[tip: sched/urgent] sched/mm_cid: Prevent NULL mm dereference in sched_mm_cid_after_execve()

["tip-bot2 for Cong Wang" <tip-bot2@linutronix.de>]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     2bdf777410dc6e022d1081885ff34673b5dfee99
Gitweb:        https://git.kernel.org/tip/2bdf777410dc6e022d1081885ff34673b5dfee99
Author:        Cong Wang <cwang@multikernel.io>
AuthorDate:    Tue, 23 Dec 2025 13:51:13 -08:00
Committer:     Thomas Gleixner <tglx@kernel.org>
CommitterDate: Fri, 09 Jan 2026 13:02:57 +01:00

sched/mm_cid: Prevent NULL mm dereference in sched_mm_cid_after_execve()

sched_mm_cid_after_execve() is called in bprm_execve()'s cleanup path even
when exec_binprm() fails. For the init task's first execve(), this causes a
problem:

  1. current->mm is NULL (kernel threads don't have an mm)
  2. sched_mm_cid_before_execve() exits early because mm is NULL
  3. exec_binprm() fails (e.g., ENOENT for missing script interpreter)
  4. sched_mm_cid_after_execve() is called with mm still NULL
  5. sched_mm_cid_fork() is called unconditionally, triggering WARN_ON

This is easily reproduced by booting with an init that is a shell script
(#!/bin/sh) where the interpreter doesn't exist in the initramfs.

Fix this by checking if t->mm is NULL before calling sched_mm_cid_fork(),
matching the behavior of sched_mm_cid_before_execve() which already
handles this case via sched_mm_cid_exit()'s early return.

Fixes: b0c3d51b54f8 ("sched/mmcid: Provide precomputed maximal value")
Signed-off-by: Cong Wang <cwang@multikernel.io>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://patch.msgid.link/20251223215113.639686-1-xiyou.wangcong@gmail.com
---
 kernel/sched/core.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 41ba0be..60afadb 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10694,10 +10694,11 @@ void sched_mm_cid_before_execve(struct task_struct *t)
 	sched_mm_cid_exit(t);
 }
 
-/* Reactivate MM CID after successful execve() */
+/* Reactivate MM CID after execve() */
 void sched_mm_cid_after_execve(struct task_struct *t)
 {
-	sched_mm_cid_fork(t);
+	if (t->mm)
+		sched_mm_cid_fork(t);
 }
 
 static void mm_cid_work_fn(struct work_struct *work)