linux-kernel - Re: [PATCH] wifi: brcmfmac: pcie: handle randbuf allocation failure

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Date: Wed, 06 Mar 2024 12:07:41 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Kalle Valo" <kvalo@...nel.org>,
 "Arend van Spriel" <arend.vanspriel@...adcom.com>
Cc: "Duoming Zhou" <duoming@....edu.cn>, linux-kernel@...r.kernel.org,
 "Konrad Dybcio" <konrad.dybcio@...aro.org>,
 "Hans de Goede" <hdegoede@...hat.com>, minipli@...ecurity.net,
 linux-wireless@...r.kernel.org, brcm80211@...ts.linux.dev,
 brcm80211-dev-list.pdl@...adcom.com
Subject: Re: [PATCH] wifi: brcmfmac: pcie: handle randbuf allocation failure

On Wed, Mar 6, 2024, at 11:53, Kalle Valo wrote:
> Arend van Spriel <arend.vanspriel@...adcom.com> writes:
>
>> On 3/1/2024 2:51 PM, Duoming Zhou wrote:
>>> The kzalloc() in brcmf_pcie_download_fw_nvram() will return
>>> null if the physical memory has run out. As a result, if we
>>> use get_random_bytes() to generate random bytes in the randbuf,
>>> the null pointer dereference bug will happen.
>>> Return -ENOMEM from brcmf_pcie_download_fw_nvram() if kzalloc()
>>> fails for randbuf.
>>> Fixes: 91918ce88d9f ("wifi: brcmfmac: pcie: Provide a buffer of
>>> random bytes to the device")
>>
>> Looks good to me. Looking for kernel guideline about stack usage to
>> determine whether it would be ok to just use buffer on stack. Does
>> anyone know. This one is 256 bytes so I guess the allocation is
>> warranted here.
>
> Arnd, what do you suggest? Do we have any documentation or guidelines
> anywhere?

I don't think we have anything document about this. I usually
consider anything more than half a kilobyte as excessive,
even though the warning limit is higher.

256 bytes is usually fine, but in this case I would split out
the basic block that does this into a separate function
so it does not share the stack frame with other leaf functions
below brcmf_pcie_download_fw_nvram(). It might also be justified
to then mark it as noinline_for_stack.

      Arnd