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[PATCH 0/7] Mitigation against unsafe data speculation (CVE-2017-5753)
- From: Richard Earnshaw <Richard dot Earnshaw at arm dot com>
- To: gcc-patches at gcc dot gnu dot org
- Cc: Richard Earnshaw <Richard dot Earnshaw at arm dot com>, richard dot earnshaw at arm dot com
- Date: Mon, 9 Jul 2018 17:38:12 +0100
- Subject: [PATCH 0/7] Mitigation against unsafe data speculation (CVE-2017-5753)
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
- The speculation constraints could only cover the immediately preceding
branch, which often did not fit well with the structure of the existing
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
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.
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
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
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
Richard Earnshaw (7):
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