r323155 - Introduce the "retpoline" x86 mitigation technique for variant #2 of the speculative execution vulnerabilities disclosed today, specifically identified by CVE-2017-5715, "Branch Target Injection", and is one of the two halves to Spectre..

Thu Feb 1 14:53:34 PST 2018

I backported these to the release_50 branch in the following revisions:

llvm r324007, r324009
cfe r324012
lld r324025, r324026

The regression test suites pass on my machine with the exception of two
pre-existing failures. I also built my small retpoline execution test that
does a variety of indirect calls and tail calls, and it executes
successfully and the only indirect jumps in it come from crt0.o. That's the
same as what we get with trunk.

We might want to do more extensive testing of the flag with the 5.0
release, but I'm not sure what's next exactly.

On Thu, Feb 1, 2018 at 11:09 AM, Chandler Carruth <chandlerc at gmail.com>
wrote:

> +Hans Wennborg <hans at chromium.org> +tstellar at redhat.com
> <tstellar at redhat.com>
>
> So the retpoline patch series we should get back ported start with this
> revision and have two follow ups:
> r323155
> r323288
> r323915
>
> +Reid Kleckner <rnk at google.com> was going to look at doing the (likely
> more involved) backport to the 5 branch....
>
> On Mon, Jan 22, 2018 at 2:06 PM Chandler Carruth via cfe-commits <
> cfe-commits at lists.llvm.org> wrote:
>
>> Author: chandlerc
>> Date: Mon Jan 22 14:05:25 2018
>> New Revision: 323155
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=323155&view=rev
>> Log:
>> Introduce the "retpoline" x86 mitigation technique for variant #2 of the
>> speculative execution vulnerabilities disclosed today, specifically
>> identified by CVE-2017-5715, "Branch Target Injection", and is one of the
>> two halves to Spectre..
>>
>> Summary:
>> First, we need to explain the core of the vulnerability. Note that this
>> is a very incomplete description, please see the Project Zero blog post
>> for details:
>> https://googleprojectzero.blogspot.com/2018/01/reading-
>> privileged-memory-with-side.html
>>
>> The basis for branch target injection is to direct speculative execution
>> of the processor to some "gadget" of executable code by poisoning the
>> prediction of indirect branches with the address of that gadget. The
>> gadget in turn contains an operation that provides a side channel for
>> reading data. Most commonly, this will look like a load of secret data
>> followed by a branch on the loaded value and then a load of some
>> predictable cache line. The attacker then uses timing of the processors
>> cache to determine which direction the branch took *in the speculative
>> execution*, and in turn what one bit of the loaded value was. Due to the
>> nature of these timing side channels and the branch predictor on Intel
>> processors, this allows an attacker to leak data only accessible to
>> a privileged domain (like the kernel) back into an unprivileged domain.
>>
>> The goal is simple: avoid generating code which contains an indirect
>> branch that could have its prediction poisoned by an attacker. In many
>> cases, the compiler can simply use directed conditional branches and
>> a small search tree. LLVM already has support for lowering switches in
>> this way and the first step of this patch is to disable jump-table
>> lowering of switches and introduce a pass to rewrite explicit indirectbr
>> sequences into a switch over integers.
>>
>> However, there is no fully general alternative to indirect calls. We
>> introduce a new construct we call a "retpoline" to implement indirect
>> calls in a non-speculatable way. It can be thought of loosely as
>> a trampoline for indirect calls which uses the RET instruction on x86.
>> Further, we arrange for a specific call->ret sequence which ensures the
>> processor predicts the return to go to a controlled, known location. The
>> retpoline then "smashes" the return address pushed onto the stack by the
>> call with the desired target of the original indirect call. The result
>> is a predicted return to the next instruction after a call (which can be
>> used to trap speculative execution within an infinite loop) and an
>> actual indirect branch to an arbitrary address.
>>
>> On 64-bit x86 ABIs, this is especially easily done in the compiler by
>> using a guaranteed scratch register to pass the target into this device.
>> For 32-bit ABIs there isn't a guaranteed scratch register and so several
>> different retpoline variants are introduced to use a scratch register if
>> one is available in the calling convention and to otherwise use direct
>> stack push/pop sequences to pass the target address.
>>
>> This "retpoline" mitigation is fully described in the following blog
