+ fix-the-racy-usage-of-task_locktsk-group_leader-in-sys_prlimit64-paths.patch added to mm-nonmm-unstable branch

+ fix-the-racy-usage-of-task_locktsk-group_leader-in-sys_prlimit64-paths.patch added to mm-nonmm-unstable branch

[Andrew Morton]

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/fix-the-racy-usage-of-task_locktsk-group_leader-in-sys_prlimit64-paths.patch

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