keys: safe concurrent user->{session,uid}_keyring access

The current code can perform concurrent updates and reads on
user->session_keyring and user->uid_keyring. Add a comment to
struct user_struct to document the nontrivial locking semantics, and use
READ_ONCE() for unlocked readers and smp_store_release() for writers to
prevent memory ordering issues.

Fixes: 69664cf16af4 ("keys: don't generate user and user session keyrings unless they're accessed")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: James Morris <james.morris@microsoft.com>

3 files changed, 27 insertions, 16 deletions

diff --git a/include/linux/sched/user.h b/include/linux/sched/user.h
index c7b5f86b91a1..468d2565a9fe 100644
--- a/include/linux/sched/user.h
+++ b/include/linux/sched/user.h
@@ -31,6 +31,13 @@ struct user_struct {
         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
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index bd7243cb4c85..f05f7125a7d5 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -58,7 +58,7 @@ int install_user_keyrings(void)
 
         kenter("%p{%u}", user, uid);
 
-        if (user->uid_keyring && user->session_keyring) {
+        if (READ_ONCE(user->uid_keyring) && READ_ONCE(user->session_keyring)) {
                 kleave(" = 0 [exist]");
                 return 0;
         }
@@ -111,8 +111,10 @@ int install_user_keyrings(void)
                 }
 
                 /* install the keyrings */
-                user->uid_keyring = uid_keyring;
-                user->session_keyring = session_keyring;
+                /* paired with READ_ONCE() */
+                smp_store_release(&user->uid_keyring, uid_keyring);
+                /* paired with READ_ONCE() */
+                smp_store_release(&user->session_keyring, session_keyring);
         }
 
         mutex_unlock(&key_user_keyring_mutex);
@@ -340,6 +342,7 @@ void key_fsgid_changed(struct task_struct *tsk)
 key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
 {
         key_ref_t key_ref, ret, err;
+        const struct cred *cred = ctx->cred;
 
         /* we want to return -EAGAIN or -ENOKEY if any of the keyrings were
          * searchable, but we failed to find a key or we found a negative key;
@@ -353,9 +356,9 @@ key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
         err = ERR_PTR(-EAGAIN);
 
         /* search the thread keyring first */
-        if (ctx->cred->thread_keyring) {
+        if (cred->thread_keyring) {
                 key_ref = keyring_search_aux(
-                        make_key_ref(ctx->cred->thread_keyring, 1), ctx);
+                        make_key_ref(cred->thread_keyring, 1), ctx);
                 if (!IS_ERR(key_ref))
                         goto found;
 
@@ -371,9 +374,9 @@ key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
         }
 
         /* search the process keyring second */
-        if (ctx->cred->process_keyring) {
+        if (cred->process_keyring) {
                 key_ref = keyring_search_aux(
-                        make_key_ref(ctx->cred->process_keyring, 1), ctx);
+                        make_key_ref(cred->process_keyring, 1), ctx);
                 if (!IS_ERR(key_ref))
                         goto found;
 
@@ -392,9 +395,9 @@ key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
         }
 
         /* search the session keyring */
-        if (ctx->cred->session_keyring) {
+        if (cred->session_keyring) {
                 key_ref = keyring_search_aux(
-                        make_key_ref(ctx->cred->session_keyring, 1), ctx);
+                        make_key_ref(cred->session_keyring, 1), ctx);
 
                 if (!IS_ERR(key_ref))
                         goto found;
@@ -413,9 +416,9 @@ key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
                 }
         }
         /* or search the user-session keyring */
-        else if (ctx->cred->user->session_keyring) {
+        else if (READ_ONCE(cred->user->session_keyring)) {
                 key_ref = keyring_search_aux(
-                        make_key_ref(ctx->cred->user->session_keyring, 1),
+                        make_key_ref(READ_ONCE(cred->user->session_keyring), 1),
                         ctx);
                 if (!IS_ERR(key_ref))
                         goto found;
@@ -602,7 +605,7 @@ try_again:
                                 goto error;
                         goto reget_creds;
                 } else if (ctx.cred->session_keyring ==
-                           ctx.cred->user->session_keyring &&
+                           READ_ONCE(ctx.cred->user->session_keyring) &&
                            lflags & KEY_LOOKUP_CREATE) {
                         ret = join_session_keyring(NULL);
                         if (ret < 0)
@@ -616,7 +619,7 @@ try_again:
                 break;
 
         case KEY_SPEC_USER_KEYRING:
-                if (!ctx.cred->user->uid_keyring) {
+                if (!READ_ONCE(ctx.cred->user->uid_keyring)) {
                         ret = install_user_keyrings();
                         if (ret < 0)
                                 goto error;
@@ -628,7 +631,7 @@ try_again:
                 break;
 
         case KEY_SPEC_USER_SESSION_KEYRING:
-                if (!ctx.cred->user->session_keyring) {
+                if (!READ_ONCE(ctx.cred->user->session_keyring)) {
                         ret = install_user_keyrings();
                         if (ret < 0)
                                 goto error;
diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index db72dc4d7639..75d87f9e0f49 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -293,11 +293,12 @@ static int construct_get_dest_keyring(struct key **_dest_keyring)
                         /* fall through */
                 case KEY_REQKEY_DEFL_USER_SESSION_KEYRING:
                         dest_keyring =
-                                key_get(cred->user->session_keyring);
+                                key_get(READ_ONCE(cred->user->session_keyring));
                         break;
 
                 case KEY_REQKEY_DEFL_USER_KEYRING:
-                        dest_keyring = key_get(cred->user->uid_keyring);
+                        dest_keyring =
+                                key_get(READ_ONCE(cred->user->uid_keyring));
                         break;
 
                 case KEY_REQKEY_DEFL_GROUP_KEYRING: