From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757160Ab0G2LrH (ORCPT ); Thu, 29 Jul 2010 07:47:07 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42821 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753404Ab0G2LrE (ORCPT ); Thu, 29 Jul 2010 07:47:04 -0400 Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells Subject: [PATCH 2/2] CRED: Fix __task_cred()'s lockdep check and banner comment To: torvalds@osdl.org, akpm@linux-foundation.org Cc: linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, David Howells , Jiri Olsa , "Paul E. McKenney" Date: Thu, 29 Jul 2010 12:45:55 +0100 Message-ID: <20100729114555.29508.4525.stgit@warthog.procyon.org.uk> In-Reply-To: <20100729114549.29508.44899.stgit@warthog.procyon.org.uk> References: <20100729114549.29508.44899.stgit@warthog.procyon.org.uk> User-Agent: StGIT/0.14.3 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Fix __task_cred()'s lockdep check by removing the following validation condition: lockdep_tasklist_lock_is_held() as commit_creds() does not take the tasklist_lock, and nor do most of the functions that call it, so this check is pointless and it can prevent detection of the RCU lock not being held if the tasklist_lock is held. Instead, add the following validation condition: task->exit_state >= 0 to permit the access if the target task is dead and therefore unable to change its own credentials. Fix __task_cred()'s comment to: (1) discard the bit that says that the caller must prevent the target task from being deleted. That shouldn't need saying. (2) Add a comment indicating the result of __task_cred() should not be passed directly to get_cred(), but rather than get_task_cred() should be used instead. Also put a note into the documentation to enforce this point there too. Signed-off-by: David Howells Acked-by: Jiri Olsa Cc: Paul E. McKenney --- Documentation/credentials.txt | 3 +++ include/linux/cred.h | 15 ++++++++++----- include/linux/sched.h | 1 + 3 files changed, 14 insertions(+), 5 deletions(-) diff --git a/Documentation/credentials.txt b/Documentation/credentials.txt index a2db352..995baf3 100644 --- a/Documentation/credentials.txt +++ b/Documentation/credentials.txt @@ -417,6 +417,9 @@ reference on them using: This does all the RCU magic inside of it. The caller must call put_cred() on the credentials so obtained when they're finished with. + [*] Note: The result of __task_cred() should not be passed directly to + get_cred() as this may race with commit_cred(). + There are a couple of convenience functions to access bits of another task's credentials, hiding the RCU magic from the caller: diff --git a/include/linux/cred.h b/include/linux/cred.h index ce40cbc..4d2c395 100644 --- a/include/linux/cred.h +++ b/include/linux/cred.h @@ -274,13 +274,18 @@ static inline void put_cred(const struct cred *_cred) * @task: The task to query * * Access the objective credentials of a task. The caller must hold the RCU - * readlock. + * readlock or the task must be dead and unable to change its own credentials. * - * The caller must make sure task doesn't go away, either by holding a ref on - * task or by holding tasklist_lock to prevent it from being unlinked. + * The result of this function should not be passed directly to get_cred(); + * rather get_task_cred() should be used instead. */ -#define __task_cred(task) \ - ((const struct cred *)(rcu_dereference_check((task)->real_cred, rcu_read_lock_held() || lockdep_tasklist_lock_is_held()))) +#define __task_cred(task) \ + ({ \ + const struct task_struct *__t = (task); \ + rcu_dereference_check(__t->real_cred, \ + rcu_read_lock_held() || \ + task_is_dead(__t)); \ + }) /** * get_current_cred - Get the current task's subjective credentials diff --git a/include/linux/sched.h b/include/linux/sched.h index 747fcae..0478888 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -214,6 +214,7 @@ extern char ___assert_task_state[1 - 2*!!( #define task_is_traced(task) ((task->state & __TASK_TRACED) != 0) #define task_is_stopped(task) ((task->state & __TASK_STOPPED) != 0) +#define task_is_dead(task) ((task)->exit_state != 0) #define task_is_stopped_or_traced(task) \ ((task->state & (__TASK_STOPPED | __TASK_TRACED)) != 0) #define task_contributes_to_load(task) \