KEYS: Fix race between updating and finding a negative key

Consolidate KEY_FLAG_INSTANTIATED, KEY_FLAG_NEGATIVE and the rejection
error into one field such that:

 (1) The instantiation state can be modified/read atomically.

 (2) The error can be accessed atomically with the state.

 (3) The error isn't stored unioned with the payload pointers.

This deals with the problem that the state is spread over three different
objects (two bits and a separate variable) and reading or updating them
atomically isn't practical, given that not only can uninstantiated keys
change into instantiated or rejected keys, but rejected keys can also turn
into instantiated keys - and someone accessing the key might not be using
any locking.

The main side effect of this problem is that what was held in the payload
may change, depending on the state.  For instance, you might observe the
key to be in the rejected state.  You then read the cached error, but if
the key semaphore wasn't locked, the key might've become instantiated
between the two reads - and you might now have something in hand that isn't
actually an error code.

The state is now KEY_IS_UNINSTANTIATED, KEY_IS_POSITIVE or a negative error
code if the key is negatively instantiated.  The key_is_instantiated()
function is replaced with key_is_positive() to avoid confusion as negative
keys are also 'instantiated'.

Additionally, barriering is included:

 (1) Order payload-set before state-set during instantiation.

 (2) Order state-read before payload-read when using the key.

Further separate barriering is necessary if RCU is being used to access the
payload content after reading the payload pointers.

Fixes: 146aa8b1453b ("KEYS: Merge the type-specific data with the payload data")
Cc: stable@vger.kernel.org # v4.4+
Reported-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>

12 files changed, 49 insertions, 39 deletions

diff --git a/security/keys/big_key.c b/security/keys/big_key.c
index e607830b6154..929e14978c42 100644
--- a/security/keys/big_key.c
+++ b/security/keys/big_key.c
@@ -247,7 +247,7 @@ void big_key_revoke(struct key *key)
 
         /* clear the quota */
         key_payload_reserve(key, 0);
-        if (key_is_instantiated(key) &&
+        if (key_is_positive(key) &&
             (size_t)key->payload.data[big_key_len] > BIG_KEY_FILE_THRESHOLD)
                 vfs_truncate(path, 0);
 }
@@ -279,7 +279,7 @@ void big_key_describe(const struct key *key, struct seq_file *m)
 
         seq_puts(m, key->description);
 
-        if (key_is_instantiated(key))
+        if (key_is_positive(key))
                 seq_printf(m, ": %zu [%s]",
                            datalen,
                            datalen > BIG_KEY_FILE_THRESHOLD ? "file" : "buff");
diff --git a/security/keys/encrypted-keys/encrypted.c b/security/keys/encrypted-keys/encrypted.c
index 535db141f4da..d92cbf9687c3 100644
--- a/security/keys/encrypted-keys/encrypted.c
+++ b/security/keys/encrypted-keys/encrypted.c
@@ -854,7 +854,7 @@ static int encrypted_update(struct key *key, struct key_preparsed_payload *prep)
         size_t datalen = prep->datalen;
         int ret = 0;
 
-        if (test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+        if (key_is_negative(key))
                 return -ENOKEY;
         if (datalen <= 0 || datalen > 32767 || !prep->data)
                 return -EINVAL;
diff --git a/security/keys/gc.c b/security/keys/gc.c
index 87cb260e4890..f01d48cb3de1 100644
--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -129,15 +129,15 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
         while (!list_empty(keys)) {
                 struct key *key =
                         list_entry(keys->next, struct key, graveyard_link);
+                short state = key->state;
+
                 list_del(&key->graveyard_link);
 
                 kdebug("- %u", key->serial);
                 key_check(key);
 
                 /* Throw away the key data if the key is instantiated */
-                if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags) &&
-                    !test_bit(KEY_FLAG_NEGATIVE, &key->flags) &&
-                    key->type->destroy)
+                if (state == KEY_IS_POSITIVE && key->type->destroy)
                         key->type->destroy(key);
 
                 security_key_free(key);
@@ -151,7 +151,7 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
                 }
 
                 atomic_dec(&key->user->nkeys);
-                if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
+                if (state != KEY_IS_UNINSTANTIATED)
                         atomic_dec(&key->user->nikeys);
 
                 key_user_put(key->user);
diff --git a/security/keys/key.c b/security/keys/key.c
index eb914a838840..9385e7cc710f 100644
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -402,6 +402,18 @@ int key_payload_reserve(struct key *key, size_t datalen)
 EXPORT_SYMBOL(key_payload_reserve);
 
