Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

Dan Williams
2018-01-05T18:05:34Z

On Fri, Jan 5, 2018 at 8:44 AM, Dan Williams <dan.j.williams@intel.com> wrote:
> On Fri, Jan 5, 2018 at 6:40 AM, Mark Rutland <mark.rutland@arm.com> wrote:
>> On Thu, Jan 04, 2018 at 02:09:52PM -0800, Dan Williams wrote:
>>> On Thu, Jan 4, 2018 at 3:47 AM, Mark Rutland <mark.rutland@arm.com> wrote:
>>> > Hi Dan,
>>> >
>>> > Thanks for these examples.
>>> >
>>> > On Thu, Jan 04, 2018 at 03:10:51AM +0000, Williams, Dan J wrote:
>>> >> Note, the following is Elena's work, I'm just helping poke the upstream
>>> >> discussion along while she's offline.
>>> >>
>>> >> Elena audited the static analysis reports down to the following
>>> >> locations where userspace provides a value for a conditional branch and
>>> >> then later creates a dependent load on that same userspace controlled
>>> >> value.
>>> >>
>>> >> 'osb()', observable memory barrier, resolves to an lfence on x86. My
>>> >> concern with these changes is that it is not clear what content within
>>> >> the branch block is of concern. Peter's 'if_nospec' proposal combined
>>> >> with Mark's 'nospec_load' would seem to clear that up, from a self
>>> >> documenting code perspective, and let archs optionally protect entire
>>> >> conditional blocks or individual loads within those blocks.
>>> >
>>> > I'm a little concerned by having to use two helpers that need to be paired. It
>>> > means that we have to duplicate the bounds information, and I suspect in
>>> > practice that they'll often be paired improperly.
>>>
>>> Hmm, will they be often mispaired? All of the examples to date the
>>> load occurs in the same code block as the bound checking if()
>>> statement.
>>
>> Practically speaking, barriers get misused all the time, and having a
>> single helper minimizes the risk or misuse.
>
> I agree, but this is why if_nospec hides the barrier / makes it implicit.
>
>>
>>> > I think that we can avoid those problems if we use nospec_ptr() on its own. It
>>> > returns NULL if the pointer would be out-of-bounds, so we can use it in the
>>> > if-statement.
>>> >
>>> > On x86, that can contain the bounds checks followed be an OSB / lfence, like
>>> > if_nospec(). On arm we can use our architected sequence. In either case,
>>> > nospec_ptr() returns NULL if the pointer would be out-of-bounds.
>>> >
>>> > Then, rather than sequences like:
>>> >
>>> > if_nospec(idx < max) {
>>> > val = nospec_array_load(array, idx, max);
>>> > ...
>>> > }
>>> >
>>> > ... we could have:
>>> >
>>> > if ((elem_ptr = nospec_array_ptr(array, idx, max)) {
>>> > val = *elem_ptr;
>>> > ...
>>> > }
>>> >
>>> > ... which also means we don't have to annotate every single load in the branch
>>> > if the element is a structure with multiple fields.
>>>
>>> We wouldn't need to annotate each load in that case, right? Just the
>>> instance that uses idx to calculate an address.
>>
>> Correct.
>>
>>> if_nospec (idx < max_idx) {
>>> elem_ptr = nospec_array_ptr(array, idx, max);
>>> val = elem_ptr->val;
>>> val2 = elem_ptr->val2;
>>> }
>>>
>>> > Does that sound workable to you?
>>>
>>> Relative to the urgency of getting fixes upstream I'm ok with whatever
>>> approach generates the least debate from sub-system maintainers.
>>>
>>> One concern is that on x86 the:
>>>
>>> if ((elem_ptr = nospec_array_ptr(array, idx, max)) {
>>>
>>> ...approach produces two conditional branches whereas:
>>>
>>> if_nospec (idx < max_idx) {
>>> elem_ptr = nospec_array_ptr(array, idx, max);
>>>
>>> ....can be optimized to one conditional with the barrier.
>>
>> Do you mean because the NULL check is redundant? I was hoping that the
>> compiler would have the necessary visibility to fold that with the
>> bounds check (on the assumption that the array base must not be NULL).
>
> ...but there's legitimately 2 conditionals one to control the
> non-speculative flow, and one to sink the speculation ala the
> array_access() approach from Linus. Keeping those separate seems to
> lead to less change in the suspected areas. In any event I'll post the
> reworked patches and we can iterate from there.

Disregard this, I'm going ahead with your new nospec_array_ptr()
helper as it falls out nicely and seems to be equally self documenting
as 'if_nospec'. Since I was already done with the if_nospec +
nospec_array_load conversions it's not much more work to go back and
roll the nospec_array_ptr conversion on top.