linux-kernel - Re: [PATCH][next] PCI/P2PDMA: Fix undefined behavior bug in struct pci_p2pdma

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Message-ID: <e76ff7de-8c42-4393-36e2-b37ffcb58149@deltatee.com>
Date:   Mon, 2 Oct 2023 12:46:04 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] PCI/P2PDMA: Fix undefined behavior bug in struct
 pci_p2pdma_pagemap



On 2023-10-02 12:40, Gustavo A. R. Silva wrote:
> 
> 
> On 10/2/23 18:07, Logan Gunthorpe wrote:
>>
>>
>> On 2023-10-01 15:08, Gustavo A. R. Silva wrote:
>>> `struct dev_pagemap` is a flexible structure, which means that it
>>> contains a flexible-array member at the bottom. This could potentially
>>> lead to an overwrite of the objects following `pgmap` in `struct
>>> pci_p2pdma_pagemap`, when `nr_range > 1`.
>>>
>>> Fix this by placing the declaration of object `pgmap` at the end of
>>> `struct pci_p2pdma_pagemap`.
>>>
>>> -Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
>>> ready to enable it globally.
>>>
>>> Fixes: 0afea3814358 ("PCI/P2PDMA: Add provider's pci_dev to pci_p2pdma_pagemap struct")
>>> Fixes: a6e6fe6549f6 ("PCI/P2PDMA: Introduce private pagemap structure")
>>> Cc: stable@...r.kernel.org
>>> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
>>
>>
>> Makes sense to me, thanks.
>>
>> Although, I'm not sure the fixes tags are appropriate. The
>> flexible-array member was introduced in 5.10 (b7b3c01b19) and both the
>> "fixed" commits predate that change by a number of releases.
> 
> You're right. I'll remove those tags.
> 
>>
>> Also, it's probably worth noting in the commit message that the p2pdma
>> code hardcodes nr_ranges to 1 (in pci_p2pdma_add_resource); so there is
>> no way to actually hit any bug with the current code.
> 
> Yep. I mention that in this part "This could potentially lead to an
> overwrite of the objects following `pgmap` in `struct pci_p2pdma_pagemap`,
> when `nr_range > 1`."

Yes, but the commit message is not clear that nr_range can never be >1
in the code as it currently is.

Logan