 /*
+ * Change the key state to being instantiated.
+ */
+static void mark_key_instantiated(struct key *key, int reject_error)
+{
+        /* Commit the payload before setting the state; barrier versus
+         * key_read_state().
+         */
+        smp_store_release(&key->state,
+                          (reject_error < 0) ? reject_error : KEY_IS_POSITIVE);
+}
+
+/*
  * Instantiate a key and link it into the target keyring atomically.  Must be
  * called with the target keyring's semaphore writelocked.  The target key's
  * semaphore need not be locked as instantiation is serialised by
@@ -424,14 +436,14 @@ static int __key_instantiate_and_link(struct key *key,
         mutex_lock(&key_construction_mutex);
 
         /* can't instantiate twice */
-        if (!test_bit(KEY_FLAG_INSTANTIATED, &key->flags)) {
+        if (key->state == KEY_IS_UNINSTANTIATED) {
                 /* instantiate the key */
                 ret = key->type->instantiate(key, prep);
 
                 if (ret == 0) {
                         /* mark the key as being instantiated */
                         atomic_inc(&key->user->nikeys);
-                        set_bit(KEY_FLAG_INSTANTIATED, &key->flags);
+                        mark_key_instantiated(key, 0);
 
                         if (test_and_clear_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags))
                                 awaken = 1;
@@ -577,13 +589,10 @@ int key_reject_and_link(struct key *key,
         mutex_lock(&key_construction_mutex);
 
         /* can't instantiate twice */
-        if (!test_bit(KEY_FLAG_INSTANTIATED, &key->flags)) {
+        if (key->state == KEY_IS_UNINSTANTIATED) {
                 /* mark the key as being negatively instantiated */
                 atomic_inc(&key->user->nikeys);
-                key->reject_error = -error;
-                smp_wmb();
-                set_bit(KEY_FLAG_NEGATIVE, &key->flags);
-                set_bit(KEY_FLAG_INSTANTIATED, &key->flags);
+                mark_key_instantiated(key, -error);
                 now = current_kernel_time();
                 key->expiry = now.tv_sec + timeout;
                 key_schedule_gc(key->expiry + key_gc_delay);
@@ -752,8 +761,8 @@ static inline key_ref_t __key_update(key_ref_t key_ref,
 
         ret = key->type->update(key, prep);
         if (ret == 0)
-                /* updating a negative key instantiates it */
-                clear_bit(KEY_FLAG_NEGATIVE, &key->flags);
+                /* Updating a negative key positively instantiates it */
+                mark_key_instantiated(key, 0);
 
         up_write(&key->sem);
 
@@ -986,8 +995,8 @@ int key_update(key_ref_t key_ref, const void *payload, size_t plen)
 
         ret = key->type->update(key, &prep);
         if (ret == 0)
-                /* updating a negative key instantiates it */
-                clear_bit(KEY_FLAG_NEGATIVE, &key->flags);
+                /* Updating a negative key positively instantiates it */
+                mark_key_instantiated(key, 0);
 
         up_write(&key->sem);
 
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 365ff85d7e27..76d22f726ae4 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -766,10 +766,9 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
 
         key = key_ref_to_ptr(key_ref);
 
-        if (test_bit(KEY_FLAG_NEGATIVE, &key->flags)) {
-                ret = -ENOKEY;
-                goto error2;
-        }
+        ret = key_read_state(key);
+        if (ret < 0)
+                goto error2; /* Negatively instantiated */
 
         /* see if we can read it directly */
         ret = key_permission(key_ref, KEY_NEED_READ);
@@ -901,7 +900,7 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
                 atomic_dec(&key->user->nkeys);
                 atomic_inc(&newowner->nkeys);
 
-                if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags)) {
+                if (key->state != KEY_IS_UNINSTANTIATED) {
                         atomic_dec(&key->user->nikeys);
                         atomic_inc(&newowner->nikeys);
                 }
diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index 4fa82a8a9c0e..06173b091a74 100644
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -414,7 +414,7 @@ static void keyring_describe(const struct key *keyring, struct seq_file *m)
         else
                 seq_puts(m, "[anon]");
 
-        if (key_is_instantiated(keyring)) {
+        if (key_is_positive(keyring)) {
                 if (keyring->keys.nr_leaves_on_tree != 0)
                         seq_printf(m, ": %lu", keyring->keys.nr_leaves_on_tree);
                 else
@@ -553,7 +553,8 @@ static int keyring_search_iterator(const void *object, void *iterator_data)
 {
         struct keyring_search_context *ctx = iterator_data;
         const struct key *key = keyring_ptr_to_key(object);
-        unsigned long kflags = key->flags;
+        unsigned long kflags = READ_ONCE(key->flags);
+        short state = READ_ONCE(key->state);
 
         kenter("{%d}", key->serial);
 
