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Subject: Re: [PATCH v1] trace: Fix race in trace_open and buffer resize call
To:     Greg KH <gregkh@linuxfoundation.org>,
        Steven Rostedt <rostedt@goodmis.org>
Cc:     Denis Efremov <efremov@linux.com>, linux-kernel@vger.kernel.org,
        linux-arm-msm@vger.kernel.org, stable@vger.kernel.org,
        Julia Lawall <julia.lawall@inria.fr>
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From:   Gaurav Kohli <gkohli@codeaurora.org>
Message-ID: <8e17ad41-b62b-5d39-82ef-3ee6ea9f4278@codeaurora.org>
Date:   Fri, 22 Jan 2021 16:55:29 +0530
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On 1/22/2021 4:29 PM, Greg KH wrote:
> On Thu, Jan 21, 2021 at 03:37:32PM -0500, Steven Rostedt wrote:
>> On Thu, 21 Jan 2021 23:15:22 +0300
>> Denis Efremov <efremov@linux.com> wrote:
>>
>>> On 1/21/21 10:09 PM, Steven Rostedt wrote:
>>>> On Thu, 21 Jan 2021 17:30:40 +0300
>>>> Denis Efremov <efremov@linux.com> wrote:
>>>>    
>>>>> Hi,
>>>>>
>>>>> This patch (CVE-2020-27825) was tagged with
>>>>> Fixes: b23d7a5f4a07a ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")
>>>>>
>>>>> I'm not an expert here but it seems like b23d7a5f4a07a only refactored
>>>>> ring_buffer_reset_cpu() by introducing reset_disabled_cpu_buffer() without
>>>>> significant changes. Hence, mutex_lock(&buffer->mutex)/mutex_unlock(&buffer->mutex)
>>>>> can be backported further than b23d7a5f4a07a~ and to all LTS kernels. Is
>>>>> b23d7a5f4a07a the actual cause of the bug?
>>>>>   
>>>>
>>>> Ug, that looks to be a mistake. Looking back at the thread about this:
>>>>
>>>>    https://lore.kernel.org/linux-arm-msm/20200915141304.41fa7c30@gandalf.local.home/
>>>
>>> I see from the link that it was planned to backport the patch to LTS kernels:
>>>
>>>> Actually we are seeing issue in older kernel like 4.19/4.14/5.4 and there below patch was not
>>>> present in stable branches:
>>>> Commit b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")
>>>
>>> The point is that it's not backported yet. Maybe because of Fixes tag. I've discovered
>>> this while trying to formalize CVE-2020-27825 bug in cvehound
>>> https://github.com/evdenis/cvehound/blob/master/cvehound/cve/CVE-2020-27825.cocci
>>>
>>> I think that the backport to the 4.4+ should be something like:
>>>
>>> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
>>> index 547a3a5ac57b..2171b377bbc1 100644
>>> --- a/kernel/trace/ring_buffer.c
>>> +++ b/kernel/trace/ring_buffer.c
>>> @@ -4295,6 +4295,8 @@ void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu)
>>>   	if (!cpumask_test_cpu(cpu, buffer->cpumask))
>>>   		return;
>>>   
>>> +	mutex_lock(&buffer->mutex);
>>> +
>>>   	atomic_inc(&buffer->resize_disabled);
>>>   	atomic_inc(&cpu_buffer->record_disabled);
>>>   
>>> @@ -4317,6 +4319,8 @@ void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu)
>>>   
>>>   	atomic_dec(&cpu_buffer->record_disabled);
>>>   	atomic_dec(&buffer->resize_disabled);
>>> +
>>> +	mutex_unlock(&buffer->mutex);
>>>   }
>>>   EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);
>>>   
>>
>> That could possibly work.

Yes, this will work, As i have tested similar patch for internal testing 
for kernel branches like 5.4/4.19.

> 
> Ok, so what can I do here?  Can someone resend this as a backport to the
> other stable kernels in this way so that I can queue it up?
> 
> thanks,
> 
> greg k-h
> 

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