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Re: [PATCH 0/7] Mitigation against unsafe data speculation (CVE-2017-5753)


On Tue, Jul 10, 2018 at 12:53 PM Richard Earnshaw (lists)
<Richard.Earnshaw@arm.com> wrote:
>
> On 10/07/18 11:10, Richard Biener wrote:
> > On Tue, Jul 10, 2018 at 10:39 AM Richard Earnshaw (lists)
> > <Richard.Earnshaw@arm.com> wrote:
> >>
> >> On 10/07/18 08:19, Richard Biener wrote:
> >>> On Mon, Jul 9, 2018 at 6:39 PM Richard Earnshaw
> >>> <Richard.Earnshaw@arm.com> wrote:
> >>>>
> >>>>
> >>>> The patches I posted earlier this year for mitigating against
> >>>> CVE-2017-5753 (Spectre variant 1) attracted some useful feedback, from
> >>>> which it became obvious that a rethink was needed.  This mail, and the
> >>>> following patches attempt to address that feedback and present a new
> >>>> approach to mitigating against this form of attack surface.
> >>>>
> >>>> There were two major issues with the original approach:
> >>>>
> >>>> - The speculation bounds were too tightly constrained - essentially
> >>>>   they had to represent and upper and lower bound on a pointer, or a
> >>>>   pointer offset.
> >>>> - The speculation constraints could only cover the immediately preceding
> >>>>   branch, which often did not fit well with the structure of the existing
> >>>>   code.
> >>>>
> >>>> An additional criticism was that the shape of the intrinsic did not
> >>>> fit particularly well with systems that used a single speculation
> >>>> barrier that essentially had to wait until all preceding speculation
> >>>> had to be resolved.
> >>>>
> >>>> To address all of the above, these patches adopt a new approach, based
> >>>> in part on a posting by Chandler Carruth to the LLVM developers list
> >>>> (https://lists.llvm.org/pipermail/llvm-dev/2018-March/122085.html),
> >>>> but which we have extended to deal with inter-function speculation.
> >>>> The patches divide the problem into two halves.
> >>>>
> >>>> The first half is some target-specific code to track the speculation
> >>>> condition through the generated code to provide an internal variable
> >>>> which can tell us whether or not the CPU's control flow speculation
> >>>> matches the data flow calculations.  The idea is that the internal
> >>>> variable starts with the value TRUE and if the CPU's control flow
> >>>> speculation ever causes a jump to the wrong block of code the variable
> >>>> becomes false until such time as the incorrect control flow
> >>>> speculation gets unwound.
> >>>>
> >>>> The second half is that a new intrinsic function is introduced that is
> >>>> much simpler than we had before.  The basic version of the intrinsic
> >>>> is now simply:
> >>>>
> >>>>       T var = __builtin_speculation_safe_value (T unsafe_var);
> >>>>
> >>>> Full details of the syntax can be found in the documentation patch, in
> >>>> patch 1.  In summary, when not speculating the intrinsic returns
> >>>> unsafe_var; when speculating then if it can be shown that the
> >>>> speculative flow has diverged from the intended control flow then zero
> >>>> is returned.  An optional second argument can be used to return an
> >>>> alternative value to zero.  The builtin may cause execution to pause
> >>>> until the speculation state is resolved.
> >>>
> >>> So a trivial target implementation would be to emit a barrier and then
> >>> it would always return unsafe_var and never zero.  What I don't understand
> >>> fully is what users should do here, thus what the value of ever returning
> >>> "unsafe" is.  Also I wonder why the API is forcing you to single-out a
> >>> special value instead of doing
> >>>
> >>>  bool safe = __builtin_speculation_safe_value_p (T unsafe_value);
> >>>  if (!safe)
> >>>    /* what now? */
> >>>
> >>> I'm only guessing that the correct way to handle "unsafe" is basically
> >>>
> >>>  while (__builtin_speculation_safe_value (val) == 0)
> >>>     ;
> >>>
> >>> use val, it's now safe
> >>
> >> No, a safe version of val is returned, not a bool telling you it is now
> >> safe to use the original.
