[PATCH 2/2] fix the racy usage of task_lock(tsk->group_leader) in sys_prlimit64() paths

Oleg Nesterov
2025-09-14T11:11:11Z

The usage of task_lock(tsk->group_leader) in sys_prlimit64()->do_prlimit()
path is very broken.

sys_prlimit64() does get_task_struct(tsk) but this only protects task_struct
itself. If tsk != current and tsk is not a leader, this process can exit/exec
and task_lock(tsk->group_leader) may use the already freed task_struct.

Another problem is that sys_prlimit64() can race with mt-exec which changes
->group_leader. In this case do_prlimit() may take the wrong lock, or (worse)
->group_leader may change between task_lock() and task_unlock().

Change sys_prlimit64() to take tasklist_lock when necessary. This is not
nice, but I don't see a better fix for -stable.

Cc: stable@vger.kernel.org
Fixes: c022a0acad53 ("rlimits: implement prlimit64 syscall")
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/sys.c | 22 ++++++++++++++++++++--
1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 1e28b40053ce..36d66ff41611 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1734,6 +1734,7 @@ SYSCALL_DEFINE4(prlimit64, pid_t, pid, unsigned int, resource,
struct rlimit old, new;
struct task_struct *tsk;
unsigned int checkflags = 0;
+ bool need_tasklist;
int ret;

if (old_rlim)
@@ -1760,8 +1761,25 @@ SYSCALL_DEFINE4(prlimit64, pid_t, pid, unsigned int, resource,
get_task_struct(tsk);
rcu_read_unlock();

- ret = do_prlimit(tsk, resource, new_rlim ? &new : NULL,
- old_rlim ? &old : NULL);
+ need_tasklist = !same_thread_group(tsk, current);
+ if (need_tasklist) {
+ /*
+ * Ensure we can't race with group exit or de_thread(),
+ * so tsk->group_leader can't be freed or changed until
+ * read_unlock(tasklist_lock) below.
+ */
+ read_lock(&tasklist_lock);
+ if (!pid_alive(tsk))
+ ret = -ESRCH;
+ }
+
+ if (!ret) {
+ ret = do_prlimit(tsk, resource, new_rlim ? &new : NULL,
+ old_rlim ? &old : NULL);
+ }
+
+ if (need_tasklist)
+ read_unlock(&tasklist_lock);

if (!ret && old_rlim) {
rlim_to_rlim64(&old, &old64);
--
2.25.1.362.g51ebf55