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

[Waiman Long <longman@redhat.com>]

On 12/2/22 05:18, Will Deacon wrote:
> On Thu, Dec 01, 2022 at 12:03:39PM -0500, Waiman Long wrote:
>> 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:-)
> Right, but I actually took the time to hit the same race for the other
> affinity mask field so it seems a bit narrow-minded for us just to fix the
> one issue.

I focused on this particular one because of a double-free bug report 
from David. What other fields have you found to be subjected to data race?

>
>>>> 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.
> Sorry, I meant let's wrap this access in the data_race() macro and add a
> comment so that KCSAN won't report the false positive.

Good point. I should have done that.

Thanks,
Longman