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

Wed Mar 16 09:38:30 PDT 2016

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 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>
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
>
> 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
> (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>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: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/rL263446
>>
>>
>>
>> On Fri, Mar 11, 2016 at 9:57 AM, Sanjay Patel < <spatel at rotateright.com>
>> 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 <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: <llvm-dev-bounces at lists.llvm.org>
>> 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
>>
>>
>>
>>
>>
>
>
>
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