Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

[Andy Lutomirski]

> On Jun 7, 2019, at 12:49 PM, Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> 
> On Fri, 2019-06-07 at 11:29 -0700, Andy Lutomirski wrote:
>>> On Jun 7, 2019, at 10:59 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>>> 
>>>> On 6/7/19 10:43 AM, Peter Zijlstra wrote:
>>>> I've no idea what the kernel should do; since you failed to answer the
>>>> question what happens when you point this to garbage.
>>>> 
>>>> Does it then fault or what?
>>> 
>>> Yeah, I think you'll fault with a rather mysterious CR2 value since
>>> you'll go look at the instruction that faulted and not see any
>>> references to the CR2 value.
>>> 
>>> I think this new MSR probably needs to get included in oops output when
>>> CET is enabled.
>> 
>> This shouldnât be able to OOPS because it only happens at CPL 3, right?  We
>> should put it into core dumps, though.
>> 
>>> 
>>> Why don't we require that a VMA be in place for the entire bitmap?
>>> Don't we need a "get" prctl function too in case something like a JIT is
>>> running and needs to find the location of this bitmap to set bits itself?
>>> 
>>> Or, do we just go whole-hog and have the kernel manage the bitmap
>>> itself. Our interface here could be:
>>> 
>>>   prctl(PR_MARK_CODE_AS_LEGACY, start, size);
>>> 
>>> and then have the kernel allocate and set the bitmap for those code
>>> locations.
>> 
>> Given that the format depends on the VA size, this might be a good idea.  I
>> bet we can reuse the special mapping infrastructure for this â the VMA could
>> be a MAP_PRIVATE special mapping named [cet_legacy_bitmap] or similar, and we
>> can even make special rules to core dump it intelligently if needed.  And we
>> can make mremap() on it work correctly if anyone (CRIU?) cares.
>> 
>> Hmm.  Can we be creative and skip populating it with zeros?  The CPU should
>> only ever touch a page if we miss an ENDBR on it, so, in normal operation, we
>> donât need anything to be there.  We could try to prevent anyone from
>> *reading* it outside of ENDBR tracking if we want to avoid people accidentally
>> wasting lots of memory by forcing it to be fully populated when the read it.
>> 
>> The one downside is this forces it to be per-mm, but that seems like a
>> generally reasonable model anyway.
>> 
>> This also gives us an excellent opportunity to make it read-only as seen from
>> userspace to prevent exploits from just poking it full of ones before
>> redirecting execution.
> 
> GLIBC sets bits only for legacy code, and then makes the bitmap read-only.  That
> avoids most issues:

How does glibc know the linear address space size?  We donât want LA64 to break old binaries because the address calculation changed.