Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok

Alexei Starovoitov
2018-01-06T20:09:20Z

On Sat, Jan 06, 2018 at 07:55:51PM +0000, Alan Cox wrote:
> > cpus execute what they see. speculative execution does the same
> > except results are not committed to visible registers and stay
> > in renanmed/shadow set. There is no 'undo' of the speculative execution.
> > The whole issue is that cache and branch predictor don't have
> > a shadow unlike registers.
>
> Can I suggest you read something like "Exploitig Value Locaity to Exceed
> The Dataflow Limit" by Lipasti and Shen 1996.

thanks for the pointer.
A quote from above paper:
"Value prediction consists of predicting entire 32- and 64-bit register values
based on previously-seen values"

> In other words there are at least two problems with Linus proposal
>
> 1. The ffff/0000 mask has to be generated and that has to involve
> speculative flows.

to answer above and Thomas's
"For one particular architecture and that's not a solution for generic code."

The following:
#define array_access(base, idx, max) ({ \
union { typeof(base[0]) _val; unsigned long _bit; } __u;\
unsigned long _i = (idx); \
unsigned long _m = (max); \
unsigned long _mask = ~(long)(_m - 1 - _i) >> 63; \
__u._val = base[_i & _mask]; \
__u._bit &= _mask; \
__u._val; })

is generic and no speculative flows.

> 2. There are processors on the planet that may speculate not just what
> instruction to execute but faced with a stall on an input continue by
> using an educated guess at the value that will appear at the input in
> future.

correct. that's why earlier I mentioned that "if 'mask' cannot
be influenced by attacker".
Even if 'mask' in 'index & mask' example is a stall the educated
guess will come from the prior value (according to the quoted paper)

To be honest I haven't read that particular paper in the past, but
abstracts fits my understanding and this array_access() proposal.
Thanks for the pointer. Will read it fully to make sure
I didn't miss anything.