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Date:   Thu, 12 Jan 2023 01:34:29 +0000 (GMT)
From:   "Maciej W. Rozycki" <macro@orcam.me.uk>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] x86: Disable kernel stack offset randomization for
 !TSC
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On Tue, 10 Jan 2023, Jason A. Donenfeld wrote:

> > Index: linux-macro/arch/x86/include/asm/entry-common.h
> > ===================================================================
> > --- linux-macro.orig/arch/x86/include/asm/entry-common.h
> > +++ linux-macro/arch/x86/include/asm/entry-common.h
> > @@ -5,6 +5,7 @@
> >  #include <linux/randomize_kstack.h>
> >  #include <linux/user-return-notifier.h>
> >  
> > +#include <asm/cpufeature.h>
> >  #include <asm/nospec-branch.h>
> >  #include <asm/io_bitmap.h>
> >  #include <asm/fpu/api.h>
> > @@ -85,7 +86,8 @@ static inline void arch_exit_to_user_mod
> >  	 * 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);
> 
> What would happen if you just called `get_random_u8()` here?

 Thank you for your input.  I've had a look at the function and it seems a 
bit heavyweight compared to a mere single CPU instruction, but I guess why 
not.  Do you have any performance figures (in terms of CPU cycles) for the 
usual cases?  Offhand I'm not sure how I could benchmark it myself.

 I have made a patch and of course it makes the system boot too, although 
it's not clear to me how I can actually verify randomisation works.  I can 
assume it does I suppose.

  Maciej