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Re: [PATCH v2 00/19] prevent bounds-check bypass via speculative execution

On Thu, Jan 11, 2018 at 5:19 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Jan 11, 2018 at 4:46 PM, Dan Williams <dan.j.williams@xxxxxxxxx> 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.