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Date:   Thu, 18 Mar 2021 10:49:50 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>, Jessica Yu <jeyu@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Tejun Heo <tj@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        bpf <bpf@...r.kernel.org>, linux-hardening@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        PCI <linux-pci@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 02/17] cfi: add __cficanonical

On Thu, Mar 18, 2021 at 10:11 AM Sami Tolvanen <samitolvanen@...gle.com> 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@...gle.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>   # pci.h

Irrelevant to this series, but I checked when the FN attr was first
available in clang; clang-10. (That's the minimum supported version of
clang for the kernel, and this series depends on LTO which depends on
clang-12, so no additional guards are necessary).

Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

> ---
>  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 6de9d0c9377e..adbe76b203e2 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -63,3 +63,4 @@
>  #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);                                              \
>         }                                                               \
> --
> 2.31.0.291.g576ba9dcdaf-goog
>


-- 
Thanks,
~Nick Desaulniers

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