Re: x86/clearregs: Register sanitizing at kernel entry for speculation hygiene

[Andy Lutomirski]

> On Jan 9, 2018, at 5:03 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> 
> This patch kit implements clearing of all unused registers on kernel entries,
> including system calls and all exceptions and interrupt.
> 
> This doesn't fix any known issue, but will make it harder in general
> to exploit the kernel with speculation because it will be harder
> to get user controlled values into kernel code.

I don't like this at all.  Once upon a time, Linux syscalls were supposed to be fast.  Then we learned about the Meltdown screwup, so we mostly fixed it for real upstream and the distroa seriously half-arsed their own fixes [1].  This came with a big performance cost, but it can be turned off on non-busted hardware.  So be it.

But now we're proposing to throw out the whole fast path because it might make it a bit harder to do the most obvious attack.  Not very hard, mind you, but a little bit harder.  And there's no off switch for less-leaky hardware.  No thanks.

Meanwhile we're doing nothing whatsoever to mitigate cross-process attacks because we can't do anything about it short of turning IBRS on systemwide.