The patch titled Subject: kernel/sys.c: fix the racy usage of task_lock(tsk->group_leader) in sys_prlimit64() paths has been added to the -mm mm-nonmm-unstable branch. Its filename is fix-the-racy-usage-of-task_locktsk-group_leader-in-sys_prlimit64-paths.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-nonmm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Oleg Nesterov oleg@redhat.com Subject: kernel/sys.c: fix the racy usage of task_lock(tsk->group_leader) in sys_prlimit64() paths Date: Mon, 15 Sep 2025 14:09:17 +0200

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 ---

kernel/sys.c | 22 ++++++++++++++++++++-- 1 file changed, 20 insertions(+), 2 deletions(-)

--- a/kernel/sys.c~fix-the-racy-usage-of-task_locktsk-group_leader-in-sys_prlimit64-paths +++ a/kernel/sys.c @@ -1734,6 +1734,7 @@ SYSCALL_DEFINE4(prlimit64, pid_t, pid, u 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, u 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); _

Patches currently in -mm which might be from oleg@redhat.com are

fix-the-wrong-comment-on-task_lock-nesting-with-tasklist_lock.patch fix-the-racy-usage-of-task_locktsk-group_leader-in-sys_prlimit64-paths.patch