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

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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: 18c91bb2d8 ("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>
This commit is contained in:
Oleg Nesterov
2025-09-15 14:09:17 +02:00
committed by Andrew Morton
parent 39f17c7074
commit a15f37a401
1 changed files with 20 additions and 2 deletions
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22
kernel/sys.c
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@@ -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);