On Thu, Jan 11, 2018 at 5:19 PM, Linus Torvalds
<torva...@linux-foundation.org> wrote:
> On Thu, Jan 11, 2018 at 4:46 PM, Dan Williams <dan.j.willi...@intel.com>
> wrote:
>>
>> This series incorporates Mark Rutland's latest ARM changes and adds
>> the x86 specific implementation of 'ifence_array_ptr'. That ifence
>> based approach is provided as an opt-in fallback, but the default
>> mitigation, '__array_ptr', uses a 'mask' approach that removes
>> conditional branches instructions, and otherwise aims to redirect
>> speculation to use a NULL pointer rather than a user controlled value.
>
> Do you have any performance numbers and perhaps example code
> generation? Is this noticeable? Are there any microbenchmarks showing
> the difference between lfence use and the masking model?
I don't have performance numbers, but here's a sample code generation
from __fcheck_files, where the 'and; lea; and' sequence is portion of
array_ptr() after the mask generation with 'sbb'.
fdp = array_ptr(fdt->fd, fd, fdt->max_fds);
8e7: 8b 02 mov (%rdx),%eax
8e9: 48 39 c7 cmp %rax,%rdi
8ec: 48 19 c9 sbb %rcx,%rcx
8ef: 48 8b 42 08 mov 0x8(%rdx),%rax
8f3: 48 89 fe mov %rdi,%rsi
8f6: 48 21 ce and %rcx,%rsi
8f9: 48 8d 04 f0 lea (%rax,%rsi,8),%rax
8fd: 48 21 c8 and %rcx,%rax
> Having both seems good for testing, but wouldn't we want to pick one in the
> end?
I was thinking we'd keep it as a 'just in case' sort of thing, at
least until the 'probably safe' assumption of the 'mask' approach has
more time to settle out.
>
> Also, I do think that there is one particular array load that would
> seem to be pretty obvious: the system call function pointer array.
>
> Yes, yes, the actual call is now behind a retpoline, but that protects
> against a speculative BTB access, it's not obvious that it protects
> against the mispredict of the __NR_syscall_max comparison in
> arch/x86/entry/entry_64.S.
>
> The act of fetching code is a kind of read too. And retpoline protects
> against BTB stuffing etc, but what if the _actual_ system call
> function address is wrong (due to mis-prediction of the system call
> index check)?
>
> Should the array access in entry_SYSCALL_64_fastpath be made to use
> the masking approach?
I'll take a look. I'm firmly in the 'patch first / worry later' stance
on these investigations.
