linux-kernel - Re: [PATCH] EDAC/ghes: Do not warn when incrementing refcount on 0

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Date:   Tue, 26 Nov 2019 09:57:56 +0000
From:   John Garry <john.garry@...wei.com>
To:     Borislav Petkov <bp@...en8.de>,
        Robert Richter <rrichter@...vell.com>
CC:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        "huangming23@...wei.com" <huangming23@...wei.com>,
        "james.morse@....com" <james.morse@....com>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxarm@...wei.com" <linuxarm@...wei.com>,
        "tanxiaofei@...wei.com" <tanxiaofei@...wei.com>,
        "wanghuiqiang@...wei.com" <wanghuiqiang@...wei.com>
Subject: Re: [PATCH] EDAC/ghes: Do not warn when incrementing refcount on 0

On 22/11/2019 09:01, Borislav Petkov wrote:
> On Thu, Nov 21, 2019 at 09:36:57PM +0000, Robert Richter wrote:
>> Following warning from the refcount framework is seen during ghes
>> initialization:
>>
>>   EDAC MC0: Giving out device to module ghes_edac.c controller ghes_edac: DEV ghes (INTERRUPT)
>>   ------------[ cut here ]------------
>>   refcount_t: increment on 0; use-after-free.
>>   WARNING: CPU: 36 PID: 1 at lib/refcount.c:156 refcount_inc_checked+0x44/0x50
>> [...]
>>   Call trace:
>>    refcount_inc_checked+0x44/0x50
>>    ghes_edac_register+0x258/0x388
>>    ghes_probe+0x28c/0x5f0
>>
>> It warns if the refcount is incremented from zero. This warning is
>> reasonable as a kernel object is typically created with a refcount of
>> one and freed once the refcount is zero. Afterwards the object would
>> be "used-after-free".
>>
>> For ghes the refcount is initialized with zero, and that is why this
>> message is seen when initializing the first instance. However,
>> whenever the refcount is zero, the device will be allocated and
>> registered. Since the ghes_reg_mutex protects the refcount and
>> serializes allocation and freeing of ghes devices, a use-after-free
>> cannot happen here.
>>
>> Instead of using refcount_inc() for the first instance, use
>> refcount_set(). This can be used here because the refcount is zero at
>> this point and can not change due to its protection by the mutex.
>>
>> Reported-by: John Garry <john.garry@...wei.com>
>> Tested-by: John Garry <john.garry@...wei.com>

According to kernel dev process Doc, this should be explicitly granted, so:
Tested-by: John Garry <john.garry@...wei.com>

Thanks,
John

>> Signed-off-by: Robert Richter <rrichter@...vell.com>
>> ---
>>   drivers/edac/ghes_edac.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> Queued, thanks.
>