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    Subject[PATCH v5 04/15] cfi: Add DEFINE_CFI_IMMEDIATE_RETURN_STUB
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    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@google.com>
    Tested-by: Nick Desaulniers <ndesaulniers@google.com>
    Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
    ---
    include/asm-generic/vmlinux.lds.h | 11 +++++++++++
    include/linux/cfi.h | 13 +++++++++++++
    kernel/cfi.c | 24 +++++++++++++++++++++++-
    3 files changed, 47 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..19f74af8eac2 100644
    --- a/include/linux/cfi.h
    +++ b/include/linux/cfi.h
    @@ -20,6 +20,17 @@ 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 \
    + __section(".cfi_excluded_stubs") = (void *)&fn
    +
    #ifdef CONFIG_CFI_CLANG_SHADOW

    extern void cfi_module_add(struct module *mod, unsigned long base_addr);
    @@ -35,6 +46,8 @@ 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
    --
    2.33.0.1079.g6e70778dc9-goog
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