Re: [PATCH 02/17] cfi: add __cficanonical

Bjorn Helgaas
2021-03-12T20:29:17Z

On Thu, Mar 11, 2021 at 04:49:04PM -0800, Sami Tolvanen wrote:
> With CONFIG_CFI_CLANG, the compiler replaces a function address taken
> in C code with the address of a local jump table entry, which passes
> runtime indirect call checks. However, the compiler won't replace
> addresses taken in assembly code, which will result in a CFI failure
> if we later jump to such an address in instrumented C code. The code
> generated for the non-canonical jump table looks this:
>
> <noncanonical.cfi_jt>: /* In C, &noncanonical points here */
> jmp noncanonical
> ...
> <noncanonical>: /* function body */
> ...
>
> This change adds the __cficanonical attribute, which tells the
> compiler to use a canonical jump table for the function instead. This
> means the compiler will rename the actual function to <function>.cfi
> and points the original symbol to the jump table entry instead:
>
> <canonical>: /* jump table entry */
> jmp canonical.cfi
> ...
> <canonical.cfi>: /* function body */
> ...
>
> As a result, the address taken in assembly, or other non-instrumented
> code always points to the jump table and therefore, can be used for
> indirect calls in instrumented code without tripping CFI checks.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

If you need it:

Acked-by: Bjorn Helgaas <bhelgaas@google.com> # pci.h

> ---
> include/linux/compiler-clang.h | 1 +
> include/linux/compiler_types.h | 4 ++++
> include/linux/init.h | 4 ++--
> include/linux/pci.h | 4 ++--
> 4 files changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index 1ff22bdad992..c275f23ce023 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -57,3 +57,4 @@
> #endif
>
> #define __nocfi __attribute__((__no_sanitize__("cfi")))
> +#define __cficanonical __attribute__((__cfi_canonical_jump_table__))
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 796935a37e37..d29bda7f6ebd 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -246,6 +246,10 @@ struct ftrace_likely_data {
> # define __nocfi
> #endif
>
> +#ifndef __cficanonical
> +# define __cficanonical
> +#endif
> +
> #ifndef asm_volatile_goto
> #define asm_volatile_goto(x...) asm goto(x)
> #endif
> diff --git a/include/linux/init.h b/include/linux/init.h
> index b3ea15348fbd..045ad1650ed1 100644
> --- a/include/linux/init.h
> +++ b/include/linux/init.h
> @@ -220,8 +220,8 @@ extern bool initcall_debug;
> __initcall_name(initstub, __iid, id)
>
> #define __define_initcall_stub(__stub, fn) \
> - int __init __stub(void); \
> - int __init __stub(void) \
> + int __init __cficanonical __stub(void); \
> + int __init __cficanonical __stub(void) \
> { \
> return fn(); \
> } \
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 86c799c97b77..39684b72db91 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -1944,8 +1944,8 @@ enum pci_fixup_pass {
> #ifdef CONFIG_LTO_CLANG
> #define __DECLARE_PCI_FIXUP_SECTION(sec, name, vendor, device, class, \
> class_shift, hook, stub) \
> - void stub(struct pci_dev *dev); \
> - void stub(struct pci_dev *dev) \
> + void __cficanonical stub(struct pci_dev *dev); \
> + void __cficanonical stub(struct pci_dev *dev) \
> { \
> hook(dev); \
> } \
> --
> 2.31.0.rc2.261.g7f71774620-goog
>