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    Subject[PATCH v5 2/2] extable: verify address is read-only
    Date
    Provide a mechanism to check if the address of a variable is
    const or ro_after_init. It mimics the existing functions that test if an
    address is inside the kernel's text section.

    The idea is to prevent structures that are not read-only from being
    passed to functions. Other functions inside the kernel could then use
    this capability to verify that their arguments are read-only.

    This implements the first half of a suggestion made by Kees Cook for
    the Kernel Self Protection Project:

    - provide mechanism to check for ro_after_init memory areas, and
    reject structures not marked ro_after_init in vmbus_register()

    Suggested-by: Kees Cook <keescook@chromium.org>
    Signed-off-by: Eddie Kovsky <ewk@edkovsky.org>
    ---
    Changes in v5:
    - Replace __start_data_ro_after_init with __start_ro_after_init
    - Replace __end_data_ro_after_init with __end_ro_after_init
    Changes in v4:
    - Rename function core_kernel_ro_data() to core_kernel_rodata().
    Changes in v3:
    - Fix missing declaration of is_module_rodata_address()
    ---
    include/linux/kernel.h | 2 ++
    kernel/extable.c | 29 +++++++++++++++++++++++++++++
    2 files changed, 31 insertions(+)

    diff --git a/include/linux/kernel.h b/include/linux/kernel.h
    index 4c26dc3a8295..5748784ca209 100644
    --- a/include/linux/kernel.h
    +++ b/include/linux/kernel.h
    @@ -444,6 +444,8 @@ extern int core_kernel_data(unsigned long addr);
    extern int __kernel_text_address(unsigned long addr);
    extern int kernel_text_address(unsigned long addr);
    extern int func_ptr_is_kernel_text(void *ptr);
    +extern int core_kernel_rodata(unsigned long addr);
    +extern int kernel_ro_address(unsigned long addr);

    unsigned long int_sqrt(unsigned long);

    diff --git a/kernel/extable.c b/kernel/extable.c
    index 2676d7f8baf6..18c5e4dbe0fc 100644
    --- a/kernel/extable.c
    +++ b/kernel/extable.c
    @@ -154,3 +154,32 @@ int func_ptr_is_kernel_text(void *ptr)
    return 1;
    return is_module_text_address(addr);
    }
    +
    +/**
    + * core_kernel_rodata - Verify address points to read-only section
    + * @addr: address to test
    + *
    + */
    +int core_kernel_rodata(unsigned long addr)
    +{
    + if (addr >= (unsigned long)__start_rodata &&
    + addr < (unsigned long)__end_rodata)
    + return 1;
    +
    + if (addr >= (unsigned long)__start_ro_after_init &&
    + addr < (unsigned long)__end_ro_after_init)
    + return 1;
    +
    + return 0;
    +}
    +
    +/* Verify that address is const or ro_after_init. */
    +int kernel_ro_address(unsigned long addr)
    +{
    + if (core_kernel_rodata(addr))
    + return 1;
    + if (is_module_rodata_address(addr))
    + return 1;
    +
    + return 0;
    +}
    --
    2.12.2
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