On Thu, Apr 30, 2020 at 11:02 AM Aaron Ballman <aaron_at_[hidden]>
wrote:

> On Thu, Apr 30, 2020 at 1:52 PM Richard Smith via SG12
> <sg12_at_[hidden]> wrote:
> >
> > On Thu, 30 Apr 2020, 07:39 Arthur O'Dwyer via SG12, <
> sg12_at_[hidden]> wrote:
> >>
> >> On Thu, Apr 30, 2020 at 2:19 AM Jens Maurer <Jens.Maurer_at_[hidden]>
> wrote:
> >>>
> >>> On 30/04/2020 02.10, Arthur O'Dwyer via SG12 wrote:
> >>> >
> >>> > It's easy to ensure that an operation */is/* performed; all you have
> to do is insert an optimization barrier like Google Benchmark's
> DoNotOptimize, which is basically an `asm volatile("")` statement.
> >>>
> >>> Yes.
> >>> Paul's P1382 is much easier in this regard, and very close to
> traditional volatile semantics.
> >>
> >>
> >> I wouldn't call P1382 "easier" on the human programmer; it's a lot more
> fiddly than a simple "escape" or "DoNotOptimize" function that can fence
> any side-effect at all. But it's certainly close to traditional `volatile`.
> >>
> >> The equivalent of `memset(passwordbuffer, 0x00, sizeof passwordbuffer)`
> under P1382 would be
> >> char passwordbuffer[100];
> >> ...
> >> for (auto& ch : passwordbuffer) {
> >> volatile_store(&ch, '\0'); // note that volatile_store(&ch,
> 0) would not compile, and I think that's too bad
> >> }
> >> although I think P1382 needs to explicitly explain whether
> >> char sequence_of_zeros[100] = {};
> >> std::volatile_store<char[100]>(&passwordbuffer, sequence_of_zeros);
> >> is intended to be supported. (I predict it's not supported, because
> SD-8, but I'd like to see it in writing in the paper.)
> >>
> >>
> >> Tony Van Eerd suggests:
> >> > a function: volatile_clear(void *ptr, int size), that does volatile
> writes of 0 into the memory.
> >> > Is this a worthwhile step?
> >>
> >> IMHO that would be a useful addition to P1382 (given the fiddliness of
> using `volatile_store` as shown above).
> >> I would even call it `volatile_memset` with the appropriate signature,
> since we have prior art for memset.
> >>
> >> Niall Douglas suggests:
> >> > I'd gladly take into the standard an:
> >> > assume_escaped(const byte *, size_t);
> >> > I can build stuff on top of that.
> >>
> >> That signature should be simply `void assume_escaped(const void*)`.
> The `size_t` parameter is pointless, because the entire point of this
> function (from the compiler's point of view) is that everything reachable
> from that pointer must be assumed to have escaped.
> >> int a[2];
> >> int *p = &a[0];
> >> int *q = &a[1];
> >> assume_escaped(p);
> >> assume_escaped(q);
> >> Those two lines should have the same effect from the compiler's point
> of view, since the memory at `*p` is also reachable via the expression
> `*(q-1)`. If `q` escapes into
> a-function-that-the-compiler-doesn't-know-what-it-does, then the compiler
> must assume that `*(q-1)` might be read by that function.
> >> Here is Google's `DoNotOptimize(const T&)`.
> >> If we had a simple DoNotOptimize-style library function, we wouldn't
> need `volatile_store` for Miguel's purpose. But I believe Paul wants
> `volatile_store` for more "lock-free atomic access patterns" kinds of
> things; `volatile_store` has other benefits for his domain, such as
> non-tearing.
> >>
> >>
> >>> > memset(passwordbuffer, 0x00, sizeof passwordbuffer);
> >>> > DoNotOptimize(&passwordbuffer); // escape this variable to
> force the memset to be respected
> >>> >
> >>> > What secure_clear is trying to do is to ensure that a "sufficiently
> dumb compiler" does not compile the above code into the equivalent of
> >>> >
> >>> > char *secondcopy = strdup(passwordbuffer); free(secondcopy); //
> oops, leaked a copy of the data onto the heap
> >>> > static char secondcopy[100]; strcpy(secondcopy,
> passwordbuffer); // oops, leaked a copy of the data into the data section
> >>> > memset(passwordbuffer, 0x00, sizeof passwordbuffer);
> >>> > DoNotOptimize(&passwordbuffer); // the memset is respected, but
> useless
> >>>
> >>> And then, the compiler does
> >>> strcpy(passwordbuffer, secondcopy);
> >>> rigbt before "passwordbuffer" ends its lifetime. Also undesirable and
> >>> totally defeating the purpose, but fine under the status quo as-if
> rule.
> >>
> >>
> >> Sure; but my point was that the purpose of "not leaking the secret" has
> already been defeated, even if the compiler doesn't do what you said. As
> soon as the very first copy is made, the security has gone out the window.
> And those copies can be made under the as-if rule.
> >
> >
> > Right. Rather than pursuing an option that we don't know of any way to
> extend to being an actually secure clear mechanism, we should find some way
> to express the actually desired semantics of "don't leak the secret stuff".
> >
> > For example, suppose we had a function attribute:
> >
> > [[secure_clear]]
> >
> > ... with no normative semantics (because it has no effect on the
> abstract machine), but with recommended behaviour that it ensures to the
> extent possible that data used in the function is not accessible after it
> terminates, by any means -- in particular, that any stack space or
> registers used by the function are cleared when the function returns or
> propagates an exception. That seems implementable, and seems like it would
> actually give the desired depth of protection.
>
> I'm not certain I agree, because implementations are free to
> effectively ignore the attribute because a correct program would
> remain correct in the absence of the attribute. Maybe we would want to
> nam it 'secure_clear' as a keyword attribute, similar to what we did
> for alignas (and for the same reasons).
>

Given the way we describe the abstract machine, I think this is simply
fundamental. We cannot give a guarantee here because the behavior of the
physical machine is outside our scope and the behavior of the abstract
machine is unaffected by the attribute. It seems entirely appropriate to me
for this to be in the domain of quality-of-implementation -- whether and to
what extent an implementation can bound the behavior of a program whose
behavior is undefined isn't something we can reasonably regulate.

(Sure, a conforming implementation can ignore the attribute. A constant
expression evaluator certainly would. And that seems fine, because that
implementation is emulating the abstract machine and tightly bounding the
possible effects in the presence of undefined behavior, so (assuming the
implementation itself doesn't have bugs) it therefore presents no risk of
leaking secrets to later parts of the program execution.)


> ~Aaron
>
> >
> > We'd still need some way to securely clear heap allocations. Maybe the
> proposed std::secure_clear function would be useful for that, but there
> might be an argument that what we actually want is a secure allocator. For
> example, maybe you don't want your secrets written to a page file -- we
> could take care of that for the stack automatically via the function
> attribute, but probably not for the heap if the regular allocator is used.
> And you probably don't want a read-after-free elsewhere in your program to
> be able to read your secure secrets, which would be the case if you used
> the regular allocator.
> >
> >>> > The above is permitted today under the as-if rule; but secure_clear
> is asking for some way to /*ensure, formally,*/ that it /*never */happens.
> >>> >
> >>> > Ensuring that something /doesn't /happen is much much harder than
> ensuring that it /does /happen.
> >>>
> >>> Agreed.
> >>
> >>
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