[cfe-dev] [llvm-dev] the as-if rule / perf vs. security

Stephen Canon via cfe-dev cfe-dev at lists.llvm.org
Wed Mar 16 11:39:30 PDT 2016


Hypothesizing about architectures with larger-than-cacheline vectors, there’s also the possibility of changing externally observable behavior by loading a catchline-sized donut hole.

I believe that a very conservative set of criteria that nonetheless license this almost all the time is something like

1. both ends of vector are used
2. vector is smaller than a cacheline
3. not FP, or arch doesn’t raise flags on FP loads (most)
4. not volatile or atomic

– Steve

> On Mar 16, 2016, at 2:28 PM, Sanjay Patel via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> We are careful not to try this optimization where it would extend the range of loaded memory; this is purely for what I call a "load doughnut". :)
> Reading past either specified edge would be very bad because it could cause a memory fault / exception where there was none in the original program. That's definitely not legal.
> 
> On Wed, Mar 16, 2016 at 12:20 PM, Craig, Ben <ben.craig at codeaurora.org <mailto:ben.craig at codeaurora.org>> wrote:
> I'm having a hard time finding any problems here, at least as long as the value is in the middle.  I wouldn't expect the contents of x[2] to affect the timing or power usage of anything.  I guess there would be  a minor "bad" side effect in that a memory read watchpoint would trigger with the 128 bit load that wouldn't be there with the 32-bit loads.  I think it is semantically very similar to this situation as well...
> v4i32 first_call(int *x) { //use all of the array
>    int f0 = x[0];
>    int f1 = x[1];
>    int f2 = x[2];
>    int f3 = x[3];
>    return (v4i32) { f0, f1, f2, f3 };
> }
> v4i32 second_call(int *x) { //use some of the array
>    int s0 = x[0];
>    int s1 = x[1];
>    int s2 = 0;
>    int s3 = x[3];
>    return (v4i32) { s0, s1, s2, s3 };
> }
> first_call(x);
> second_call(x);
> The implementation isn't going to zero out the stack in between those calls, so for a short period of time, the memory location of s2 will contain x[2].
> 
> I'm less sure if the gaps are on the edges.  I'm worried that you might ending up crossing some important address boundary if you look at something earlier or later than what the user requested.
> 
> 
> On 3/16/2016 11:38 AM, Sanjay Patel wrote:
>> Hi Ben -
>> 
>> Thanks for your response. For the sake of argument, let's narrow the scope of the problem to eliminate some of the variables you have rightfully cited. 
>> 
>> Let's assume we're not dealing with volatiles, atomics, or FP operands. We'll even guarantee that the extra loaded value is never used. This is, in fact, the scenario that  <http://reviews.llvm.org/rL263446>http://reviews.llvm.org/rL263446 <http://reviews.llvm.org/rL263446> is concerned with.
>> 
>> Related C example:
>> 
>> typedef int v4i32 __attribute__((__vector_size__(16)));
>> 
>> // Load some almost-consecutive ints as a vector.
>> v4i32 foo(int *x) {
>>    int x0 = x[0];
>>    int x1 = x[1];
>> // int x2 = x[2];   // U can't touch this? 
>>    int x3 = x[3];
>>    return (v4i32) { x0, x1, 0, x3 };
>> }
>> 
>> For x86, we notice that we have nearly a v4i32 vector's worth of loads, so we just turn that into a vector load and mask out the element that's getting set to zero:
>>     movups    (%rdi), %xmm0            ; load 128-bits instead of three 32-bit elements
>>     andps    LCPI0_0(%rip), %xmm0 ; put zero bits into the 3rd element of the vector
>> 
>> Should that optimization be disabled by a hypothetical -fextra-secure flag?
>> 
>> 
>> 
>> On Wed, Mar 16, 2016 at 7:59 AM, Craig, Ben <ben.craig at codeaurora.org <mailto:ben.craig at codeaurora.org>> wrote:
>> Regarding accessing extra data, there are at least some limits as to what can be accessed.  You can't generate extra loads or stores to volatiles.  You can't generate extra stores to atomics, even if the extra stores appear to be the same value as the old value.
