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Date:   Fri, 13 Jan 2023 15:53:45 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Borislav Petkov <bp@alien8.de>
Cc:     "Maciej W. Rozycki" <macro@orcam.me.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86: Disable kernel stack offset randomization for !TSC
Message-ID: <202301131547.81D41D8@keescook>
References: <alpine.DEB.2.21.2301081919550.65308@angie.orcam.me.uk>
 <Y7sfdxJ5/2DSQK/l@zn.tnic>
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On Sun, Jan 08, 2023 at 08:55:06PM +0100, Borislav Petkov wrote:
> On Sun, Jan 08, 2023 at 07:42:54PM +0000, Maciej W. Rozycki wrote:
> > For x86 kernel stack offset randomization uses the RDTSC instruction, 
> > which causes an invalid opcode exception with hardware that does not 
> > implement this instruction:
> 
> Out of pure curiosity, what hw is that?
> 
> Also, I guess the fix should be something like this instead:
> 
> ---
> diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
> index 117903881fe4..57b372ca6ce7 100644
> --- a/arch/x86/include/asm/entry-common.h
> +++ b/arch/x86/include/asm/entry-common.h
> @@ -85,7 +85,8 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
>  	 * Therefore, final stack offset entropy will be 5 (x86_64) or
>  	 * 6 (ia32) bits.
>  	 */
> -	choose_random_kstack_offset(rdtsc() & 0xFF);
> +	if (cpu_feature_enabled(X86_FEATURE_TSC))
> +		choose_random_kstack_offset(rdtsc() & 0xFF);
>  }
>  #define arch_exit_to_user_mode_prepare arch_exit_to_user_mode_prepare

I think probably the entire feature should just be disabled in early
boot, checking for TSC, and then using:

	if (!cpu_feature_enabled(X86_FEATURE_TSC))
		static_branch_disable(&randomize_kstack_offset);

I'm not sure the best place to put that where it's early but late enough
that static branches are working...

-- 
Kees Cook