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

[David Laight <David.Laight@ACULAB.COM>]

From: Ingo Molnar
> Sent: 10 January 2023 10:47
> 
> 
> * Maciej W. Rozycki <macro@orcam.me.uk> wrote:
> 
> > Jason,
> >
> >  Would you mind commenting on the below?
> >
> > On Mon, 9 Jan 2023, Ingo Molnar 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:
> > >
> > > > @@ -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);
> > > >  }
> > >
> > > While this is an obscure corner case, falling back to 0 offset silently
> > > feels a bit wrong - could we at least attempt to generate some
> > > unpredictability in this case?
> > >
> > > It's not genuine entropy, but we could pass in a value that varies from
> > > task to task and which is not an 'obviously known' constant value like the
> > > 0 fallback?
> > >
> > > For example the lowest 8 bits of the virtual page number of the current
> > > task plus the lowest 8 bits of jiffies should vary from task to task, has
> > > some time dependence and is cheap to compute:
> > >
> > > 	(((unsigned long)current >> 12) + jiffies) & 0xFF
> > >
> > > This combined with the per-CPU forward storage of previous offsets:
> > >
> > > #define choose_random_kstack_offset(rand) do {                          \
> > >         if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
> > >                                 &randomize_kstack_offset)) {            \
> > >                 u32 offset = raw_cpu_read(kstack_offset);               \
> > >                 offset ^= (rand);                                       \
> > >                 raw_cpu_write(kstack_offset, offset);                   \
> > >         }                                                               \
> > >
> > > Should make this reasonably hard to guess for long-running tasks even if
> > > there's no TSC - and make it hard to guess even for tasks whose creation an
> > > attacker controls, unless there's an info-leak to rely on.
> >
> > Sure, I'm fine implementing it, even in such a way so as not to cause a
> > code size/performance regression for X86_TSC configurations.  But is the
> > calculation really unpredictable enough? [...]
> 
> It's not binary: it's obviously not as good as a TSC, but my point is that
> 'something cheap & variable' is clearly better than 'zero offset all the
> time'.

Does it really matter if running on anything as old as a real 486?
In reality they'll only be used for testing.
There are more modern 486-class cpu for embedded use, but they
almost certainly have a TSC.

	David

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