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

[Greg KH <gregkh@linuxfoundation.org>]

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