>> 
>> As for determining where the perf vs. security line should be drawn, I would argue that most compilers have gone too far on the perf side while optimizing undefined behavior.  Dead store elimination leaving passwords in memory, integer overflow checks getting optimized out, and NULL checks optimized away.  Linus Torvalds was complaining about those just recently on this list, and while I don't share his tone, I agree with him regarding the harm these optimizations can cause.
>> 
>> If I'm understanding correctly, for your specific cases, you are wondering if it is fine to load and operate on a floating point value that the user did not specifically request you to operate on.  This could cause (at least) two different problems.  First, it could cause a floating point exception.  I think the danger of the floating point exception should rule out loading values the user didn't request.  Second, loading values the user didn't specify could enable a timing attack.  The timing attack is scary, but I don't think it is something we can really fix in the general case.  As long as individual assembly instructions have impractical-to-predict execution times, we will be at the mercy of the current hardware state.  There are timing attacks that can determine TLS keys in a different VM instance based off of how quickly loads in the current process execute.  If our worst timing attack problems are floating point denormalization issues, then I think we are in a pretty good state.
>> 
>> 
>> On 3/15/2016 10:46 AM, Sanjay Patel via llvm-dev wrote:
>>> [cc'ing cfe-dev because this may require some interpretation of language law]
>>> 
>>> My understanding is that the compiler has the freedom to access extra data in C/C++ (not sure about other languages); AFAIK, the LLVM LangRef is silent about this. In C/C++, this is based on the "as-if rule":
>>> http://en.cppreference.com/w/cpp/language/as_if <http://en.cppreference.com/w/cpp/language/as_if>
>>> 
>>> So the question is: where should the optimizer draw the line with respect to perf vs. security if it involves operating on unknown data? Are there guidelines that we can use to decide this?
>>> 
>>> The masked load transform referenced below is not unique in accessing / operating on unknown data. In addition to the related scalar loads -> vector load transform that I've mentioned earlier in this thread, see for example:
>>> https://llvm.org/bugs/show_bug.cgi?id=20358 <https://llvm.org/bugs/show_bug.cgi?id=20358>
>>> (and the security paper and patch review linked there)
>>> 
>>> 
>>> On Mon, Mar 14, 2016 at 10:26 PM, Shahid, Asghar-ahmad <Asghar-ahmad.Shahid at amd.com <mailto:Asghar-ahmad.Shahid at amd.com>> wrote:
>>> Hi Sanjay,
>>> 
>>>  
>>> >The real question I have is whether it is legal to read the extra memory, regardless of whether this is a masked load or
>>> 
>>> >something else.
>>> 
>>> No, It is not legal AFAIK because by doing that we are exposing the content of the memory which programmer
>>> 
>>> does not intend to. This may be vulnerable for exploitation.
>>> 
>>>  
>>> Regards,
>>> 
>>> Shahid
>>> 
>>>  
>>>  
>>> From: llvm-dev [mailto: <mailto:llvm-dev-bounces at lists.llvm.org>llvm-dev-bounces at lists.llvm.org <mailto:llvm-dev-bounces at lists.llvm.org>] On Behalf Of Sanjay Patel via llvm-dev
>>> Sent: Monday, March 14, 2016 10:37 PM
>>> To: Nema, Ashutosh
>>> Cc: llvm-dev
>>> Subject: Re: [llvm-dev] masked-load endpoints optimization
>>> 
>>>  
>>> I checked in a patch to do this transform for x86-only for now:
>>> http://reviews.llvm.org/D18094 <http://reviews.llvm.org/D18094> / http://reviews.llvm.org/rL263446 <http://reviews.llvm.org/rL263446>
>>>  
>>> On Fri, Mar 11, 2016 at 9:57 AM, Sanjay Patel < <mailto:spatel at rotateright.com>spatel at rotateright.com <mailto:spatel at rotateright.com>> wrote:
>>> 
>>> Thanks, Ashutosh.
>>> 
