Re: [BUG] mei: a possible use-after-free caused by concurrency execution

[Tuo Li <islituo@gmail.com>]

On 2025/1/16 17:51, Greg KH wrote:
> On Thu, Jan 16, 2025 at 05:26:33PM +0800, Tuo Li wrote:
>> Hello,
>>
>> Our static analysis tool has identified a potential use-after-free caused
>> by concurrency execution in drivers/misc/mei/main.c.
>>
>> Consider the following execution scenario:
>> (The line numbers can be referred to
>> https://elixir.bootlin.com/linux/v6.12/source/drivers/misc/mei/main.c)
>>
>>   mei_release()                            //Line 112
>>     cl = file->private_data;               //Line 114
>>     mutex_lock(&dev->device_lock);         //Line 123
>>     kfree(cl);                             //Line 149
>>     file->private_data = NULL;             //Line 152
>>     mutex_unlock(&dev->device_lock);       //Line 154
>>
>>   mei_read()                               //Line 169
>>     cl = file->private_data;               //Line 172
>>     mutex_lock(&dev->device_lock);         //Line 184
>>     cb = mei_cl_read_cb(cl, file);         //Line 200
>>     cl_dbg(dev, cl, ...);                  //Line 275
>>     mutex_unlock(&dev->device_lock);       //Line 276
>>
>> If mei_release() and mei_read() can execute concurrently and the execution
>> order is 114, 172, 123, 149 (free), 152, 154, 184, 200 (use), 275 (use),
>> 276, a possible use-after-free can occur.
> 
> How can release run at the same time read happens?  release only happens
> after all references are dropped, right?
> 
>> Our static analysis tool reports this use-after-free when analyzing Linux
>> 6.12. The tool deduces lock() and unlock() pairs with alias analysis. It
>> then applies data flow analysis to detect use-after-free across
>> synchronization points.
>>
>> I am not quite sure whether this possible use-after-free is real and how to
>> fix it if it is real.
> 
> Test it and see!  And if you feel a fix is needed, please provide a
> patch.
> 
> thanks,
> 
> greg k-h

Hello,

I really appreciate the feedback! I apologize for the inconvenience this
report has caused.

Sincerely,
Tuo Li