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SubjectRe: [PATCH v2] x86: Disable kernel stack offset randomization for !TSC
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

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