RE: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

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

From: Reshetova, Elena
> Sent: 02 May 2019 09:16
...
> > I'm also guessing that get_cpu_var() disables pre-emption?
> 
> Yes, in my understanding:
> 
> #define get_cpu_var(var)						\
> (*({									\
> 	preempt_disable();						\
> 	this_cpu_ptr(&var);						\
> }))
> 
> > This code could probably run 'fast and loose' and just ignore
> > the fact that pre-emption would have odd effects.
> > All it would do is perturb the randomness!
> 
> Hm.. I see your point, but I am wondering what the odd effects might
> be.. i.e. can we end up using the same random bits twice for two or more
> different syscalls and attackers can try to trigger this situation?

To trigger it you'd need to arrange for an interrupt in the right
timing window to cause another process to run.
There are almost certainly easier ways to break things.

I think the main effects would be the increment writing to a different
cpu local data (causing the same data to be used again and/or skipped)
and the potential for updating the random buffer on the 'wrong cpu'.

So something like:
	/* We don't really care if the update is written to the 'wrong'
	 * cpu or if the vale comes from the wrong buffer. */
	offset = *this_cpu_ptr(&cpu_syscall_rand_offset);
	*this_cpu_ptr(&cpu_syscall_rand_offset) = offset + 1;
	
	if ((offset &= 4095)) return this_cpu_ptr(&cpu_syscall_rand_buffer)[offset];

	buffer = get_cpu_var((&cpu_syscall_rand_buffer);
	get_random_bytes();
	val = buffer[0];
	/* maybe set cpu_syscall_rand_offset to 1 */
	put_cpu_var();
	return val;

The whole thing might even work with a global buffer!

	David


	

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