linux-kernel - Re: [RFC PATCH] initramfs: Add size validation to prevent tmpfs exhaustion

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Message-ID: <872d008e-8412-4351-a954-29ee7c7c8315@kernel.org>
Date: Tue, 18 Mar 2025 12:55:29 +0100
From: Krzysztof Kozlowski <krzk@...nel.org>
To: David Disseldorp <ddiss@...e.de>,
 Stephen Eta Zhou <stephen.eta.zhou@...look.com>
Cc: "jsperbeck@...gle.com" <jsperbeck@...gle.com>,
 "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
 "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
 "lukas@...ner.de" <lukas@...ner.de>,
 "wufan@...ux.microsoft.com" <wufan@...ux.microsoft.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC PATCH] initramfs: Add size validation to prevent tmpfs
 exhaustion

On 17/03/2025 08:21, David Disseldorp wrote:
>> From 3499daeb5caf934f08a485027b5411f9ef82d6be Mon Sep 17 00:00:00 2001
>> From: Stephen Eta Zhou <stephen.eta.zhou@...look.com>
>> Date: Fri, 14 Mar 2025 12:32:59 +0800
>> Subject: [PATCH] initramfs: Add size validation to prevent tmpfs exhaustion
>>
>> When initramfs is loaded into a small memory environment, if its size
>> exceeds the tmpfs max blocks limit, the loading will fail. Additionally,
>> if the required blocks are close to the tmpfs max blocks boundary,
>> subsequent drivers or subsystems using tmpfs may fail to initialize.
>>
>> To prevent this, the size limit is set to half of tmpfs max blocks.
>> This ensures that initramfs can complete its mission without exhausting
>> tmpfs resources, as user-space programs may also rely on tmpfs after boot.
>>
>> This patch adds a validation mechanism to check the decompressed size
>> of initramfs based on its compression type and ratio. If the required
>> blocks exceed half of the tmpfs max blocks limit, the loading will be
>> aborted with an appropriate error message, exposing the issue early
>> and preventing further escalation.
> 
> This behaviour appears fragile and quite arbitrary. I don't think
> initramfs should be responsible for making any of these decisions.
> 
> Why can't the init binary make the decision of whether or not the amount
> of free memory remaining is sufficient for user-space, instead of this
> magic 50% limit?
> 
> What are you trying to achieve by failing in this way before initramfs
> extraction instead of during / after?

Just in case before anyone tries to actually apply it: the entire patch
has both corrupted header and actual patch is corrupted - all
indentation messed.

Best regards,
Krzysztof