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

Kees Cook
2025-02-19T19:33:16Z

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@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