>> post: https://support.google.com/faqs/answer/7625886
>>
>> We also support a target feature that disables emission of the retpoline
>> thunk by the compiler to allow for custom thunks if users want them.
>> These are particularly useful in environments like kernels that
>> routinely do hot-patching on boot and want to hot-patch their thunk to
>> different code sequences. They can write this custom thunk and use
>> `-mretpoline-external-thunk` *in addition* to `-mretpoline`. In this
>> case, on x86-64 thu thunk names must be:
>> ```
>>   __llvm_external_retpoline_r11
>> ```
>> or on 32-bit:
>> ```
>>   __llvm_external_retpoline_eax
>>   __llvm_external_retpoline_ecx
>>   __llvm_external_retpoline_edx
>>   __llvm_external_retpoline_push
>> ```
>> And the target of the retpoline is passed in the named register, or in
>> the case of the `push` suffix on the top of the stack via a `pushl`
>> instruction.
>>
>> There is one other important source of indirect branches in x86 ELF
>> binaries: the PLT. These patches also include support for LLD to
>> generate PLT entries that perform a retpoline-style indirection.
>>
>> The only other indirect branches remaining that we are aware of are from
>> precompiled runtimes (such as crt0.o and similar). The ones we have
>> found are not really attackable, and so we have not focused on them
>> here, but eventually these runtimes should also be replicated for
>> retpoline-ed configurations for completeness.
>>
>> For kernels or other freestanding or fully static executables, the
>> compiler switch `-mretpoline` is sufficient to fully mitigate this
>> particular attack. For dynamic executables, you must compile *all*
>> libraries with `-mretpoline` and additionally link the dynamic
>> executable and all shared libraries with LLD and pass `-z retpolineplt`
>> (or use similar functionality from some other linker). We strongly
>> recommend also using `-z now` as non-lazy binding allows the
>> retpoline-mitigated PLT to be substantially smaller.
>>
>> When manually apply similar transformations to `-mretpoline` to the
>> Linux kernel we observed very small performance hits to applications
>> running typical workloads, and relatively minor hits (approximately 2%)
>> even for extremely syscall-heavy applications. This is largely due to
>> the small number of indirect branches that occur in performance
>> sensitive paths of the kernel.
>>
>> When using these patches on statically linked applications, especially
>> C++ applications, you should expect to see a much more dramatic
>> performance hit. For microbenchmarks that are switch, indirect-, or
>> virtual-call heavy we have seen overheads ranging from 10% to 50%.
>>
>> However, real-world workloads exhibit substantially lower performance
>> impact. Notably, techniques such as PGO and ThinLTO dramatically reduce
>> the impact of hot indirect calls (by speculatively promoting them to
>> direct calls) and allow optimized search trees to be used to lower
>> switches. If you need to deploy these techniques in C++ applications, we
>> *strongly* recommend that you ensure all hot call targets are statically
>> linked (avoiding PLT indirection) and use both PGO and ThinLTO. Well
>> tuned servers using all of these techniques saw 5% - 10% overhead from
>> the use of retpoline.
>>
>> We will add detailed documentation covering these components in
>> subsequent patches, but wanted to make the core functionality available
>> as soon as possible. Happy for more code review, but we'd really like to
>> get these patches landed and backported ASAP for obvious reasons. We're
>> planning to backport this to both 6.0 and 5.0 release streams and get
>> a 5.0 release with just this cherry picked ASAP for distros and vendors.
>>
>> This patch is the work of a number of people over the past month: Eric,
>> Reid,
>> Rui, and myself. I'm mailing it out as a single commit due to the time
>> sensitive nature of landing this and the need to backport it. Huge thanks
>> to
>> everyone who helped out here, and everyone at Intel who helped out in
>> discussions about how to craft this. Also, credit goes to Paul Turner (at
>> Google, but not an LLVM contributor) for much of the underlying retpoline
>> design.
>>
>> Reviewers: echristo, rnk, ruiu, craig.topper, DavidKreitzer
>>
>> Subscribers: sanjoy, emaste, mcrosier, mgorny, mehdi_amini, hiraditya,
>> llvm-commits
>>
>> Differential Revision: https://reviews.llvm.org/D41723
>>
>> Modified:
>>     cfe/trunk/include/clang/Driver/Options.td
>>     cfe/trunk/lib/Basic/Targets/X86.cpp
>>     cfe/trunk/lib/Basic/Targets/X86.h
>>     cfe/trunk/test/Driver/x86-target-features.c
>>
>> Modified: cfe/trunk/include/clang/Driver/Options.td
>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/
