答复: [External Mail][PATCH-tip] sched: Fix use-after-free bug in dup_user_cpus_ptr()

[David Wang 王标]

Hi, Waiman

We use 140 devices to test this patch 72 hours.  The issue can not be reproduced.  If no this patch,  the issue can be reproduced.
Could you help merge this patch to mailine?

https://lore.kernel.org/all/20221125023943.1118603-1-longman@xxxxxxxxxx/

If this patch is applied to the maintainer's tree,  we can request google to help cherrypick to ACK to fix issue.

Thanks

-----邮件原件-----
发件人: Waiman Long <longman@xxxxxxxxxx>
发送时间: 2022年11月28日 9:45
收件人: Ingo Molnar <mingo@xxxxxxxxxx>; Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Juri Lelli <juri.lelli@xxxxxxxxxx>; Vincent Guittot <vincent.guittot@xxxxxxxxxx>; Dietmar Eggemann <dietmar.eggemann@xxxxxxx>; Steven Rostedt <rostedt@xxxxxxxxxxx>; Ben Segall <bsegall@xxxxxxxxxx>; Mel Gorman <mgorman@xxxxxxx>; Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
抄送: Phil Auld <pauld@xxxxxxxxxx>; Wenjie Li <wenjieli@xxxxxxxxxxxxxxxx>; David Wang 王标 <wangbiao3@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Waiman Long <longman@xxxxxxxxxx>; stable@xxxxxxxxxxxxxxx
主题: [External Mail][PATCH-tip] sched: Fix use-after-free bug in dup_user_cpus_ptr()

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Since commit 07ec77a1d4e8 ("sched: Allow task CPU affinity to be restricted on asymmetric systems"), the setting and clearing of user_cpus_ptr are done under pi_lock for arm64 architecture. However,
dup_user_cpus_ptr() accesses user_cpus_ptr without any lock protection. When racing with the clearing of user_cpus_ptr in __set_cpus_allowed_ptr_locked(), it can lead to user-after-free and double-free in arm64 kernel.

Commit 8f9ea86fdf99 ("sched: Always preserve the user requested
cpumask") fixes this problem as user_cpus_ptr, once set, will never be cleared in a task's lifetime. However, this bug was re-introduced in commit 851a723e45d1 ("sched: Always clear user_cpus_ptr in
do_set_cpus_allowed()") which allows the clearing of user_cpus_ptr in do_set_cpus_allowed(). This time, it will affect all arches.

Fix this bug by always clearing the user_cpus_ptr of the newly cloned/forked task before the copying process starts and check the user_cpus_ptr state of the source task under pi_lock.

Note to stable, this patch won't be applicable to stable releases.
Just copy the new dup_user_cpus_ptr() function over.

Fixes: 07ec77a1d4e8 ("sched: Allow task CPU affinity to be restricted on asymmetric systems")
Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
CC: stable@xxxxxxxxxxxxxxx
Reported-by: David Wang 王标 <wangbiao3@xxxxxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 kernel/sched/core.c | 32 ++++++++++++++++++++++++++++----
 1 file changed, 28 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 8df51b08bb38..f2b75faaf71a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2624,19 +2624,43 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)  int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
                      int node)
 {
+       cpumask_t *user_mask;
        unsigned long flags;

+       /*
+        * Always clear dst->user_cpus_ptr first as their user_cpus_ptr's
+        * may differ by now due to racing.
+        */
+       dst->user_cpus_ptr = NULL;
+
+       /*
+        * This check is racy and losing the race is a valid situation.
+        * It is not worth the extra overhead of taking the pi_lock on
+        * every fork/clone.
+        */
        if (!src->user_cpus_ptr)
                return 0;

-       dst->user_cpus_ptr = kmalloc_node(cpumask_size(), GFP_KERNEL, node);
-       if (!dst->user_cpus_ptr)
+       user_mask = kmalloc_node(cpumask_size(), GFP_KERNEL, node);
+       if (!user_mask)
                return -ENOMEM;

-       /* Use pi_lock to protect content of user_cpus_ptr */
+       /*
+        * Use pi_lock to protect content of user_cpus_ptr
+        *
+        * Though unlikely, user_cpus_ptr can be reset to NULL by a concurrent
+        * do_set_cpus_allowed().
+        */
        raw_spin_lock_irqsave(&src->pi_lock, flags);
-       cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
+       if (src->user_cpus_ptr) {
+               swap(dst->user_cpus_ptr, user_mask);
+               cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
+       }
        raw_spin_unlock_irqrestore(&src->pi_lock, flags);
+
+       if (unlikely(user_mask))
+               kfree(user_mask);
+
        return 0;
 }

--
2.31.1

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