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SubjectRe: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function
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> On Jun 14, 2019, at 3:06 PM, Dave Hansen <dave.hansen@intel.com> wrote:
>
>> On 6/14/19 2:34 PM, Yu-cheng Yu wrote:
>> On Fri, 2019-06-14 at 13:57 -0700, Dave Hansen wrote:
>>>> I have a related question:
>>>>
>>>> Do we allow the application to read the bitmap, or any fault from the
>>>> application on bitmap pages?
>>>
>>> We have to allow apps to read it. Otherwise they can't execute
>>> instructions.
>>
>> What I meant was, if an app executes some legacy code that results in bitmap
>> lookup, but the bitmap page is not yet populated, and if we then populate that
>> page with all-zero, a #CP should follow. So do we even populate that zero page
>> at all?
>>
>> I think we should; a #CP is more obvious to the user at least.
>
> Please make an effort to un-Intel-ificate your messages as much as
> possible. I'd really prefer that folks say "missing end branch fault"
> rather than #CP. I had to Google "#CP".
>
> I *think* you are saying that: The *only* lookups to this bitmap are on
> "missing end branch" conditions. Normal, proper-functioning code
> execution that has ENDBR instructions in it will never even look at the
> bitmap. The only case when we reference the bitmap locations is when
> the processor is about do do a "missing end branch fault" so that it can
> be suppressed. Any population with the zero page would be done when
> code had already encountered a "missing end branch" condition, and
> populating with a zero-filled page will guarantee that a "missing end
> branch fault" will result. You're arguing that we should just figure
> this out at fault time and not ever reach the "missing end branch fault"
> at all.
>
> Is that right?
>
> If so, that's an architecture subtlety that I missed until now and which
> went entirely unmentioned in the changelog and discussion up to this
> point. Let's make sure that nobody else has to walk that path by
> improving our changelog, please.
>
> In any case, I don't think this is worth special-casing our zero-fill
> code, FWIW. It's not performance critical and not worth the complexity.
> If apps want to handle the signals and abuse this to fill space up with
> boring page table contents, they're welcome to. There are much easier
> ways to consume a lot of memory.

Isn’t it a special case either way? Either we look at CR2 and populate a page, or we look at CR2 and the “tracker” state and send a different signal. Admittedly the former is very common in the kernel.

>
>>> We don't have to allow them to (popuating) fault on it. But, if we
>>> don't, we need some kind of kernel interface to avoid the faults.
>>
>> The plan is:
>>
>> * Move STACK_TOP (and vdso) down to give space to the bitmap.
>
> Even for apps with 57-bit address spaces?
>
>> * Reserve the bitmap space from (mm->start_stack + PAGE_SIZE) to cover a code
>> size of TASK_SIZE_LOW, which is (TASK_SIZE_LOW / PAGE_SIZE / 8).
>
> The bitmap size is determined by CR4.LA57, not the app. If you place
> the bitmap here, won't references to it for high addresses go into the
> high address space?
>
> Specifically, on a CR4.LA57=0 system, we have 48 bits of address space,
> so 128TB for apps. You are proposing sticking the bitmap above the
> stack which is near the top of that 128TB address space. But on a
> 5-level paging system with CR4.LA57=1, there could be valid data at
> 129GB. Is there something keeping that data from being mistaken for
> being part of the bitmap?
>

I think we need to make the vma be full sized — it should cover the entire range that the CPU might access. If that means it spans the 48-bit boundary, so be it.

> Also, if you're limiting it to TASK_SIZE_LOW, please don't forget that
> this is yet another thing that probably won't work with the vsyscall
> page. Please make sure you consider it and mention it in your next post.

Why not? The vsyscall page is at a negative address.

>
>> * Mmap the space only when the app issues the first mark-legacy prctl. This
>> avoids the core-dump issue for most apps and the accounting problem that
>> MAP_NORESERVE probably won't solve

What happens if there’s another VMA there by the time you map it?
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