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[PATCH 0/3] [gcc-7 backport] Add __builtin_load_no_speculate

This is a back-port of the __builtin-load_no_speculate builtin to the GCC-7

Recently, Google Project Zero disclosed several classes of attack
against speculative execution. One of these, known as variant-1
(CVE-2017-5753), allows explicit bounds checks to be bypassed under
speculation, providing an arbitrary read gadget. Further details can
be found on the GPZ blog [1] and the documentation that is included
with the first patch.

This patch set adds a new builtin function for GCC to provide a
mechanism for limiting speculation by a CPU after a bounds-checked
memory access.  I've tried to design this in such a way that it can be
used for any target where this might be necessary.  The patch set
provides a generic implementation of the builtin and then
target-specific support for Arm and AArch64.  Other architectures can
utilize the internal infrastructure as needed.

Most of the details of the builtin and the hooks that need to be
implemented to support it are described in the updates to the manual,
but a short summary is given below.

TYP __builtin_load_no_speculate
        (const volatile TYP *ptr,
         const volatile void *lower,
         const volatile void *upper,
         TYP failval,
         const volatile void *cmpptr)

Where TYP can be any integral type (signed or unsigned char, int,
short, long, etc) or any pointer type.

The builtin implements the following logical behaviour:

inline TYP __builtin_load_no_speculate
         (const volatile TYP *ptr,
          const volatile void *lower,
          const volatile void *upper,
          TYP failval,
          const volatile void *cmpptr)
  TYP result;

  if (cmpptr >= lower && cmpptr < upper)
    result = *ptr;
    result = failval;
  return result;

in addition the specification of the builtin ensures that future
speculation using *ptr may only continue iff cmpptr lies within the
bounds specified.

Some optimizations are permitted to make the builtin easier to use.
The final two arguments can both be omitted (c++ style): failval will
default to 0 in this case and if cmpptr is omitted ptr will be used
for expansions of the range check.  In addition either lower or upper
(but not both) may be a literal NULL and the expansion will then
ignore that boundary condition when expanding.

The patch set is constructed as follows:
1 - generic modifications to GCC providing the builtin function for all
    architectures and expanding to an implementation that gives the
    logical behaviour of the builtin only.  A warning is generated if
    this expansion path is used that code will execute correctly but
    without providing protection against speculative use.
2 - AArch64 support
3 - AArch32 support (arm) for A32 and thumb2 states.

These patches can be used with the header file that Arm recently
published here:

Kernel patches are also being developed, eg:  The intent is that eventually
code like this will be able to use support directly from the compiler
in a portable manner.

Similar patches are also being developed for LLVM and will be posted
to their development lists shortly.

[1] More information on the topic can be found here:
Arm specific information can be found here:

Richard Earnshaw (3):
  [builtins] Generic support for __builtin_load_no_speculate()
  [aarch64] Implement support for __builtin_load_no_speculate.
  [arm] Implement support for the de-speculation intrinsic

 gcc/builtin-types.def         |  16 +++++
 gcc/builtins.c                |  99 +++++++++++++++++++++++++
 gcc/builtins.def              |  22 ++++++
 gcc/c-family/c-common.c       | 164 ++++++++++++++++++++++++++++++++++++++++++
 gcc/c-family/c-cppbuiltin.c   |   5 +-
 gcc/config/aarch64/aarch64.c  |  91 +++++++++++++++++++++++
 gcc/config/aarch64/ |  28 ++++++++
 gcc/config/arm/arm.c          | 108 ++++++++++++++++++++++++++++
 gcc/config/arm/         |  40 ++++++++++-
 gcc/config/arm/     |   1 +
 gcc/doc/cpp.texi              |   4 ++
 gcc/doc/extend.texi           |  53 ++++++++++++++
 gcc/doc/tm.texi               |   6 ++
 gcc/doc/            |   2 +
 gcc/target.def                |  20 ++++++
 gcc/targhooks.c               |  67 ++++++++++++++++-
 gcc/targhooks.h               |   3 +
 17 files changed, 726 insertions(+), 3 deletions(-)

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