>>> Yes, either TTI or TLI could be used to limit the transform if we do it in CGP rather than the DAG.
>>> 
>>> The real question I have is whether it is legal to read the extra memory, regardless of whether this is a masked load or something else.
>>> 
>>> Note that the x86 backend already does this, so either my proposal is ok for x86, or we're already doing an illegal optimization:
>>> 
>>> 
>>> define <4 x i32> @load_bonus_bytes(i32* %addr1, <4 x i32> %v) {
>>>   %ld1 = load i32, i32* %addr1
>>>   %addr2 = getelementptr i32, i32* %addr1, i64 3
>>>   %ld2 = load i32, i32* %addr2
>>>   %vec1 = insertelement <4 x i32> undef, i32 %ld1, i32 0
>>>   %vec2 = insertelement <4 x i32> %vec1, i32 %ld2, i32 3
>>>   ret <4 x i32> %vec2
>>> }
>>> 
>>> $ ./llc -o - loadcombine.ll 
>>> ...
>>>     movups    (%rdi), %xmm0
>>>     retq
>>> 
>>> 
>>>  
>>> On Thu, Mar 10, 2016 at 10:22 PM, Nema, Ashutosh < <mailto:Ashutosh.Nema at amd.com>Ashutosh.Nema at amd.com <mailto:Ashutosh.Nema at amd.com>> wrote:
>>> 
>>> This looks interesting, the main motivation appears to be replacing masked vector load with a general vector load followed by a select.
>>> 
>>>  
>>> Observed masked vector loads are in general expensive in comparison with a vector load.
>>> 
>>>  
>>> But if first & last element of a masked vector load are guaranteed to be accessed then it can be transformed to a vector load.
>>> 
>>>  
>>> In opt this can be driven by TTI, where the benefit of this transformation should be checked.
>>> 
>>>  
>>> Regards,
>>> 
>>> Ashutosh
>>> 
>>>  
>>> From: llvm-dev [mailto: <mailto:llvm-dev-bounces at lists.llvm.org>llvm-dev-bounces at lists.llvm.org <mailto:llvm-dev-bounces at lists.llvm.org>] On Behalf Of Sanjay Patel via llvm-dev
>>> Sent: Friday, March 11, 2016 3:37 AM
>>> To: llvm-dev
>>> Subject: [llvm-dev] masked-load endpoints optimization
>>> 
>>>  
>>> If we're loading the first and last elements of a vector using a masked load [1], can we replace the masked load with a full vector load?
>>> 
>>> "The result of this operation is equivalent to a regular vector load instruction followed by a ‘select’ between the loaded and the passthru values, predicated on the same mask. However, using this intrinsic prevents exceptions on memory access to masked-off lanes."
>>> 
>>> I think the fact that we're loading the endpoints of the vector guarantees that a full vector load can't have any different faulting/exception behavior on x86 and most (?) other targets. We would, however, be reading memory that the program has not explicitly requested.
>>> 
>>> IR example:
>>> 
>>> define <4 x i32> @maskedload_endpoints(<4 x i32>* %addr, <4 x i32> %v) {
>>> 
>>>   ; load the first and last elements pointed to by %addr and shuffle those into %v
>>> 
>>>   %res = call <4 x i32> @llvm.masked.load.v4i32(<4 x i32>* %addr, i32 4, <4 x i1> <i1 1, i1 0, i1 0, i1 1>, <4 x i32> %v)
>>>   ret <4 x i32> %res
>>> }
>>> 
>>> would become something like:
>>> 
>>> 
>>> define <4 x i32> @maskedload_endpoints(<4 x i32>* %addr, <4 x i32> %v) {
>>> 
>>>   %vecload = load <4 x i32>, <4 x i32>* %addr, align 4
>>> 
>>>   %sel = select <4 x i1> <i1 1, i1 0, i1 0, i1 1>, <4 x i32> %vecload, <4 x i32> %v
>>> 
>>>   ret <4 x i32> %sel
>>> }
>>> 
>>> If this isn't valid as an IR optimization, would it be acceptable as a DAG combine with target hook to opt in?
>>> 
>>> 
>>> [1]  <http://llvm.org/docs/LangRef.html#llvm-masked-load-intrinsics> <http://llvm.org/docs/LangRef.html#llvm-masked-load-intrinsics>http://llvm.org/docs/LangRef.html#llvm-masked-load-intrinsics <http://llvm.org/docs/LangRef.html#llvm-masked-load-intrinsics>
>>>  
>>>  
>>> 
>>> 
>>> 
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>> 
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>> 
> 
> -- 
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> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
> 
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