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

Oleg Nesterov
2025-02-19T21:01:23Z

Forgot to say...

with or without this patch the usage of ptrace_relation->tracer/tracee
doesn't look right (safe) to me... but probably I missed something and
this is another story. Currently I am trying to audit the usage of
task->group_leader.

On 02/19, Oleg Nesterov wrote:
>
> On 02/19, Kees Cook wrote:
> >
> > 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 ?
>
> Well, almost, but this has nothing to do with tsk->sighand->siglock...
>
> task->group_leader can only be changed by thread exec, when a non leader
> thread does exec, see de_thread(). But de_thread() can't succeed and change
> ->group_leader until all other threads exit, see the "Kill all other threads
> in the thread group" code in de_thread(). The "current" task can't exit, so
> current->group_leader is stable.
>
> Note also that we already have a lot of current->group_leader users which
> don't use rcu/get_task_struct.
>
> That said, we have a lot of buggy users of tsk->group_leader when
> same_thread_group(tsk, current) != true ;) For example, sys_prlimit64().
> And note that rcu_read_lock/get_task_struct can't help in this case.
> I am going to send some fixes.
>
> Oleg.
>
> >
> > -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
> >