linux-kernel - Re: [PATCH v2 0/2] net: sched: act_ife: initialize struct tc

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Message-ID: <20251106133116.4895-1-vnranganath.20@gmail.com>
Date: Thu,  6 Nov 2025 19:01:12 +0530
From: Ranganath V N <vnranganath.20@...il.com>
To: horms@...nel.org
Cc: davem@...emloft.net,
	david.hunter.linux@...il.com,
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	xiyou.wangcong@...il.com
Subject: Re: [PATCH v2 0/2] net: sched: act_ife: initialize struct tc_ife to fix KMSAN kernel-infoleak

On 11/6/25 00:43, Simon Horman wrote:
> On Wed, Nov 05, 2025 at 10:09:37AM -0500, Jamal Hadi Salim wrote:
>> On Wed, Nov 5, 2025 at 7:59 AM Simon Horman <horms@...nel.org> wrote:
>>>
>>> On Wed, Nov 05, 2025 at 03:33:58PM +0530, Ranganath V N wrote:
>>>> On 11/4/25 19:38, Simon Horman wrote:
>>>>> On Sat, Nov 01, 2025 at 06:04:46PM +0530, Ranganath V N wrote:
>>>>>> Fix a KMSAN kernel-infoleak detected  by the syzbot .
>>>>>>
>>>>>> [net?] KMSAN: kernel-infoleak in __skb_datagram_iter
>>>>>>
>>>>>> In tcf_ife_dump(), the variable 'opt' was partially initialized using a
>>>>>> designatied initializer. While the padding bytes are reamined
>>>>>> uninitialized. nla_put() copies the entire structure into a
>>>>>> netlink message, these uninitialized bytes leaked to userspace.
>>>>>>
>>>>>> Initialize the structure with memset before assigning its fields
>>>>>> to ensure all members and padding are cleared prior to beign copied.
>>>>>
>>>>> Perhaps not important, but this seems to only describe patch 1/2.
>>>>>
>>>>>>
>>>>>> Signed-off-by: Ranganath V N <vnranganath.20@...il.com>
>>>>>
>>>>> Sorry for not looking more carefully at v1.
>>>>>
>>>>> The presence of this padding seems pretty subtle to me.
>>>>> And while I agree that your change fixes the problem described.
>>>>> I wonder if it would be better to make things more obvious
>>>>> by adding a 2-byte pad member to the structures involved.
>>>>
>>>> Thanks for the input.
>>>>
>>>> One question — even though adding a 2-byte `pad` field silences KMSAN,
>>>> would that approach be reliable across all architectures?
>>>> Since the actual amount and placement of padding can vary depending on
>>>> structure alignment and compiler behavior, I’m wondering if this would only
>>>> silence the report on certain builds rather than fixing the root cause.
>>>>
>>>> The current memset-based initialization explicitly clears all bytes in the
>>>> structure (including any compiler-inserted padding), which seems safer and
>>>> more consistent across architectures.
>>>>
>>>> Also, adding a new member — even a padding field — could potentially alter
>>>> the structure size or layout as seen from user space. That might
>>>> unintentionally affect existing user-space expectations.
>>>>
>>>> Do you think relying on a manual pad field is good enough?
>>>
>>> I think these are the right questions to ask.
>>>
>>> My thinking is that structures will be padded to a multiple
>>> of either 4 or 8 bytes, depending on the architecture.
>>>
>>> And my observation is that that the unpadded length of both of the structures
>>> in question are 22 bytes. And that on x86_64 they are padded to 24 bytes.
>>> Which is divisible by both 4 and 8. So I assume this will be consistent
>>> for all architectures. If so, I think this would address the questions you
>>> raised.
>>>
>>> I do, however, agree that your current memset-based approach is safer
>>> in the sense that it carries a lower risk of breaking things because
>>> it has fewer assumptions (that we have thought of so far).
>>
>> +1
>> My view is lets fix the immediate leak issue with the memset, and a
>> subsequent patch can add the padding if necessary.
>
> Sure, no objections from my side.

Thanks for the clarification.

I'll send the new patch series(v3) with fix(missed ;)
I'll keep the current change limited to the memset fix to resolve the
issue. Also, I've noticed that similar uninitialized structure
patterns exist in a few other locations in the net code.

Thanks for the review.