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

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Date:   Fri, 22 Nov 2019 10:01:23 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Robert Richter <rrichter@...vell.com>
Cc:     "john.garry@...wei.com" <john.garry@...wei.com>,
        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 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>
> Signed-off-by: Robert Richter <rrichter@...vell.com>
> ---
>  drivers/edac/ghes_edac.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Queued, thanks.

-- 
Regards/Gruss,
    Boris.

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