> >
> > OK, so making the old value dead is required to preserve the desired
> > dataflow.
> >
> > But how should I use the special value that signaled "failure"?
> >
> > Obviously the user isn't supposed to simply replace 'val' with
> >
> >  val = __builtin_speculation_safe_value (val);
> >
> > to make it speculation-proof.  So - how should the user _use_ this
> > builtin?  The docs do not say anything about this but says the
> > very confusing
> >
> > +The function may use target-dependent speculation tracking state to cause
> > +@var{failval} to be returned when it is known that speculative
> > +execution has incorrectly predicted a conditional branch operation.
> >
> > because speculation is about executing instructions as if they were
> > supposed to be executed.  Once it is known the prediciton was wrong
> > no more "wrong" instructions will be executed but a previously
> > speculated instruction cannot know it was "falsely" speculated.
> >
> > Does the above try to say that the function may return failval if the
> > instruction is currently executed speculatively instead?  That would
> > make sense to me.  And return failval independent of if the speculation
> > later turns out to be correct or not.
> >
> >>  You must use the sanitized version in future,
> >> not the unprotected version.
> >>
> >>
> >> So the usage is going to be more like:
> >>
> >> val = __builtin_speculation_safe_value (val);  // Overwrite val with a
> >> sanitized version.
> >>
> >> You have to use the cleaned up version, the unclean version is still
> >> vulnerable to incorrect speculation.
> >
> > But then where does failval come into play?  The above cannot be correct
> > if failval matters.
>
> failval only comes into play if the CPU is speculating incorrectly.
> It's just a safe value that some CPUs might use until such time as the
> speculation gets unwound.  In most cases it can be zero.  Perhaps a more
> concrete example would be something like.
>
>
> void **mem;
>
> void* f(unsigned untrusted)
> {
>   if (untrusted < 100)
>     return mem[untrusted];
>   return NULL;
> }
>
> This can be rewritten as either:
>
> void *mem;
>
> void* f(unsigned untrusted)
> {
>   if (untrusted < 100)
>     return mem[__builtin_speculation_safe_value (untrusted)];
>   return NULL;
> }
>
> if leaking mem[0] is not a problem; or if you're really paranoid:
>
> void* f(unsigned untrusted)
> {
>   if (untrusted < 100)
>     return *__builtin_speculation_safe_value (mem + untrusted);
>   return NULL;
> }
>
> which becomes a NULL pointer dereference if the speculation is
> incorrect.  The programmer doesn't need to test this code (any test
> would likely be useless anyway as the CPU would predict the outcome and
> speculate based on such a prediction).  They do need to think a bit
> about what might leak if the CPU does miss-speculate, hence the two
> examples above.
>
> A more complex case, where you might want a different failure value can
> come up if you have a mask operation.  A slightly convoluted example
> (derived from a real example we found in the Linux kernel) might be
>
> if (mode32)
>   mask_from = 0x100000000;
> else
>   mask_from = 0;
>
> ... // some code
>
> return mem[untrusted_val & (mask_from - 1)];
>
> It would probably be better to place the speculation cleanup nearer the
> initialization of mask_from, especially if mask_from could be used more
> than once; but in that case 0 is less safe than any other (since 0-1 is
> all bits 1).  In this case you might write:
>
> if (mode32)
>   mask_from = 0x100000000;
> else
>   mask_from = 0;
>
> mask_from = __builtin_speculation_safe_value (mask_from, 1);
>
> ... // some code
>
> return mem[untrusted_val & (mask_from - 1)];
>
> And in this case if the speculation is incorrect the (mask_from - 1)
> expression becomes 0 and the whole range is protected.
>
> Note that speculation is fine if it is correct, we don't want to hold
> things up in that case since it will happen anyway.  It's only a problem
> if the speculation is incorrect :-)

