[PATCH] Keys: Fix race between two instantiators of a key

Add a revocation notification method to the key type and calls it whilst
the key's semaphore is still write-locked after setting the revocation
flag.

The patch then uses this to maintain a reference on the task_struct of the
process that calls request_key() for as long as the authorisation key
remains unrevoked.

This fixes a potential race between two processes both of which have
assumed the authority to instantiate a key (one may have forked the other
for example).  The problem is that there's no locking around the check for
revocation of the auth key and the use of the task_struct it points to, nor
does the auth key keep a reference on the task_struct.

Access to the "context" pointer in the auth key must thenceforth be done
with the auth key semaphore held.  The revocation method is called with the
target key semaphore held write-locked and the search of the context
process's keyrings is done with the auth key semaphore read-locked.

The check for the revocation state of the auth key just prior to searching
it is done after the auth key is read-locked for the search.  This ensures
that the auth key can't be revoked between the check and the search.

The revocation notification method is added so that the context task_struct
can be released as soon as instantiation happens rather than waiting for
the auth key to be destroyed, thus avoiding the unnecessary pinning of the
requesting process.

Signed-off-by: David Howells <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: dhowells

Documentation/keys.txt

@@ -964,6 +964,16 @@ The structure has a number of fields, some of which are mandatory:
      It is not safe to sleep in this method; the caller may hold spinlocks.
 
 
+ (*) void (*revoke)(struct key *key);
+
+     This method is optional.  It is called to discard part of the payload
+     data upon a key being revoked.  The caller will have the key semaphore
+     write-locked.
+
+     It is safe to sleep in this method, though care should be taken to avoid
+     a deadlock against the key semaphore.
+
+
  (*) void (*destroy)(struct key *key);
 
      This method is optional. It is called to discard the payload data on a key

include/linux/key.h

@@ -205,6 +205,11 @@ struct key_type {
 	/* match a key against a description */
 	int (*match)(const struct key *key, const void *desc);
 
+	/* clear some of the data from a key on revokation (optional)
+	 * - the key's semaphore will be write-locked by the caller
+	 */
+	void (*revoke)(struct key *key);
+
 	/* clear the data from a key (optional) */
 	void (*destroy)(struct key *key);
 

security/keys/key.c

@@ -907,6 +907,10 @@ void key_revoke(struct key *key)
 	 * it */
 	down_write(&key->sem);
 	set_bit(KEY_FLAG_REVOKED, &key->flags);
+
+	if (key->type->revoke)
+		key->type->revoke(key);
+
 	up_write(&key->sem);
 
 } /* end key_revoke() */

security/keys/process_keys.c

@@ -391,6 +391,8 @@ key_ref_t search_process_keyrings(struct key_type *type,
 	struct request_key_auth *rka;
 	key_ref_t key_ref, ret, err;
 
+	might_sleep();
+
 	/* 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;
 	 * otherwise we want to return a sample error (probably -EACCES) if
@@ -496,27 +498,35 @@ key_ref_t search_process_keyrings(struct key_type *type,
 	 */
 	if (context->request_key_auth &&
 	    context == current &&
-	    type != &key_type_request_key_auth &&
-	    key_validate(context->request_key_auth) == 0
+	    type != &key_type_request_key_auth
 	    ) {
-		rka = context->request_key_auth->payload.data;
+		/* defend against the auth key being revoked */
+		down_read(&context->request_key_auth->sem);
 
-		key_ref = search_process_keyrings(type, description, match,
-						  rka->context);
+		if (key_validate(context->request_key_auth) == 0) {
+			rka = context->request_key_auth->payload.data;
 
-		if (!IS_ERR(key_ref))
-			goto found;
+			key_ref = search_process_keyrings(type, description,
+							  match, rka->context);
 
