Re: [PATCH 2/2] mm_for_maps: take ->cred_guard_mutex to fix the race with exec

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Oleg Nesterov (oleg@redhat.com):
> The problem is minor, but without ->cred_guard_mutex held we can race
> with exec() and get the new ->mm but check old creds.
> 
> Now we do not need to re-check task->mm after ptrace_may_access(), it
> can't be changed to the new mm under us.
> 
> Strictly speaking, this also fixes another very minor problem. Unless
> security check fails or the task exits mm_for_maps() should never
> return NULL, the caller should get either old or new ->mm.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: Serge Hallyn <serue@us.ibm.com>

> ---
> 
>  fs/proc/base.c |   22 +++++++++++-----------
>  1 file changed, 11 insertions(+), 11 deletions(-)
> 
> --- WAIT/fs/proc/base.c~2_CRED_MUTEX	2009-07-10 02:05:14.000000000 +0200
> +++ WAIT/fs/proc/base.c	2009-07-10 03:23:01.000000000 +0200
> @@ -234,19 +234,19 @@ static int check_mem_permission(struct t
> 
>  struct mm_struct *mm_for_maps(struct task_struct *task)
>  {
> -	struct mm_struct *mm = get_task_mm(task);
> +	struct mm_struct *mm;
> 
> -	if (mm && mm != current->mm) {
> -		/*
> -		 * task->mm can be changed before security check,
> -		 * in that case we must notice the change after.
> -		 */
> -		if (!ptrace_may_access(task, PTRACE_MODE_READ) ||
> -		    mm != task->mm) {
> -			mmput(mm);
> -			mm = NULL;
> -		}
> +	if (mutex_lock_killable(&task->cred_guard_mutex))
> +		return NULL;
> +
> +	mm = get_task_mm(task);
> +	if (mm && mm != current->mm &&
> +			!ptrace_may_access(task, PTRACE_MODE_READ)) {
> +		mmput(mm);
> +		mm = NULL;
>  	}
> +	mutex_unlock(&task->cred_guard_mutex);
> +
>  	return mm;
>  }
>