linux-kernel - Re: [PATCH v4 1/3] randomize_kstack: Maintain kstack

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Message-ID: <85d0d013-eca2-4b9f-bee3-d583d0eeb99e@intel.com>
Date: Mon, 19 Jan 2026 08:10:42 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Huacai Chen <chenhuacai@...nel.org>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Michael Ellerman <mpe@...erman.id.au>, Paul Walmsley <pjw@...nel.org>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Kees Cook <kees@...nel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 Arnd Bergmann <arnd@...db.de>, Mark Rutland <mark.rutland@....com>,
 "Jason A. Donenfeld" <Jason@...c4.com>, Ard Biesheuvel <ardb@...nel.org>,
 Jeremy Linton <jeremy.linton@....com>,
 David Laight <david.laight.linux@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 loongarch@...ts.linux.dev, linuxppc-dev@...ts.ozlabs.org,
 linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
 linux-hardening@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v4 1/3] randomize_kstack: Maintain kstack_offset per task

On 1/19/26 05:01, Ryan Roberts wrote:
...
> Cc: stable@...r.kernel.org

Since this doesn't fix any known functional issues, if it were me, I'd
leave stable@ alone. It isn't clear that this is stable material.

> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1591,6 +1591,10 @@ struct task_struct {
>  	unsigned long			prev_lowest_stack;
>  #endif
>  
> +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
> +	u32				kstack_offset;
> +#endif
> +
>  #ifdef CONFIG_X86_MCE
>  	void __user			*mce_vaddr;

Nit: This seems to be throwing a u32 potentially in between a couple of
void*/ulong sized objects.

It probably doesn't matter with struct randomization and it's really
hard to get right among the web of task_struct #ifdefs. But, it would be
nice to at _least_ nestle this next to another int-sized thing.

Does it really even need to be 32 bits? x86 has this comment:

>         /*
>          * This value will get limited by KSTACK_OFFSET_MAX(), which is 10
>          * bits. The actual entropy will be further reduced by the compiler
>          * when applying stack alignment constraints (see cc_stack_align4/8 in
>          * arch/x86/Makefile), which will remove the 3 (x86_64) or 2 (ia32)
>          * low bits from any entropy chosen here.
>          *
>          * Therefore, final stack offset entropy will be 7 (x86_64) or
>          * 8 (ia32) bits.
>          */