@@ -597,9 +598,8 @@ static int keyring_search_iterator(const void *object, void *iterator_data)
 
         if (ctx->flags & KEYRING_SEARCH_DO_STATE_CHECK) {
                 /* we set a different error code if we pass a negative key */
-                if (kflags & (1 << KEY_FLAG_NEGATIVE)) {
-                        smp_rmb();
-                        ctx->result = ERR_PTR(key->reject_error);
+                if (state < 0) {
+                        ctx->result = ERR_PTR(state);
                         kleave(" = %d [neg]", ctx->skipped_ret);
                         goto skipped;
                 }
diff --git a/security/keys/proc.c b/security/keys/proc.c
index de834309d100..4089ce1f7757 100644
--- a/security/keys/proc.c
+++ b/security/keys/proc.c
@@ -182,6 +182,7 @@ static int proc_keys_show(struct seq_file *m, void *v)
         unsigned long timo;
         key_ref_t key_ref, skey_ref;
         char xbuf[16];
+        short state;
         int rc;
 
         struct keyring_search_context ctx = {
@@ -236,17 +237,19 @@ static int proc_keys_show(struct seq_file *m, void *v)
                         sprintf(xbuf, "%luw", timo / (60*60*24*7));
         }
 
+        state = key_read_state(key);
+
 #define showflag(KEY, LETTER, FLAG) \
         (test_bit(FLAG, &(KEY)->flags) ? LETTER : '-')
 
         seq_printf(m, "%08x %c%c%c%c%c%c%c %5d %4s %08x %5d %5d %-9.9s ",
                    key->serial,
-                   showflag(key, 'I', KEY_FLAG_INSTANTIATED),
+                   state != KEY_IS_UNINSTANTIATED ? 'I' : '-',
                    showflag(key, 'R', KEY_FLAG_REVOKED),
                    showflag(key, 'D', KEY_FLAG_DEAD),
                    showflag(key, 'Q', KEY_FLAG_IN_QUOTA),
                    showflag(key, 'U', KEY_FLAG_USER_CONSTRUCT),
-                   showflag(key, 'N', KEY_FLAG_NEGATIVE),
+                   state < 0 ? 'N' : '-',
                    showflag(key, 'i', KEY_FLAG_INVALIDATED),
                    refcount_read(&key->usage),
                    xbuf,
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index 293d3598153b..740affd65ee9 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -730,7 +730,7 @@ try_again:
 
         ret = -EIO;
         if (!(lflags & KEY_LOOKUP_PARTIAL) &&
-            !test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
+            key_read_state(key) == KEY_IS_UNINSTANTIATED)
                 goto invalid_key;
 
         /* check the permissions */
diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index 63e63a42db3c..e8036cd0ad54 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -595,10 +595,9 @@ int wait_for_key_construction(struct key *key, bool intr)
                           intr ? TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE);
         if (ret)
                 return -ERESTARTSYS;
-        if (test_bit(KEY_FLAG_NEGATIVE, &key->flags)) {
-                smp_rmb();
-                return key->reject_error;
-        }
+        ret = key_read_state(key);
+        if (ret < 0)
+                return ret;
         return key_validate(key);
 }
 EXPORT_SYMBOL(wait_for_key_construction);
diff --git a/security/keys/request_key_auth.c b/security/keys/request_key_auth.c
index 6ebf1af8fce9..424e1d90412e 100644
--- a/security/keys/request_key_auth.c
+++ b/security/keys/request_key_auth.c
@@ -73,7 +73,7 @@ static void request_key_auth_describe(const struct key *key,
 
         seq_puts(m, "key:");
         seq_puts(m, key->description);
-        if (key_is_instantiated(key))
+        if (key_is_positive(key))
                 seq_printf(m, " pid:%d ci:%zu", rka->pid, rka->callout_len);
 }
 
diff --git a/security/keys/trusted.c b/security/keys/trusted.c
index ddfaebf60fc8..bd85315cbfeb 100644
--- a/security/keys/trusted.c
+++ b/security/keys/trusted.c
@@ -1066,7 +1066,7 @@ static int trusted_update(struct key *key, struct key_preparsed_payload *prep)
         char *datablob;
         int ret = 0;
 
-        if (test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+        if (key_is_negative(key))
                 return -ENOKEY;
         p = key->payload.data[0];
         if (!p->migratable)
diff --git a/security/keys/user_defined.c b/security/keys/user_defined.c
index 3d8c68eba516..9f558bedba23 100644
--- a/security/keys/user_defined.c
+++ b/security/keys/user_defined.c
@@ -114,7 +114,7 @@ int user_update(struct key *key, struct key_preparsed_payload *prep)
 
         /* attach the new data, displacing the old */
         key->expiry = prep->expiry;
-        if (!test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+        if (key_is_positive(key))
                 zap = dereference_key_locked(key);
         rcu_assign_keypointer(key, prep->payload.data[0]);
         prep->payload.data[0] = NULL;
@@ -162,7 +162,7 @@ EXPORT_SYMBOL_GPL(user_destroy);
 void user_describe(const struct key *key, struct seq_file *m)
 {
         seq_puts(m, key->description);
-        if (key_is_instantiated(key))
+        if (key_is_positive(key))
                 seq_printf(m, ": %u", key->datalen);
 }