linux-kernel - Re: [PATCH] x86: Disable kernel stack offset randomization for !TSC

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Message-ID: <alpine.DEB.2.21.2301082001540.65308@angie.orcam.me.uk>
Date:   Sun, 8 Jan 2023 20:04:14 +0000 (GMT)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     Borislav Petkov <bp@...en8.de>
cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] x86: Disable kernel stack offset randomization for
 !TSC

On Sun, 8 Jan 2023, Borislav Petkov 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?

processor	: 0
vendor_id	: GenuineIntel
cpu family	: 4
model		: 3
model name	: 486 DX/2
stepping	: 5
fdiv_bug	: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 1
wp		: yes
flags		: fpu vme cpuid
bugs		: itlb_multihit
bogomips	: 32.35
clflush size	: 32
cache_alignment	: 32
address sizes	: 32 bits physical, 32 bits virtual
power management:

> 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);

 I'll check if that works, thanks for the suggestion.

  Maciej