linux-kernel - Re: ptrace && cred_exec_mutex (Was: [PATCH] ptrace: tracehook_unsafe

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Date:	Sat, 25 Apr 2009 19:25:24 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, James Morris <jmorris@...ei.org>,
	Hugh Dickins <hugh@...itas.com>
Subject: Re: ptrace && cred_exec_mutex (Was: [PATCH] ptrace:
	tracehook_unsafe_exec: remove the stale comment)

On 04/25, David Howells wrote:
>
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > Yes. Except it looks like ->cred_exec_mutex is never used in fact.
>
> I must to be missing something...  I see that:
>
> 	int ptrace_attach(struct task_struct *task)
> 	{
> 	...
> 		/* Protect exec's credential calculations against our interference;
> 		 * SUID, SGID and LSM creds get determined differently under ptrace.
> 		 */
> 		retval = mutex_lock_interruptible(&current->cred_exec_mutex);
> 	...
> 	}
>
> And:
>
> 	int do_execve(...)
> 	{
> 	...
> 		retval = mutex_lock_interruptible(&current->cred_exec_mutex);
> 		if (retval < 0)
> 			goto out_free;
> 	...
> 	}

Sorry David, I was very unclear.

These 2 current's are different tasks, and hence we take to unrelated locks.

We can never block taking current->cred_exec_mutex because nobody else
touches this mutex, we always use current. This means this lock is "nop".

Unless I missed something, ptrace_attach() should take task->cred_exec_mutex.

Oleg.

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