linux-kernel - Re: [PATCH] PCI/AER: potential dereference of null pointer

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Date:   Sat, 11 Dec 2021 11:54:50 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Jiasheng Jiang <jiasheng@...as.ac.cn>
Cc:     ruscur@...sell.cc, oohall@...il.com, bhelgaas@...gle.com,
        linuxppc-dev@...ts.ozlabs.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, Rajat Jain <rajatja@...gle.com>
Subject: Re: [PATCH] PCI/AER: potential dereference of null pointer

[+cc Rajat, author of aer_stats:
db89ccbe52c7 ("PCI/AER: Define aer_stats structure for AER capable devices"
81aa5206f9a7 ("PCI/AER: Add sysfs attributes to provide AER stats and breakdown"]

On Thu, Dec 09, 2021 at 05:45:56PM +0800, Jiasheng Jiang wrote:
> he return value of kzalloc() needs to be checked.
> To avoid use of null pointer in case of the failure of alloc.
> 
> Fixes: db89ccbe52c7 ("PCI/AER: Define aer_stats structure for AER capable devices")
> Signed-off-by: Jiasheng Jiang <jiasheng@...as.ac.cn>
> ---
>  drivers/pci/pcie/aer.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/pci/pcie/aer.c b/drivers/pci/pcie/aer.c
> index ec943cee5ecc..d04303edf468 100644
> --- a/drivers/pci/pcie/aer.c
> +++ b/drivers/pci/pcie/aer.c
> @@ -376,6 +376,8 @@ void pci_aer_init(struct pci_dev *dev)
>  		return;
>  
>  	dev->aer_stats = kzalloc(sizeof(struct aer_stats), GFP_KERNEL);
> +	if (!dev->aer_stats)
> +		return;

Did you actually trip over a null pointer dereference, and if so,
where was it?

I think the intent here was that aer_stats is a non-essential feature,
and if we can't allocate space to keep the statistics, we can still
use the device without the stats.

I *think* all the users of dev->aer_stats check for NULL before
dereferencing it, but if you found a case that doesn't do that, we
should definitely fix it.

In a few cases (aer_stats_dev_attr, aer_stats_rootport_attr), the
check isn't obvious -- it happens in aer_stats_attrs_are_visible().
If aer_stats_attrs_are_visible() finds that aer_stats is NULL, those
sysfs attributes should not be visible, and the corresponding *_show()
functions should never be called.

>  	/*
>  	 * We save/restore PCI_ERR_UNCOR_MASK, PCI_ERR_UNCOR_SEVER,
> -- 
> 2.25.1
>