linux-kernel - Re: [PATCH v2 2/2] mm/memblock: only mark/clear KHO scratch memory when needed

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <86bjknyxgu.fsf@kernel.org>
Date: Thu, 27 Nov 2025 22:30:57 +0100
From: Pratyush Yadav <pratyush@...nel.org>
To: Usama Arif <usamaarif642@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,  rppt@...nel.org,
  kas@...nel.org,  changyuanl@...gle.com,  graf@...zon.com,
  leitao@...ian.org,  thevlad@...a.com,  pratyush@...nel.org,
  dave.hansen@...ux.intel.com,  linux-mm@...ck.org,
  linux-kernel@...r.kernel.org,  kernel-team@...a.com
Subject: Re: [PATCH v2 2/2] mm/memblock: only mark/clear KHO scratch memory
 when needed

On Thu, Nov 27 2025, Usama Arif wrote:

> On 27/11/2025 20:55, Andrew Morton wrote:
>> On Thu, 27 Nov 2025 20:33:20 +0000 Usama Arif <usamaarif642@...il.com> wrote:
>> 
>>> The scratch memory for kexec handover is used to bootstrap the
>>> kexec'ed kernel. It is only needed when CONFIG_KEXEC_HANDOVER
>>> is enabled and only if it is a KHO boot. Add checks to prevent
>>> marking a KHO scratch region unless needed.
>> 
>> What effect does this change have?  Lessened memory consumption,
>> presumably.  Of what magnitude and for what time period?
>
> For some context, this came out of https://lore.kernel.org/all/ba690e06-c2a1-4d2e-9428-9ca2ea9f2b86@gmail.com/
> (I should have probably added that in the commit message..)
> We are experiencing several warnings a day in meta fleet due to a warning introduced
> in that patch. We dont have CONFIG_KEXEC_HANDOVER enabled in the fleet. The IMA memory
> seems to conincide with the 1st MB, but as Mike pointed out they are different arrays
> so this scratch memory is likely not a cause of the warnings. But it is not useful (and
> was a bit confusing) seeing KHO scratch memory being marked even when KHO is disabled.

Yeah, it is not yet clear if this is really the root cause for your
issue.

>
> The imapct is as you said, but its only marked for a very short period of time.
> I think a better reason for this patch is just to not mark the memory at all when KHO
> is disabled (or not in use) for clarity.

Yeah, I don't think it will have much of a difference in practice, but I
do think it is a good correctness fix. Marking the lower 1M as scratch
is a hack to get around the limitations with KHO, and we should not be
doing that when KHO isn't involved.

-- 
Regards,
Pratyush Yadav