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

[Craig, Ben via cfe-dev]

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 <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: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/rL263446
>
>     On Fri, Mar 11, 2016 at 9:57 AM, Sanjay Patel
>     <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
>     <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: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
>
>
>
>
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