On 02/19, Kees Cook wrote:

On Wed, Feb 19, 2025 at 05:14:17PM +0100, Oleg Nesterov wrote:

quoted

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

quoted

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