Re: [PATCH] cred - synchronize rcu before releasing cred

[Jiri Olsa]

On Wed, Jul 28, 2010 at 02:17:27PM +0100, David Howells wrote:
> 
> The attached patch should suffice to fix get_task_cred(), and should render
> Jiri's patch unnecessary.
> 
> David
> ---
> From: David Howells <dhowells@xxxxxxxxxx>
> Subject: [PATCH] CRED: Move get_task_cred() out of line and make it use atomic_inc_not_zero()
> 
> It's possible for get_task_cred() as it currently stands to 'corrupt' a set of
> credentials by incrementing their usage count after their replacement by the
> task being accessed.
> 
> What happens is that get_task_cred() engages the RCU read lock, accesses the cred
> 
> 
> 	TASK_1			TASK_2			RCU_CLEANER
> 	-->get_task_cred(TASK_2)
> 	rcu_read_lock()
> 	__cred = __task_cred(TASK_2)
> 				-->commit_creds()
> 				old_cred = TASK_2->real_cred
> 				TASK_2->real_cred = ...
> 				put_cred(old_cred)
> 				  call_rcu(old_cred)
> 		[__cred->usage == 0]
> 	get_cred(__cred)
> 		[__cred->usage == 1]
> 	rcu_read_unlock()
> 							-->put_cred_rcu()
> 							[__cred->usage == 1]
> 							panic()
> 
> However, since a tasks credentials are generally not changed very often, we can
> reasonably make use of a loop involving reading the creds pointer and using
> atomic_inc_not_zero() to attempt to increment it if it hasn't already hit zero.
> 
> If successful, we can safely return the credentials in the knowledge that, even
> if the task we're accessing has released them, they haven't gone to the RCU
> cleanup code.

looks ok, I changed the task_state to use this and I'm running the
bug reproducer... so far so good ;)

wbr,
jirka
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