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Date:   Tue, 14 Sep 2021 12:36:11 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     x86@...nel.org, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH v3 04/16] cfi: Add DEFINE_CFI_IMMEDIATE_RETURN_STUB

On Tue, Sep 14, 2021 at 12:10 PM Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> This change introduces the DEFINE_CFI_IMMEDIATE_RETURN_STUB macro,
> which defines a stub function that immediately returns and when
> defined in the core kernel, always passes indirect call checking
> with CONFIG_CFI_CLANG. Note that this macro should only be used when
> a stub cannot be called using the correct function type.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  include/asm-generic/vmlinux.lds.h | 11 +++++++++++
>  include/linux/cfi.h               | 14 ++++++++++++++
>  kernel/cfi.c                      | 24 +++++++++++++++++++++++-
>  3 files changed, 48 insertions(+), 1 deletion(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index f2984af2b85b..5b77284f7221 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -407,6 +407,16 @@
>         KEEP(*(.static_call_tramp_key))                                 \
>         __stop_static_call_tramp_key = .;
>
> +#ifdef CONFIG_CFI_CLANG
> +#define CFI_EXCLUDED_DATA                                              \
> +       . = ALIGN(8);                                                   \
> +       __start_cfi_excluded = .;                                       \
> +       KEEP(*(.cfi_excluded_stubs))                                    \
> +       __stop_cfi_excluded = .;
> +#else
> +#define CFI_EXCLUDED_DATA
> +#endif
> +
>  /*
>   * Allow architectures to handle ro_after_init data on their
>   * own by defining an empty RO_AFTER_INIT_DATA.
> @@ -430,6 +440,7 @@
>                 __start_rodata = .;                                     \
>                 *(.rodata) *(.rodata.*)                                 \
>                 SCHED_DATA                                              \
> +               CFI_EXCLUDED_DATA                                       \
>                 RO_AFTER_INIT_DATA      /* Read only after init */      \
>                 . = ALIGN(8);                                           \
>                 __start___tracepoints_ptrs = .;                         \
> diff --git a/include/linux/cfi.h b/include/linux/cfi.h
> index 879744aaa6e0..9ebf67a0d421 100644
> --- a/include/linux/cfi.h
> +++ b/include/linux/cfi.h
> @@ -20,6 +20,18 @@ extern void __cfi_check(uint64_t id, void *ptr, void *diag);
>  #define __CFI_ADDRESSABLE(fn, __attr) \
>         const void *__cfi_jt_ ## fn __visible __attr = (void *)&fn
>
> +/*
> + * Defines a stub function that returns immediately, and when defined and
> + * referenced in the core kernel, always passes CFI checking. This should
> + * be used only for stubs that cannot be called using the correct function
> + * pointer type, which should be rare.
> + */
> +#define DEFINE_CFI_IMMEDIATE_RETURN_STUB(fn) \
> +       void fn(void) { return; } \
> +       const void *__cfi_excl_ ## fn __visible \
> +               __attribute__((__section__(".cfi_excluded_stubs"))) \

Can we use __section from include/linux/compiler_attributes.h here
rather than open coding the function attribute?

> +               = (void *)&fn
> +
>  #ifdef CONFIG_CFI_CLANG_SHADOW
>
>  extern void cfi_module_add(struct module *mod, unsigned long base_addr);
> @@ -35,6 +47,8 @@ static inline void cfi_module_remove(struct module *mod, unsigned long base_addr
>  #else /* !CONFIG_CFI_CLANG */
>
>  #define __CFI_ADDRESSABLE(fn, __attr)
> +#define DEFINE_CFI_IMMEDIATE_RETURN_STUB(fn) \
> +       void fn(void) { return; }
>
>  #endif /* CONFIG_CFI_CLANG */
>
> diff --git a/kernel/cfi.c b/kernel/cfi.c
> index 9594cfd1cf2c..8d931089141b 100644
> --- a/kernel/cfi.c
> +++ b/kernel/cfi.c
> @@ -278,12 +278,34 @@ static inline cfi_check_fn find_module_check_fn(unsigned long ptr)
>         return fn;
>  }
>
> +extern unsigned long __start_cfi_excluded[];
> +extern unsigned long __stop_cfi_excluded[];
> +
> +static inline bool is_cfi_excluded(unsigned long ptr)
> +{
> +       unsigned long *p = __start_cfi_excluded;
> +
> +       for ( ; p < __stop_cfi_excluded; ++p)
> +               if (*p == ptr)
> +                       return true;
> +
> +       return false;
> +}
> +
> +static void __cfi_pass(uint64_t id, void *ptr, void *diag)
> +{
> +}
> +
>  static inline cfi_check_fn find_check_fn(unsigned long ptr)
>  {
>         cfi_check_fn fn = NULL;
>
> -       if (is_kernel_text(ptr))
> +       if (is_kernel_text(ptr)) {
> +               if (unlikely(is_cfi_excluded(ptr)))
> +                       return __cfi_pass;
> +
>                 return __cfi_check;
> +       }
>
>         /*
>          * Indirect call checks can happen when RCU is not watching. Both
> --
> 2.33.0.309.g3052b89438-goog
>


-- 
Thanks,
~Nick Desaulniers

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