Re: [kernel-hardening] [RFC 1/7] arch: add __ro_mostly_after_init section marker

Ard Biesheuvel
2017-02-19T11:24:08Z

On 19 February 2017 at 10:04, Hoeun Ryu <hoeun.ryu@gmail.com> wrote:
> After `__ro_after_init` marker is included in kernel, many kernel data
> objects can be read-only-after-init. But there are many other places that
> would be good to read-only-after-init but `__ro_after_init` can not be simply
> applicable to them because they should be writable at some points, which are
> during module_init/exit or dynamic de/registration for a specific subsystem.
> `__ro_mostly_after_init` is basically the same to `__ro_after_init`. The
> section is mapped as read-only after kernel init. The different thing is
> this section is temporarily mapped as read-write during module_init/exit and
> de/registration of a subsystem using set_ro_mostly_after_init_rw/ro pair.
> Use `__ro_mostly_after_init` as a way to mark such memory instead when
> `__ro_after_init` is not applicable because the memory should be writable
> at the described points of time. They are read-only right after kernel init
> and writable temporarily only during module_init/exit and dynamic
> de/registration for a subsystem.
>
> Signed-off-by: Hoeun Ryu <hoeun.ryu@gmail.com>
> ---
> include/asm-generic/sections.h | 1 +
> include/asm-generic/vmlinux.lds.h | 10 ++++++++++
> include/linux/cache.h | 11 +++++++++++
> 3 files changed, 22 insertions(+)
>
> diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
> index 4df64a1..16a6f21 100644
> --- a/include/asm-generic/sections.h
> +++ b/include/asm-generic/sections.h
> @@ -34,6 +34,7 @@ extern char __bss_start[], __bss_stop[];
> extern char __init_begin[], __init_end[];
> extern char _sinittext[], _einittext[];
> extern char __start_data_ro_after_init[], __end_data_ro_after_init[];
> +extern char __start_data_ro_mostly_after_init[], __end_data_ro_mostly_after_init[];
> extern char _end[];
> extern char __per_cpu_load[], __per_cpu_start[], __per_cpu_end[];
> extern char __kprobes_text_start[], __kprobes_text_end[];
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 4e09b28..cc5f44e 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -265,6 +265,15 @@
> __end_data_ro_after_init = .;
> #endif
>
> +#ifndef RO_MOSTLY_AFTER_INIT_DATA
> +#define RO_MOSTLY_AFTER_INIT_DATA(align) \
> + . = ALIGN(align); \
> + VMLINUX_SYMBOL(__start_data_ro_mostly_after_init) = .; \
> + *(.data..ro_mostly_after_init) \
> + . = ALIGN(align); \
> + VMLINUX_SYMBOL(__end_data_ro_mostly_after_init) = .;
> +#endif
> +
> /*
> * Read only Data
> */
> @@ -275,6 +284,7 @@
> *(.rodata) *(.rodata.*) \
> RO_AFTER_INIT_DATA /* Read only after init */ \
> KEEP(*(__vermagic)) /* Kernel version magic */ \
> + RO_MOSTLY_AFTER_INIT_DATA(align) \

You can't really drop this in the middle of a section like this. On
arm64, we try very hard to use a minimal segment alignment of 64 KB
(of 2 MB if DEBUG_ALIGN_RODATA=y), to ensure that the TLB footprint of
the kernel image is minimized.

So this should be a separate section in the arm64 linker script.

> . = ALIGN(8); \
> VMLINUX_SYMBOL(__start___tracepoints_ptrs) = .; \
> KEEP(*(__tracepoints_ptrs)) /* Tracepoints: pointer array */ \
> diff --git a/include/linux/cache.h b/include/linux/cache.h
> index 1be04f8..fd1cb9b 100644
> --- a/include/linux/cache.h
> +++ b/include/linux/cache.h
> @@ -30,6 +30,17 @@
> #define __ro_after_init __attribute__((__section__(".data..ro_after_init")))
> #endif
>
> +/*
> + * __ro_mostly_after_init is almost like __ro_after_init.
> + * but __ro_mostly_after_init section is temporarily writable only during
> + * module_init/exit or dynamic de/registeration of a subsystem using
> + * set_ro_mostly_after_init_rw/ro pair.
> + */
> +#ifndef __ro_mostly_after_init
> +#define __ro_mostly_after_init \
> + __attribute__((__section__(".data..ro_mostly_after_init")))
> +#endif
> +
> #ifndef ____cacheline_aligned
> #define ____cacheline_aligned __attribute__((__aligned__(SMP_CACHE_BYTES)))
> #endif
> --
> 2.7.4
>