linux-kernel - Re: [PATCH v6 2/2] x86/MCE/AMD: Fix memory leak when `threshold_create

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Date:   Mon, 4 Apr 2022 00:43:47 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>
Cc:     Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Tony Luck <tony.luck@...el.com>,
        Yazen Ghannam <yazen.ghannam@....com>,
        Linux Edac Mailing List <linux-edac@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Stable Kernel <stable@...r.kernel.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>,
        x86 Mailing List <x86@...nel.org>
Subject: Re: [PATCH v6 2/2] x86/MCE/AMD: Fix memory leak when
 `threshold_create_bank()` fails

On 4/4/22 12:03 AM, Thomas Gleixner wrote:
> On Tue, Mar 29 2022 at 17:47, Ammar Faizi wrote:
> 
>> In mce_threshold_create_device(), if threshold_create_bank() fails, the
>> @bp will be leaked, because the call to mce_threshold_remove_device()
>> will not free the @bp. mce_threshold_remove_device() frees
>> @threshold_banks. At that point, the @bp has not been written to
>> @threshold_banks, @threshold_banks is NULL, so the call is just a nop.
>>
>> Fix this by extracting the cleanup part into a new static function
>> __threshold_remove_device(), then call it from create/remove device
>> functions.
> 
> The way simpler fix is to move
> 
>>   	}
>>   	this_cpu_write(threshold_banks, bp);
> 
> before the loop. That's safe because the banks cannot yet be reached via
> an MCE as the vector is not yet enabled:
>    
>>   	if (thresholding_irq_en)
>>   		mce_threshold_vector = amd_threshold_interrupt;
Thomas,

I did like what you said (in the patch v4), but after Yazen and Borislav
reviewed it, we got a conclusion that it's not safe.

See [1] and [2] for the full message.

[1]: https://lore.kernel.org/lkml/YkFsQhpGGXIFTMyp@zn.tnic/
[2]: https://lore.kernel.org/lkml/Yh+oyD%2F5M3TW5ZMM@yaz-ubuntu/

Yazen, Borislav, please take a deeper look on this again. I will send
a v7 revision to really make it simpler by moving that "per-CPU var write"
before the loop.

Thanks!

-- 
Ammar Faizi