linux-kernel - Re: [patch 1/9] sys: Fix missing rcu protection for __task

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20091210012517.GF6938@linux.vnet.ibm.com>
Date:	Wed, 9 Dec 2009 17:25:17 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...sign.ru>,
	Al Viro <viro@...iv.linux.org.uk>,
	James Morris <jmorris@...ei.org>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-security-module@...r.kernel.org
Subject: Re: [patch 1/9] sys: Fix missing rcu protection for __task_cred()
	access

On Thu, Dec 10, 2009 at 12:52:51AM -0000, Thomas Gleixner wrote:
> commit c69e8d9 (CRED: Use RCU to access another task's creds and to
> release a task's own creds) added non rcu_read_lock() protected access
> to task creds of the target task in set_prio_one().
> 
> The comment above the function says:
>  * - the caller must hold the RCU read lock
> 
> The calling code in sys_setpriority does read_lock(&tasklist_lock) but
> not rcu_read_lock(). This works only when CONFIG_TREE_PREEMPT_RCU=n.
> With CONFIG_TREE_PREEMPT_RCU=y the rcu_callbacks can run in the tick
> interrupt when they see no read side critical section.

And this is called out in Documentation/RCU/checklist.txt, item #2:

2.	Do the RCU read-side critical sections make proper use of
	rcu_read_lock() and friends?  These primitives are needed
	to prevent grace periods from ending prematurely, which
	could result in data being unceremoniously freed out from
	under your read-side code, which can greatly increase the
	actuarial risk of your kernel.

	As a rough rule of thumb, any dereference of an RCU-protected
	pointer must be covered by rcu_read_lock() or rcu_read_lock_bh()
	or by the appropriate update-side lock.

> There is another instance of __task_cred() in sys_setpriority() itself
> which is equally unprotected.
> 
> Wrap the whole code section into a rcu read side critical section to
> fix this quick and dirty.
> 
> Will be revisited in course of the read_lock(&tasklist_lock) -> rcu
> crusade.

Good catch!

Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: David Howells <dhowells@...hat.com>
> Cc: James Morris <jmorris@...ei.org>
> Cc: linux-security-module@...r.kernel.org
> ---
>  kernel/sys.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> Index: linux-2.6-tip/kernel/sys.c
> ===================================================================
> --- linux-2.6-tip.orig/kernel/sys.c
> +++ linux-2.6-tip/kernel/sys.c
> @@ -163,6 +163,7 @@ SYSCALL_DEFINE3(setpriority, int, which,
>  	if (niceval > 19)
>  		niceval = 19;
> 
> +	rcu_read_lock();
>  	read_lock(&tasklist_lock);
>  	switch (which) {
>  		case PRIO_PROCESS:
> @@ -200,6 +201,7 @@ SYSCALL_DEFINE3(setpriority, int, which,
>  	}
>  out_unlock:
>  	read_unlock(&tasklist_lock);
> +	rcu_read_unlock();
>  out:
>  	return error;
>  }
> 
> 
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/