[PATCH] yama: don't abuse rcu_read_lock/get_task_struct in yama_task_prctl()

[Kees Cook]

On Wed, Feb 19, 2025 at 05:14:17PM +0100, Oleg Nesterov wrote:
> current->group_leader is stable, no need to take rcu_read_lock() and do
> get/put_task_struct().

Can you explain why this is true? In trying to figure this out again,
it seems that the only way current->group_leader can vanish is if
the entire process vanishes (fork or thread exec), in which case the
"current" in this prctl can't be happening; this appears to be locked
behind tsk->sighand->siglock ?

-Kees

> 
> Signed-off-by: Oleg Nesterov <oleg at redhat.com>
> ---
>  security/yama/yama_lsm.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
> index 1971710620c1..3d064dd4e03f 100644
> --- a/security/yama/yama_lsm.c
> +++ b/security/yama/yama_lsm.c
> @@ -222,7 +222,7 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
>  			   unsigned long arg4, unsigned long arg5)
>  {
>  	int rc = -ENOSYS;
> -	struct task_struct *myself = current;
> +	struct task_struct *myself;
>  
>  	switch (option) {
>  	case PR_SET_PTRACER:
> @@ -232,11 +232,7 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
>  		 * leader checking is handled later when walking the ancestry
>  		 * at the time of PTRACE_ATTACH check.
>  		 */
> -		rcu_read_lock();
> -		if (!thread_group_leader(myself))
> -			myself = rcu_dereference(myself->group_leader);
> -		get_task_struct(myself);
> -		rcu_read_unlock();
> +		myself = current->group_leader;
>  
>  		if (arg2 == 0) {
>  			yama_ptracer_del(NULL, myself);
> @@ -255,7 +251,6 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
>  			}
>  		}
>  
> -		put_task_struct(myself);
>  		break;
>  	}
>  
> -- 
> 2.25.1.362.g51ebf55
> 
> 

-- 
Kees Cook