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Message-Id: <FDCFFA43-5B6D-42CB-A65C-5FA485B64C7A@gmail.com>
Date: Fri, 7 Apr 2017 17:09:18 +0900
From: Ho-Eun Ryu <hoeun.ryu@...il.com>
To: Kees Cook <keescook@...omium.org>
Cc: kernel-hardening@...ts.openwall.com,
Mark Rutland <mark.rutland@....com>,
Andy Lutomirski <luto@...nel.org>,
PaX Team <pageexec@...email.hu>,
Emese Revfy <re.emese@...il.com>,
Russell King <linux@...linux.org.uk>,
x86@...nel.org,
linux-kernel@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC v2][PATCH 01/11] Introduce rare_write() infrastructure
> On 30 Mar 2017, at 3:15 AM, Kees Cook <keescook@...omium.org> wrote:
>
> Several types of data storage exist in the kernel: read-write data (.data,
> .bss), read-only data (.rodata), and RO-after-init. This introduces the
> infrastructure for another type: write-rarely, which is intended for data
> that is either only rarely modified or especially security-sensitive. The
> goal is to further reduce the internal attack surface of the kernel by
> making this storage read-only when "at rest". This makes it much harder
> to be subverted by attackers who have a kernel-write flaw, since they
> cannot directly change these memory contents.
>
> This work is heavily based on PaX and grsecurity's pax_{open,close}_kernel
> API, its __read_only annotations, its constify plugin, and the work done
> to identify sensitive structures that should be moved from .data into
> .rodata. This builds the initial infrastructure to support these kinds
> of changes, though the API and naming has been adjusted in places for
> clarity and maintainability.
>
> Variables declared with the __wr_rare annotation will be moved to the
> .rodata section if an architecture supports CONFIG_HAVE_ARCH_WRITE_RARE.
> To change these variables, either a single rare_write() macro can be used,
> or multiple uses of __rare_write(), wrapped in a matching pair of
> rare_write_begin() and rare_write_end() macros can be used. These macros
> are expanded into the arch-specific functions that perform the actions
> needed to write to otherwise read-only memory.
>
> As detailed in the Kconfig help, the arch-specific helpers have several
> requirements to make them sensible/safe for use by the kernel: they must
> not allow non-current CPUs to write the memory area, they must run
> non-preemptible to avoid accidentally leaving memory writable, and must
> be inline to avoid making them desirable ROP targets for attackers.
>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> arch/Kconfig | 25 +++++++++++++++++++++++++
> include/linux/compiler.h | 32 ++++++++++++++++++++++++++++++++
> include/linux/preempt.h | 6 ++++--
> 3 files changed, 61 insertions(+), 2 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index cd211a14a88f..5ebf62500b99 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -847,4 +847,29 @@ config STRICT_MODULE_RWX
> config ARCH_WANT_RELAX_ORDER
> bool
>
> +config HAVE_ARCH_RARE_WRITE
> + def_bool n
> + help
> + An arch should select this option if it has defined the functions
> + __arch_rare_write_begin() and __arch_rare_write_end() to
> + respectively enable and disable writing to read-only memory. The
> + routines must meet the following requirements:
> + - read-only memory writing must only be available on the current
> + CPU (to make sure other CPUs can't race to make changes too).
> + - the routines must be declared inline (to discourage ROP use).
> + - the routines must not be preemptible (likely they will call
> + preempt_disable() and preempt_enable_no_resched() respectively).
> + - the routines must validate expected state (e.g. when enabling
> + writes, BUG() if writes are already be enabled).
> +
> +config HAVE_ARCH_RARE_WRITE_MEMCPY
> + def_bool n
> + depends on HAVE_ARCH_RARE_WRITE
> + help
> + An arch should select this option if a special accessor is needed
> + to write to otherwise read-only memory, defined by the function
> + __arch_rare_write_memcpy(). Without this, the write-rarely
> + infrastructure will just attempt to write directly to the memory
> + using a const-ignoring assignment.
> +
> source "kernel/gcov/Kconfig"
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index f8110051188f..274bd03cfe9e 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -336,6 +336,38 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
> __u.__val; \
> })
>
> +/*
> + * Build "write rarely" infrastructure for flipping memory r/w
> + * on a per-CPU basis.
> + */
> +#ifndef CONFIG_HAVE_ARCH_RARE_WRITE
> +# define __wr_rare
> +# define __wr_rare_type
> +# define __rare_write(__var, __val) (__var = (__val))
> +# define rare_write_begin() do { } while (0)
> +# define rare_write_end() do { } while (0)
> +#else
> +# define __wr_rare __ro_after_init
> +# define __wr_rare_type const
> +# ifdef CONFIG_HAVE_ARCH_RARE_WRITE_MEMCPY
> +# define __rare_write_n(dst, src, len) ({ \
> + BUILD_BUG(!builtin_const(len)); \
> + __arch_rare_write_memcpy((dst), (src), (len)); \
> + })
> +# define __rare_write(var, val) __rare_write_n(&(var), &(val), sizeof(var))
> +# else
> +# define __rare_write(var, val) ((*(typeof((typeof(var))0) *)&(var)) = (val))
> +# endif
> +# define rare_write_begin() __arch_rare_write_begin()
> +# define rare_write_end() __arch_rare_write_end()
> +#endif
> +#define rare_write(__var, __val) ({ \
> + rare_write_begin(); \
> + __rare_write(__var, __val); \
> + rare_write_end(); \
> + __var; \
> +})
> +
How about we have a separate header file splitting section annotations and the actual APIs.
include/linux/compiler.h:
__wr_rare
__wr_rare_type
include/linux/rare_write.h:
__rare_write_n()
__rare_write()
rare_write_begin()
rare_write_end()
OR moving all of them to include/linux/rare_write.h.
I’m writing the arm64 port for rare_write feature and I’ve stucked in some header problems for the next version of the patch.
I need some other mmu related APIs (mostly defined in `arch/arm64/include/asm/mmu_context.h`) to implement those helpers.
but I cannot include the header in `include/linux/compiler.h` prior to definition of rare_write macros (huge compilation errors).
You know that `linux/compiler.h` header is mostly base of other headers not a user.
I have to define `__arch_rare_write_[begin/end/memcpy]()` functions as `static inline` in a header file somewhere in `arch/arm64/include/asm/`
to avoid making them ROP targets and the helpers need other mmu related APIs.
And the helpers and the mmu related APIs cannot be defined prior to definition of rare_write macros in `linux/compile.h`.
And I think I'll need `include/linux/module.h` for module related APIs like `is_module_address()` to support module address conversion
in `__arch_rare_write_memcpy()` and it’ll make the situation worse.
* make `include/linux/rare_write.h`
* the header defines rare_write APIs
* the header includes `include/asm/rare_write.h` for arch-specific helpers
* users using rare_write feature should include `linux/rare_write.h`
OR suggest other solutions please.
> #endif /* __KERNEL__ */
>
> #endif /* __ASSEMBLY__ */
> diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> index cae461224948..4fc97aaa22ea 100644
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -258,10 +258,12 @@ do { \
> /*
> * Modules have no business playing preemption tricks.
> */
> -#undef sched_preempt_enable_no_resched
> -#undef preempt_enable_no_resched
> #undef preempt_enable_no_resched_notrace
> #undef preempt_check_resched
> +#ifndef CONFIG_HAVE_ARCH_RARE_WRITE
> +#undef sched_preempt_enable_no_resched
> +#undef preempt_enable_no_resched
> +#endif
> #endif
>
> #define preempt_set_need_resched() \
> --
> 2.7.4
>
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