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SubjectRe: [PATCH v2 00/19] prevent bounds-check bypass via speculative execution
On Thu, Jan 18, 2018 at 5:18 AM, Will Deacon <> wrote:
> Hi Dan, Linus,
> On Thu, Jan 11, 2018 at 05:41:08PM -0800, Dan Williams wrote:
>> On Thu, Jan 11, 2018 at 5:19 PM, Linus Torvalds
>> <> wrote:
>> > On Thu, Jan 11, 2018 at 4:46 PM, Dan Williams <> 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.
> From the arm64 side, the only concern I have (and this actually applies to
> our CSDB sequence as well) is the calculation of the array size by the
> caller. As Linus mentioned at the end of [1], if the determination of the
> size argument is based on a conditional branch, then masking doesn't help
> because you bound within the wrong range under speculation.
> We ran into this when trying to use masking to protect our uaccess routines
> where the conditional bound is either KERNEL_DS or USER_DS. It's possible
> that a prior conditional set_fs(KERNEL_DS) could defeat the masking and so
> we'd need to throw some heavy barriers in set_fs to make it robust.

At least in the conditional mask case near set_fs() usage the approach
we are taking is to use a barrier. I.e. the following guidance from

"Basically, the rule is trivial: find all 'stac' users, and use address
masking if those users already integrate the limit check, and lfence
they don't."

...which translates to narrow the pointer for get_user() and use a
barrier for __get_user().

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