-		switch (PTR_ERR(key_ref)) {
-		case -EAGAIN: /* no key */
-			if (ret)
+			up_read(&context->request_key_auth->sem);
+
+			if (!IS_ERR(key_ref))
+				goto found;
+
+			switch (PTR_ERR(key_ref)) {
+			case -EAGAIN: /* no key */
+				if (ret)
+					break;
+			case -ENOKEY: /* negative key */
+				ret = key_ref;
 				break;
-		case -ENOKEY: /* negative key */
-			ret = key_ref;
-			break;
-		default:
-			err = key_ref;
-			break;
+			default:
+				err = key_ref;
+				break;
+			}
+		} else {
+			up_read(&context->request_key_auth->sem);
 		}
 	}
 

security/keys/request_key_auth.c

@@ -20,6 +20,7 @@
 
 static int request_key_auth_instantiate(struct key *, const void *, size_t);
 static void request_key_auth_describe(const struct key *, struct seq_file *);
+static void request_key_auth_revoke(struct key *);
 static void request_key_auth_destroy(struct key *);
 static long request_key_auth_read(const struct key *, char __user *, size_t);
 
@@ -31,6 +32,7 @@ struct key_type key_type_request_key_auth = {
 	.def_datalen	= sizeof(struct request_key_auth),
 	.instantiate	= request_key_auth_instantiate,
 	.describe	= request_key_auth_describe,
+	.revoke		= request_key_auth_revoke,
 	.destroy	= request_key_auth_destroy,
 	.read		= request_key_auth_read,
 };
@@ -91,6 +93,24 @@ static long request_key_auth_read(const struct key *key,
 
 } /* end request_key_auth_read() */
 
+/*****************************************************************************/
+/*
+ * handle revocation of an authorisation token key
+ * - called with the key sem write-locked
+ */
+static void request_key_auth_revoke(struct key *key)
+{
+	struct request_key_auth *rka = key->payload.data;
+
+	kenter("{%d}", key->serial);
+
+	if (rka->context) {
+		put_task_struct(rka->context);
+		rka->context = NULL;
+	}
+
+} /* end request_key_auth_revoke() */
+
 /*****************************************************************************/
 /*
  * destroy an instantiation authorisation token key
@@ -101,6 +121,11 @@ static void request_key_auth_destroy(struct key *key)
 
 	kenter("{%d}", key->serial);
 
+	if (rka->context) {
+		put_task_struct(rka->context);
+		rka->context = NULL;
+	}
+
 	key_put(rka->target_key);
 	kfree(rka);
 
@@ -131,14 +156,26 @@ struct key *request_key_auth_new(struct key *target, const char *callout_info)
 	 * another process */
 	if (current->request_key_auth) {
 		/* it is - use that instantiation context here too */
+		down_read(&current->request_key_auth->sem);
+
+		/* if the auth key has been revoked, then the key we're
+		 * servicing is already instantiated */
+		if (test_bit(KEY_FLAG_REVOKED,
+			     &current->request_key_auth->flags))
+			goto auth_key_revoked;
+
 		irka = current->request_key_auth->payload.data;
 		rka->context = irka->context;
 		rka->pid = irka->pid;
+		get_task_struct(rka->context);
+
+		up_read(&current->request_key_auth->sem);
 	}
 	else {
 		/* it isn't - use this process as the context */
 		rka->context = current;
 		rka->pid = current->pid;
+		get_task_struct(rka->context);
 	}
 
 	rka->target_key = key_get(target);
@@ -161,9 +198,15 @@ struct key *request_key_auth_new(struct key *target, const char *callout_info)
 	if (ret < 0)
 		goto error_inst;
 
-	kleave(" = {%d})", authkey->serial);
+	kleave(" = {%d}", authkey->serial);
 	return authkey;
 
+auth_key_revoked:
+	up_read(&current->request_key_auth->sem);
+	kfree(rka);
+	kleave("= -EKEYREVOKED");
+	return ERR_PTR(-EKEYREVOKED);
+
 error_inst:
 	key_revoke(authkey);
 	key_put(authkey);