>> clang/Driver/Options.td?rev=323155&r1=323154&r2=323155&view=diff
>> ============================================================
>> ==================
>> --- cfe/trunk/include/clang/Driver/Options.td (original)
>> +++ cfe/trunk/include/clang/Driver/Options.td Mon Jan 22 14:05:25 2018
>> @@ -2594,6 +2594,10 @@ def mshstk : Flag<["-"], "mshstk">, Grou
>>  def mno_shstk : Flag<["-"], "mno-shstk">, Group<m_x86_Features_Group>;
>>  def mibt : Flag<["-"], "mibt">, Group<m_x86_Features_Group>;
>>  def mno_ibt : Flag<["-"], "mno-ibt">, Group<m_x86_Features_Group>;
>> +def mretpoline : Flag<["-"], "mretpoline">, Group<m_x86_Features_Group>;
>> +def mno_retpoline : Flag<["-"], "mno-retpoline">,
>> Group<m_x86_Features_Group>;
>> +def mretpoline_external_thunk : Flag<["-"],
>> "mretpoline-external-thunk">, Group<m_x86_Features_Group>;
>> +def mno_retpoline_external_thunk : Flag<["-"],
>> "mno-retpoline-external-thunk">, Group<m_x86_Features_Group>;
>>
>>  // These are legacy user-facing driver-level option spellings. They are
>> always
>>  // aliases for options that are spelled using the more common Unix / GNU
>> flag
>>
>> Modified: cfe/trunk/lib/Basic/Targets/X86.cpp
>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Basic/
>> Targets/X86.cpp?rev=323155&r1=323154&r2=323155&view=diff
>> ============================================================
>> ==================
>> --- cfe/trunk/lib/Basic/Targets/X86.cpp (original)
>> +++ cfe/trunk/lib/Basic/Targets/X86.cpp Mon Jan 22 14:05:25 2018
>> @@ -787,6 +787,10 @@ bool X86TargetInfo::handleTargetFeatures
>>        HasCLZERO = true;
>>      } else if (Feature == "+rdpid") {
>>        HasRDPID = true;
>> +    } else if (Feature == "+retpoline") {
>> +      HasRetpoline = true;
>> +    } else if (Feature == "+retpoline-external-thunk") {
>> +      HasRetpolineExternalThunk = true;
>>      }
>>
>>      X86SSEEnum Level = llvm::StringSwitch<X86SSEEnum>(Feature)
>> @@ -1333,6 +1337,8 @@ bool X86TargetInfo::hasFeature(StringRef
>>        .Case("rdpid", HasRDPID)
>>        .Case("rdrnd", HasRDRND)
>>        .Case("rdseed", HasRDSEED)
>> +      .Case("retpoline", HasRetpoline)
>> +      .Case("retpoline-external-thunk", HasRetpolineExternalThunk)
>>        .Case("rtm", HasRTM)
>>        .Case("sgx", HasSGX)
>>        .Case("sha", HasSHA)
>>
>> Modified: cfe/trunk/lib/Basic/Targets/X86.h
>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Basic/
>> Targets/X86.h?rev=323155&r1=323154&r2=323155&view=diff
>> ============================================================
>> ==================
>> --- cfe/trunk/lib/Basic/Targets/X86.h (original)
>> +++ cfe/trunk/lib/Basic/Targets/X86.h Mon Jan 22 14:05:25 2018
>> @@ -97,6 +97,8 @@ class LLVM_LIBRARY_VISIBILITY X86TargetI
>>    bool HasMOVBE = false;
>>    bool HasPREFETCHWT1 = false;
>>    bool HasRDPID = false;
>> +  bool HasRetpoline = false;
>> +  bool HasRetpolineExternalThunk = false;
>>
>>    /// \brief Enumeration of all of the X86 CPUs supported by Clang.
>>    ///
>>
>> Modified: cfe/trunk/test/Driver/x86-target-features.c
>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Driver/
>> x86-target-features.c?rev=323155&r1=323154&r2=323155&view=diff
>> ============================================================
>> ==================
>> --- cfe/trunk/test/Driver/x86-target-features.c (original)
>> +++ cfe/trunk/test/Driver/x86-target-features.c Mon Jan 22 14:05:25 2018
>> @@ -129,3 +129,13 @@
>>  // RUN: %clang -target i386-unknown-linux-gnu -march=i386 -mno-rdpid %s
>> -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-RDPID %s
>>  // RDPID: "-target-feature" "+rdpid"
>>  // NO-RDPID: "-target-feature" "-rdpid"
>> +
>> +// RUN: %clang -target i386-linux-gnu -mretpoline %s -### -o %t.o 2>&1 |
>> FileCheck -check-prefix=RETPOLINE %s
>> +// RUN: %clang -target i386-linux-gnu -mno-retpoline %s -### -o %t.o
>> 2>&1 | FileCheck -check-prefix=NO-RETPOLINE %s
>> +// RETPOLINE: "-target-feature" "+retpoline"
>> +// NO-RETPOLINE: "-target-feature" "-retpoline"
>> +
>> +// RUN: %clang -target i386-linux-gnu -mretpoline
>> -mretpoline-external-thunk %s -### -o %t.o 2>&1 | FileCheck
>> -check-prefix=RETPOLINE-EXTERNAL-THUNK %s
>> +// RUN: %clang -target i386-linux-gnu -mretpoline
>> -mno-retpoline-external-thunk %s -### -o %t.o 2>&1 | FileCheck
>> -check-prefix=NO-RETPOLINE-EXTERNAL-THUNK %s
>> +// RETPOLINE-EXTERNAL-THUNK: "-target-feature"
>> "+retpoline-external-thunk"
>> +// NO-RETPOLINE-EXTERNAL-THUNK: "-target-feature"
>> "-retpoline-external-thunk"
>>
>>
>> _______________________________________________
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>> cfe-commits at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
>>
>
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