Messages in this thread | | | Subject | Re: [PATCH v2 00/14] x86: Add support for Clang CFI | From | Tom Stellard <> | Date | Mon, 23 Aug 2021 10:16:09 -0700 |
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On 8/23/21 10:13 AM, 'Sami Tolvanen' via Clang Built Linux 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 > > Version 2 depends on Clang >=14, where we fixed the issue with > referencing static functions from inline assembly. Based on the > feedback for v1, this version also changes the declaration of > functions that are not callable from C to use an opaque type, > which stops the compiler from replacing references to them. This > avoids the need to sprinkle function_nocfi() macros in the kernel > code.
How invasive are the changes in clang 14 necessary to make CFI work? Would it be possible to backport them to LLVM 13?
-Tom
> > 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-v2 > > --- > 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 (2): > x86/extable: Do not mark exception callback as CFI > x86, relocs: Ignore __typeid__ relocations > > Sami Tolvanen (12): > objtool: Add CONFIG_CFI_CLANG support > objtool: Add ASM_STACK_FRAME_NON_STANDARD > linkage: Add DECLARE_ASM_FUNC_SYMBOL > 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/mm/extable.c | 1 + > arch/x86/power/Makefile | 2 ++ > arch/x86/purgatory/Makefile | 2 +- > 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/linux/ftrace.h | 7 ++-- > include/linux/linkage.h | 13 +++++++ > include/linux/objtool.h | 6 ++++ > tools/include/linux/objtool.h | 6 ++++ > tools/objtool/arch/x86/decode.c | 16 +++++++++ > tools/objtool/elf.c | 51 +++++++++++++++++++++++++++ > tools/objtool/include/objtool/arch.h | 3 ++ > tools/objtool/include/objtool/elf.h | 2 +- > 33 files changed, 166 insertions(+), 62 deletions(-) > > > base-commit: d5ae8d7f85b7f6f6e60f1af8ff4be52b0926fde1 >
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