
+                reg_keyring = keyring_alloc(".user_reg",
+                                            user_ns->owner, INVALID_GID,
+                                            &init_cred,
+                                            KEY_POS_WRITE | KEY_POS_SEARCH |
+                                            KEY_USR_VIEW | KEY_USR_READ,
+                                            0,
+                                            NULL, NULL);
+                if (!IS_ERR(reg_keyring))
+                        smp_store_release(&user_ns->user_keyring_register,
+                                          reg_keyring);
+        }
+
+        up_write(&user_ns->keyring_sem);
+
+        /* We don't return a ref since the keyring is pinned by the user_ns */
+        return reg_keyring;
+}
+
+/*
+ * Look up the user and user session keyrings for the current process's UID,
+ * creating them if they don't exist.
  */
-int install_user_keyrings(void)
+int look_up_user_keyrings(struct key **_user_keyring,
+                          struct key **_user_session_keyring)
 {
-        struct user_struct *user;
-        const struct cred *cred;
-        struct key *uid_keyring, *session_keyring;
+        const struct cred *cred = current_cred();
+        struct user_namespace *user_ns = current_user_ns();
+        struct key *reg_keyring, *uid_keyring, *session_keyring;
         key_perm_t user_keyring_perm;
+        key_ref_t uid_keyring_r, session_keyring_r;
+        uid_t uid = from_kuid(user_ns, cred->user->uid);
         char buf[20];
         int ret;
-        uid_t uid;
 
         user_keyring_perm = (KEY_POS_ALL & ~KEY_POS_SETATTR) | KEY_USR_ALL;
-        cred = current_cred();
-        user = cred->user;
-        uid = from_kuid(cred->user_ns, user->uid);
 
-        kenter("%p{%u}", user, uid);
+        kenter("%u", uid);
 
-        if (READ_ONCE(user->uid_keyring) && READ_ONCE(user->session_keyring)) {
-                kleave(" = 0 [exist]");
-                return 0;
-        }
+        reg_keyring = get_user_register(user_ns);
+        if (IS_ERR(reg_keyring))
+                return PTR_ERR(reg_keyring);
 
-        mutex_lock(&key_user_keyring_mutex);
+        down_write(&user_ns->keyring_sem);
         ret = 0;
 
-        if (!user->uid_keyring) {
-                /* get the UID-specific keyring
-                 * - there may be one in existence already as it may have been
-                 *   pinned by a session, but the user_struct pointing to it
-                 *   may have been destroyed by setuid */
-                sprintf(buf, "_uid.%u", uid);
-
-                uid_keyring = find_keyring_by_name(buf, true);
+        /* Get the user keyring.  Note that there may be one in existence
+         * already as it may have been pinned by a session, but the user_struct
+         * pointing to it may have been destroyed by setuid.
+         */
+        snprintf(buf, sizeof(buf), "_uid.%u", uid);
+        uid_keyring_r = keyring_search(make_key_ref(reg_keyring, true),
+                                       &key_type_keyring, buf, false);
+        kdebug("_uid %p", uid_keyring_r);
+        if (uid_keyring_r == ERR_PTR(-EAGAIN)) {
+                uid_keyring = keyring_alloc(buf, cred->user->uid, INVALID_GID,
+                                            cred, user_keyring_perm,
+                                            KEY_ALLOC_UID_KEYRING |
+                                            KEY_ALLOC_IN_QUOTA,
+                                            NULL, reg_keyring);
                 if (IS_ERR(uid_keyring)) {
-                        uid_keyring = keyring_alloc(buf, user->uid, INVALID_GID,
-                                                    cred, user_keyring_perm,
-                                                    KEY_ALLOC_UID_KEYRING |
-                                                        KEY_ALLOC_IN_QUOTA,
-                                                    NULL, NULL);
-                        if (IS_ERR(uid_keyring)) {
-                                ret = PTR_ERR(uid_keyring);
-                                goto error;
-                        }
+                        ret = PTR_ERR(uid_keyring);
+                        goto error;
                 }
+        } else if (IS_ERR(uid_keyring_r)) {
+                ret = PTR_ERR(uid_keyring_r);
+                goto error;
+        } else {
+                uid_keyring = key_ref_to_ptr(uid_keyring_r);
+        }
 
