From: Waiman Long <longman@redhat.com>
To: Will Deacon <will@kernel.org>
Cc: "Ingo Molnar" <mingo@redhat.com>,
"Peter Zijlstra" <peterz@infradead.org>,
"Juri Lelli" <juri.lelli@redhat.com>,
"Vincent Guittot" <vincent.guittot@linaro.org>,
"Dietmar Eggemann" <dietmar.eggemann@arm.com>,
"Steven Rostedt" <rostedt@goodmis.org>,
"Ben Segall" <bsegall@google.com>, "Mel Gorman" <mgorman@suse.de>,
"Daniel Bristot de Oliveira" <bristot@redhat.com>,
"Phil Auld" <pauld@redhat.com>,
"Wenjie Li" <wenjieli@qti.qualcomm.com>,
"David Wang 王标" <wangbiao3@xiaomi.com>,
linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH-tip] sched: Fix use-after-free bug in dup_user_cpus_ptr()
Date: Thu, 1 Dec 2022 12:03:39 -0500 [thread overview]
Message-ID: <330989bf-0015-6d4c-9317-bfc9dba30b65@redhat.com> (raw)
In-Reply-To: <20221201134445.GC28489@willie-the-truck>
On 12/1/22 08:44, Will Deacon wrote:
> On Sun, Nov 27, 2022 at 08:44:41PM -0500, Waiman Long wrote:
>> 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@vger.kernel.org
>> Reported-by: David Wang 王标 <wangbiao3@xiaomi.com>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>> kernel/sched/core.c | 32 ++++++++++++++++++++++++++++----
>> 1 file changed, 28 insertions(+), 4 deletions(-)
> As per my comments on the previous version of this patch:
>
> https://lore.kernel.org/lkml/20221201133602.GB28489@willie-the-truck/T/#t
>
> I think there are other issues to fix when racing affinity changes with
> fork() too.
It is certainly possible that there are other bugs hiding somewhere:-)
>
>> 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;
> data_race() ?
Race is certainly possible, but the clearing of user_cpus_ptr before
will mitigate any risk.
>
>>
>> - 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);
> Isn't 'dst->user_cpus_ptr' always NULL here? Why do we need the swap()
> instead of just assigning the thing directly?
True. We still need to clear user_mask. So I used swap() instead of 2
assignment statements. I am fine to go with either way.
Cheers,
Longman
next prev parent reply other threads:[~2022-12-01 17:04 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-28 1:44 [PATCH-tip] sched: Fix use-after-free bug in dup_user_cpus_ptr() Waiman Long
2022-11-28 13:34 ` 答复: [External Mail][PATCH-tip] " David Wang 王标
2022-11-28 15:43 ` Waiman Long
2022-11-29 3:11 ` 答复: " David Wang 王标
2022-12-01 13:44 ` [PATCH-tip] " Will Deacon
2022-12-01 17:03 ` Waiman Long [this message]
2022-12-02 10:18 ` Will Deacon
2022-12-02 14:30 ` Waiman Long
2022-12-13 12:54 ` Will Deacon
2022-12-13 15:54 ` Waiman Long
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