kernel/sys.c: fix the racy usage of task_lock(tsk->group_leader) in sys_prlimit64...

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.

Link: https://lkml.kernel.org/r/20250915120917.GA27702@redhat.com
Fixes: 18c91bb2d872 ("prlimit: do not grab the tasklist_lock")
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/sys.c b/kernel/sys.c
index 1e28b40053ce206d7d0ed27e8a4fce8b616c3565..36d66ff41611764323b4be2d3089e234bc0951d8 100644 (file)
--- 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);