-                /* get a default session keyring (which might also exist
-                 * already) */
-                sprintf(buf, "_uid_ses.%u", uid);
-
-                session_keyring = find_keyring_by_name(buf, true);
+        /* Get a default session keyring (which might also exist already) */
+        snprintf(buf, sizeof(buf), "_uid_ses.%u", uid);
+        session_keyring_r = keyring_search(make_key_ref(reg_keyring, true),
+                                           &key_type_keyring, buf, false);
+        kdebug("_uid_ses %p", session_keyring_r);
+        if (session_keyring_r == ERR_PTR(-EAGAIN)) {
+                session_keyring = keyring_alloc(buf, cred->user->uid, INVALID_GID,
+                                                cred, user_keyring_perm,
+                                                KEY_ALLOC_UID_KEYRING |
+                                                KEY_ALLOC_IN_QUOTA,
+                                                NULL, NULL);
                 if (IS_ERR(session_keyring)) {
-                        session_keyring =
-                                keyring_alloc(buf, user->uid, INVALID_GID,
-                                              cred, user_keyring_perm,
-                                              KEY_ALLOC_UID_KEYRING |
-                                                  KEY_ALLOC_IN_QUOTA,
-                                              NULL, NULL);
-                        if (IS_ERR(session_keyring)) {
-                                ret = PTR_ERR(session_keyring);
-                                goto error_release;
-                        }
-
-                        /* we install a link from the user session keyring to
-                         * the user keyring */
-                        ret = key_link(session_keyring, uid_keyring);
-                        if (ret < 0)
-                                goto error_release_both;
+                        ret = PTR_ERR(session_keyring);
+                        goto error_release;
                 }
 
-                /* install the keyrings */
-                /* paired with READ_ONCE() */
-                smp_store_release(&user->uid_keyring, uid_keyring);
-                /* paired with READ_ONCE() */
-                smp_store_release(&user->session_keyring, session_keyring);
+                /* We install a link from the user session keyring to
+                 * the user keyring.
+                 */
+                ret = key_link(session_keyring, uid_keyring);
+                if (ret < 0)
+                        goto error_release_session;
+
+                /* And only then link the user-session keyring to the
+                 * register.
+                 */
+                ret = key_link(reg_keyring, session_keyring);
+                if (ret < 0)
+                        goto error_release_session;
+        } else if (IS_ERR(session_keyring_r)) {
+                ret = PTR_ERR(session_keyring_r);
+                goto error_release;
+        } else {
+                session_keyring = key_ref_to_ptr(session_keyring_r);
         }
 
-        mutex_unlock(&key_user_keyring_mutex);
+        up_write(&user_ns->keyring_sem);
+
+        if (_user_session_keyring)
+                *_user_session_keyring = session_keyring;
+        else
+                key_put(session_keyring);
+        if (_user_keyring)
+                *_user_keyring = uid_keyring;
+        else
+                key_put(uid_keyring);
         kleave(" = 0");
         return 0;
 
-error_release_both:
+error_release_session:
         key_put(session_keyring);
 error_release:
         key_put(uid_keyring);
 error:
-        mutex_unlock(&key_user_keyring_mutex);
+        up_write(&user_ns->keyring_sem);
         kleave(" = %d", ret);
         return ret;
 }
 
 /*
+ * Get the user session keyring if it exists, but don't create it if it
+ * doesn't.
+ */
+struct key *get_user_session_keyring_rcu(const struct cred *cred)
+{
+        struct key *reg_keyring = READ_ONCE(cred->user_ns->user_keyring_register);
+        key_ref_t session_keyring_r;
+        char buf[20];
+
+        struct keyring_search_context ctx = {
+                .index_key.type         = &key_type_keyring,
+                .index_key.description  = buf,
+                .cred                   = cred,
+                .match_data.cmp         = key_default_cmp,
+                .match_data.raw_data    = buf,
+                .match_data.lookup_type = KEYRING_SEARCH_LOOKUP_DIRECT,
+                .flags                  = KEYRING_SEARCH_DO_STATE_CHECK,
+        };
+
+        if (!reg_keyring)
+                return NULL;
+
+        ctx.index_key.desc_len = snprintf(buf, sizeof(buf), "_uid_ses.%u",
+                                          from_kuid(cred->user_ns,
+                                                    cred->user->uid));
+
+        session_keyring_r = keyring_search_rcu(make_key_ref(reg_keyring, true),
+                                               &ctx);
+        if (IS_ERR(session_keyring_r))
+                return NULL;
+        return key_ref_to_ptr(session_keyring_r);
+}
+
+/*
  * Install a thread keyring to the given credentials struct if it didn't have
  * one already.  This is allowed to overrun the quota.
  *
@@ -340,6 +425,7 @@ void key_fsgid_changed(struct cred *new_cred)
  */
 key_ref_t search_cred_keyrings_rcu(struct keyring_search_context *ctx)
 {
+        struct key *user_session;
         key_ref_t key_ref, ret, err;
         const struct cred *cred = ctx->cred;
 
@@ -415,10 +501,11 @@ key_ref_t search_cred_keyrings_rcu(struct keyring_search_context *ctx)
                 }
         }
         /* or search the user-session keyring */
-        else if (READ_ONCE(cred->user->session_keyring)) {
-                key_ref = keyring_search_rcu(
-                        make_key_ref(READ_ONCE(cred->user->session_keyring), 1),
-                        ctx);
+        else if ((user_session = get_user_session_keyring_rcu(cred))) {
+                key_ref = keyring_search_rcu(make_key_ref(user_session, 1),
+                                             ctx);
+                key_put(user_session);
+
                 if (!IS_ERR(key_ref))
                         goto found;
 
@@ -535,7 +622,7 @@ key_ref_t lookup_user_key(key_serial_t id, unsigned long lflags,
                                            KEYRING_SEARCH_RECURSE),
         };
         struct request_key_auth *rka;
-        struct key *key;
+        struct key *key, *user_session;
         key_ref_t key_ref, skey_ref;
         int ret;
 
