linux-kernel - Re: [RFC/RFT PATCH 0/6] Improve get_random

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Message-ID: <aSiDSp8NpqJkr1oD@J2N7QTR9R3>
Date: Thu, 27 Nov 2025 16:58:50 +0000
From: Mark Rutland <mark.rutland@....com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Ard Biesheuvel <ardb@...nel.org>, Ard Biesheuvel <ardb+git@...gle.com>,
	linux-hardening@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Kees Cook <kees@...nel.org>, Will Deacon <will@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Jeremy Linton <jeremy.linton@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [RFC/RFT PATCH 0/6] Improve get_random_u8() for use in randomize
 kstack

On Thu, Nov 27, 2025 at 03:56:59PM +0000, Ryan Roberts wrote:
> On 27/11/2025 15:03, Ard Biesheuvel wrote:
> > So the question is really whether we want to dedicate 16 bytes per
> > task for this. I wouldn't mind personally, but it is something our
> > internal QA engineers tend to obsess over.
> 
> Yeah that's a good point.

I think it's a fair point that some people will obsesses over this, but
I think the concern is misplaced.

I know that people were very happy for the kernel FPSIMD context to
disappear from task_struct, but 16 bytes is a fair amount smaller, and
I'm pretty sure we can offset that with a small/moderate amount of work.

AFAICT there are extant holes in task_struct that could easily account
for 16 bytes. I can also see a few ways to rework arm64's thread_info
and thread_struct (which are both embedded within task_struct) to save
some space.

> Is this something we could potentially keep at the start of the
> kstack? Is there any precident for keeping state there at the moment?
> For arm64, I know there is a general feeling that 16K for the stack
> more than enough (but we are stuck with it because 8K isn't quite
> enough). So it would be "for free". I guess it would be tricky to do
> this in an arch-agnostic way though...

We went out of our way to stop playing silly games like that when we
moved thread_info into task_struct; please let's not bring that back.

Mark.