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Message-ID: <00347b4a-e719-9dac-b42b-05a3f117fd46@gnuweeb.org>
Date:   Mon, 4 Apr 2022 00:45:04 +0700
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Subject: Re: [PATCH v6 2/2] x86/MCE/AMD: Fix memory leak when
 `threshold_create_bank()` fails
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From:   Ammar Faizi <ammarfaizi2@gnuweeb.org>
To:     Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>
Cc:     Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Tony Luck <tony.luck@intel.com>,
        Yazen Ghannam <yazen.ghannam@amd.com>,
        Linux Edac Mailing List <linux-edac@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Stable Kernel <stable@vger.kernel.org>,
        GNU/Weeb Mailing List <gwml@vger.gnuweeb.org>,
        x86 Mailing List <x86@kernel.org>
References: <20220329104705.65256-1-ammarfaizi2@gnuweeb.org>
 <20220329104705.65256-3-ammarfaizi2@gnuweeb.org> <87wng6ksjl.ffs@tglx>
 <215ca9e7-b719-d5b8-c6db-1d71544d47be@gnuweeb.org>
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On 4/4/22 12:43 AM, Ammar Faizi wrote:
> 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.

(only if it's really safe)

-- 
Ammar Faizi