@@ -584,20 +671,20 @@ try_again:
                 if (!ctx.cred->session_keyring) {
                         /* always install a session keyring upon access if one
                          * doesn't exist yet */
-                        ret = install_user_keyrings();
+                        ret = look_up_user_keyrings(NULL, &user_session);
                         if (ret < 0)
                                 goto error;
                         if (lflags & KEY_LOOKUP_CREATE)
                                 ret = join_session_keyring(NULL);
                         else
-                                ret = install_session_keyring(
-                                        ctx.cred->user->session_keyring);
+                                ret = install_session_keyring(user_session);
 
+                        key_put(user_session);
                         if (ret < 0)
                                 goto error;
                         goto reget_creds;
-                } else if (ctx.cred->session_keyring ==
-                           READ_ONCE(ctx.cred->user->session_keyring) &&
+                } else if (test_bit(KEY_FLAG_UID_KEYRING,
+                                    &ctx.cred->session_keyring->flags) &&
                            lflags & KEY_LOOKUP_CREATE) {
                         ret = join_session_keyring(NULL);
                         if (ret < 0)
@@ -611,26 +698,16 @@ try_again:
                 break;
 
         case KEY_SPEC_USER_KEYRING:
-                if (!READ_ONCE(ctx.cred->user->uid_keyring)) {
-                        ret = install_user_keyrings();
-                        if (ret < 0)
-                                goto error;
-                }
-
-                key = ctx.cred->user->uid_keyring;
-                __key_get(key);
+                ret = look_up_user_keyrings(&key, NULL);
+                if (ret < 0)
+                        goto error;
                 key_ref = make_key_ref(key, 1);
                 break;
 
         case KEY_SPEC_USER_SESSION_KEYRING:
-                if (!READ_ONCE(ctx.cred->user->session_keyring)) {
-                        ret = install_user_keyrings();
-                        if (ret < 0)
-                                goto error;
-                }
-
-                key = ctx.cred->user->session_keyring;
-                __key_get(key);
+                ret = look_up_user_keyrings(NULL, &key);
+                if (ret < 0)
+                        goto error;
                 key_ref = make_key_ref(key, 1);
                 break;
 
@@ -879,7 +956,7 @@ void key_change_session_keyring(struct callback_head *twork)
  */
 static int __init init_root_keyring(void)
 {
-        return install_user_keyrings();
+        return look_up_user_keyrings(NULL, NULL);
 }
 
 late_initcall(init_root_keyring);
diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index 1ffd3803ce29..9201ca96c4df 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -121,7 +121,7 @@ static int call_sbin_request_key(struct key *authkey, void *aux)
         struct request_key_auth *rka = get_request_key_auth(authkey);
         const struct cred *cred = current_cred();
         key_serial_t prkey, sskey;
-        struct key *key = rka->target_key, *keyring, *session;
+        struct key *key = rka->target_key, *keyring, *session, *user_session;
         char *argv[9], *envp[3], uid_str[12], gid_str[12];
         char key_str[12], keyring_str[3][12];
         char desc[20];
@@ -129,9 +129,9 @@ static int call_sbin_request_key(struct key *authkey, void *aux)
 
         kenter("{%d},{%d},%s", key->serial, authkey->serial, rka->op);
 
-        ret = install_user_keyrings();
+        ret = look_up_user_keyrings(NULL, &user_session);
         if (ret < 0)
-                goto error_alloc;
+                goto error_us;
 
         /* allocate a new session keyring */
         sprintf(desc, "_req.%u", key->serial);
@@ -169,7 +169,7 @@ static int call_sbin_request_key(struct key *authkey, void *aux)
 
         session = cred->session_keyring;
         if (!session)
-                session = cred->user->session_keyring;
+                session = user_session;
         sskey = session->serial;
 
         sprintf(keyring_str[2], "%d", sskey);
@@ -211,6 +211,8 @@ error_link:
         key_put(keyring);
 
 error_alloc:
+        key_put(user_session);
+error_us:
         complete_request_key(authkey, ret);
         kleave(" = %d", ret);
         return ret;
@@ -317,13 +319,15 @@ static int construct_get_dest_keyring(struct key **_dest_keyring)
 
                         /* fall through */
                 case KEY_REQKEY_DEFL_USER_SESSION_KEYRING:
-                        dest_keyring =
-                                key_get(READ_ONCE(cred->user->session_keyring));
+                        ret = look_up_user_keyrings(NULL, &dest_keyring);
+                        if (ret < 0)
+                                return ret;
                         break;
 
                 case KEY_REQKEY_DEFL_USER_KEYRING:
-                        dest_keyring =
-                                key_get(READ_ONCE(cred->user->uid_keyring));
+                        ret = look_up_user_keyrings(&dest_keyring, NULL);
+                        if (ret < 0)
+                                return ret;
                         break;
 
                 case KEY_REQKEY_DEFL_GROUP_KEYRING: