linux-kernel - Re: [PATCH v4 00/15] x86: Add support for Clang CFI

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Message-ID: <CA+icZUVC2OzVH_tzK148GA6=565S_vUbie+24YqTuiv97goR4Q@mail.gmail.com>
Date:   Fri, 1 Oct 2021 22:55:22 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>, x86@...nel.org,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v4 00/15] x86: Add support for Clang CFI

On Thu, Sep 30, 2021 at 8:38 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Thu, Sep 30, 2021 at 11:05 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
> >
> > This series adds support for Clang's Control-Flow Integrity (CFI)
> > checking to x86_64. With CFI, the compiler injects a runtime
> > check before each indirect function call to ensure the target is
> > a valid function with the correct static type. This restricts
> > possible call targets and makes it more difficult for an attacker
> > to exploit bugs that allow the modification of stored function
> > pointers. For more details, see:
> >
> >   https://clang.llvm.org/docs/ControlFlowIntegrity.html
> >
> > Note that v4 is based on tip/master. The first two patches contain
> > objtool support for CFI, the remaining patches change function
> > declarations to use opaque types, fix type mismatch issues that
> > confuse the compiler, and disable CFI where it can't be used.
> >
> > You can also pull this series from
> >
> >   https://github.com/samitolvanen/linux.git x86-cfi-v4
>
> I tested this series with near ToT clang with CONFIG_LTO_CLANG_THIN=y
> and our small buildroot-based userspace image in QEMU, then again with
> CONFIG_LTO_CLANG_FULL=y. No new warnings from objtool, everything
> seemed fine.  FWIW,
>
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
>

Hi Sami,

I tested your v4 with a selfmade PGO+ThinlTO optimized LLVM-13 toolchain.
And booted on bare metal.
My kernel-config is attached.

Thanks.

Regards,
- Sedat -

> >
> > ---
> > Changes in v4:
> > - Dropped the extable patch after the code was refactored in -tip.
> >
> > - Switched to __section() instead of open-coding the attribute.
> >
> > - Added an explicit ifdef for filtering out CC_FLAGS_CFI in
> >   purgatory for readability.
> >
> > - Added a comment to arch_cfi_jump_reloc_offset() in objtool.
> >
> > Changes in v3:
> > - Dropped Clang requirement to >= 13 after the missing compiler
> >   fix was backported there.
> >
> > - Added DEFINE_CFI_IMMEDIATE_RETURN_STUB to address the issue
> >   with tp_stub_func in kernel/tracepoint.c.
> >
> > - Renamed asm_func_t to asm_func_ptr.
> >
> > - Changed extable handlers to use __cficanonical instead of
> >   disabling CFI for fixup_exception.
> >
> > Changes in v2:
> > - Dropped the first objtool patch as the warnings were fixed in
> >   separate patches.
> >
> > - Changed fix_cfi_relocs() in objtool to not rely on jump table
> >   symbols, and to return an error if it can't find a relocation.
> >
> > - Fixed a build issue with ASM_STACK_FRAME_NON_STANDARD().
> >
> > - Dropped workarounds for inline assembly references to
> >   address-taken static functions with CFI as this was fixed in
> >   the compiler.
> >
> > - Changed the C declarations of non-callable functions to use
> >   opaque types and dropped the function_nocfi() patches.
> >
> > - Changed ARCH_SUPPORTS_CFI_CLANG to depend on Clang >=14 for
> >   the compiler fixes.
> >
> >
> > Kees Cook (1):
> >   x86, relocs: Ignore __typeid__ relocations
> >
> > Sami Tolvanen (14):
> >   objtool: Add CONFIG_CFI_CLANG support
> >   objtool: Add ASM_STACK_FRAME_NON_STANDARD
> >   linkage: Add DECLARE_ASM_FUNC_SYMBOL
> >   cfi: Add DEFINE_CFI_IMMEDIATE_RETURN_STUB
> >   tracepoint: Exclude tp_stub_func from CFI checking
> >   ftrace: Use an opaque type for functions not callable from C
> >   lkdtm: Disable UNSET_SMEP with CFI
> >   lkdtm: Use an opaque type for lkdtm_rodata_do_nothing
> >   x86: Use an opaque type for functions not callable from C
> >   x86/purgatory: Disable CFI
> >   x86, module: Ignore __typeid__ relocations
> >   x86, cpu: Use LTO for cpu.c with CFI
> >   x86, kprobes: Fix optprobe_template_func type mismatch
> >   x86, build: Allow CONFIG_CFI_CLANG to be selected
> >
> >  arch/x86/Kconfig                      |  1 +
> >  arch/x86/include/asm/ftrace.h         |  2 +-
> >  arch/x86/include/asm/idtentry.h       | 10 +++---
> >  arch/x86/include/asm/page_64.h        |  7 ++--
> >  arch/x86/include/asm/paravirt_types.h |  3 +-
> >  arch/x86/include/asm/processor.h      |  2 +-
> >  arch/x86/include/asm/proto.h          | 25 ++++++-------
> >  arch/x86/include/asm/uaccess_64.h     |  9 ++---
> >  arch/x86/kernel/alternative.c         |  2 +-
> >  arch/x86/kernel/ftrace.c              |  2 +-
> >  arch/x86/kernel/kprobes/opt.c         |  4 +--
> >  arch/x86/kernel/module.c              |  4 +++
> >  arch/x86/kernel/paravirt.c            |  4 +--
> >  arch/x86/kvm/emulate.c                |  4 +--
> >  arch/x86/kvm/kvm_emulate.h            |  9 ++---
> >  arch/x86/power/Makefile               |  2 ++
> >  arch/x86/purgatory/Makefile           |  4 +++
> >  arch/x86/tools/relocs.c               |  7 ++++
> >  arch/x86/xen/enlighten_pv.c           |  6 ++--
> >  arch/x86/xen/xen-ops.h                | 10 +++---
> >  drivers/misc/lkdtm/bugs.c             |  2 +-
> >  drivers/misc/lkdtm/lkdtm.h            |  2 +-
> >  drivers/misc/lkdtm/perms.c            |  2 +-
> >  drivers/misc/lkdtm/rodata.c           |  2 +-
> >  include/asm-generic/vmlinux.lds.h     | 11 ++++++
> >  include/linux/cfi.h                   | 13 +++++++
> >  include/linux/ftrace.h                |  7 ++--
> >  include/linux/linkage.h               | 13 +++++++
> >  include/linux/objtool.h               |  6 ++++
> >  kernel/cfi.c                          | 24 ++++++++++++-
> >  kernel/tracepoint.c                   |  5 +--
> >  tools/include/linux/objtool.h         |  6 ++++
> >  tools/objtool/arch/x86/decode.c       | 17 +++++++++
> >  tools/objtool/elf.c                   | 51 +++++++++++++++++++++++++++
> >  tools/objtool/include/objtool/arch.h  |  3 ++
> >  tools/objtool/include/objtool/elf.h   |  2 +-
> >  36 files changed, 217 insertions(+), 66 deletions(-)
> >
> >
> > base-commit: d4bfebd9ef497ee0afb498f6028a5074a6ccf307
> > --
> > 2.33.0.800.g4c38ced690-goog
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers

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