Ok, please amend at least the user-level documentation with one or two
examples like above.

>
> >
> > Does the builtin try to say that iff the current instruction is speculated
> > then it will return failval, otherwise it will return val and thus following
> > code needs to handle 'failval' in a way that is safe?
>
> No.  From an abstract machine sense the builtin only ever returns its
> first argument.  The fail value only appears if the CPU guesses the
> direction of a branch and guesses *incorrectly* (incorrect speculation)

I understand how it works when the implementation issues a fence.
How does the actual implementation (assembly) look like that makes
the CPU know whether it speculated wrongly or not at the time the
value is needed for further speculation given the *__bsfs (mem + offset)
instruction _is_ speculated as well.

Richard.

> >
> > Again, the wording "when it is known that speculative execution has
> > incorrectly predicted a conditional branch operation" is very confusing...
> >
> > Richard.
> >
> >> R.
> >>
> >>>
> >>> that is, the return value is only interesting in sofar as to whether it is equal
> >>> to val or the special value?
> >>>
> >>> That said, I wonder why we don't hide that details from the user and
> >>> provide a predicate instead.
> >>>
> >>> Richard.
> >>>
> >>>> There are seven patches in this set, as follows.
> >>>>
> >>>> 1) Introduces the new intrinsic __builtin_sepculation_safe_value.
> >>>> 2) Adds a basic hard barrier implementation for AArch32 (arm) state.
> >>>> 3) Adds a basic hard barrier implementation for AArch64 state.
> >>>> 4) Adds a new command-line option -mtrack-speculation (currently a no-op).
> >>>> 5) Disables CB[N]Z and TB[N]Z when -mtrack-speculation.
> >>>> 6) Adds the new speculation tracking pass for AArch64
> >>>> 7) Uses the new speculation tracking pass to generate CSDB-based barrier
> >>>>    sequences
> >>>>
> >>>> I haven't added a speculation-tracking pass for AArch32 at this time.
> >>>> It is possible to do this, but would require quite a lot of rework for
> >>>> the arm backend due to the limited number of registers that are
> >>>> available.
> >>>>
> >>>> Although patch 6 is AArch64 specific, I'd appreciate a review from
> >>>> someone more familiar with the branch edge code than myself.  There
> >>>> appear to be a number of tricky issues with more complex edges so I'd
> >>>> like a second opinion on that code in case I've missed an important
> >>>> case.
> >>>>
> >>>> R.
> >>>>
> >>>>
> >>>>
> >>>> Richard Earnshaw (7):
> >>>>   Add __builtin_speculation_safe_value
> >>>>   Arm - add speculation_barrier pattern
> >>>>   AArch64 - add speculation barrier
> >>>>   AArch64 - Add new option -mtrack-speculation
> >>>>   AArch64 - disable CB[N]Z TB[N]Z when tracking speculation
> >>>>   AArch64 - new pass to add conditional-branch speculation tracking
> >>>>   AArch64 - use CSDB based sequences if speculation tracking is enabled
> >>>>
> >>>>  gcc/builtin-types.def                     |   6 +
> >>>>  gcc/builtins.c                            |  57 ++++
> >>>>  gcc/builtins.def                          |  20 ++
> >>>>  gcc/c-family/c-common.c                   | 143 +++++++++
> >>>>  gcc/c-family/c-cppbuiltin.c               |   5 +-
> >>>>  gcc/config.gcc                            |   2 +-
> >>>>  gcc/config/aarch64/aarch64-passes.def     |   1 +
> >>>>  gcc/config/aarch64/aarch64-protos.h       |   3 +-
> >>>>  gcc/config/aarch64/aarch64-speculation.cc | 494 ++++++++++++++++++++++++++++++
> >>>>  gcc/config/aarch64/aarch64.c              |  88 +++++-
> >>>>  gcc/config/aarch64/aarch64.md             | 140 ++++++++-
> >>>>  gcc/config/aarch64/aarch64.opt            |   4 +
> >>>>  gcc/config/aarch64/iterators.md           |   3 +
> >>>>  gcc/config/aarch64/t-aarch64              |  10 +
> >>>>  gcc/config/arm/arm.md                     |  21 ++
> >>>>  gcc/config/arm/unspecs.md                 |   1 +
> >>>>  gcc/doc/cpp.texi                          |   4 +
> >>>>  gcc/doc/extend.texi                       |  29 ++
> >>>>  gcc/doc/invoke.texi                       |  10 +-
> >>>>  gcc/doc/md.texi                           |  15 +
> >>>>  gcc/doc/tm.texi                           |  20 ++
> >>>>  gcc/doc/tm.texi.in                        |   2 +
> >>>>  gcc/target.def                            |  23 ++
> >>>>  gcc/targhooks.c                           |  27 ++
> >>>>  gcc/targhooks.h                           |   2 +
> >>>>  gcc/testsuite/gcc.dg/spec-barrier-1.c     |  40 +++
> >>>>  gcc/testsuite/gcc.dg/spec-barrier-2.c     |  19 ++
> >>>>  gcc/testsuite/gcc.dg/spec-barrier-3.c     |  13 +
> >>>>  28 files changed, 1191 insertions(+), 11 deletions(-)
> >>>>  create mode 100644 gcc/config/aarch64/aarch64-speculation.cc
> >>>>  create mode 100644 gcc/testsuite/gcc.dg/spec-barrier-1.c
> >>>>  create mode 100644 gcc/testsuite/gcc.dg/spec-barrier-2.c
> >>>>  create mode 100644 gcc/testsuite/gcc.dg/spec-barrier-3.c
> >>
>


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