Re: [RFC v2][PATCH 01/11] Introduce rare_write() infrastructure

[Kees Cook <keescook@chromium.org>]

On Fri, Apr 7, 2017 at 1:09 AM, Ho-Eun Ryu <hoeun.ryu@gmail.com> wrote:
>
>> On 30 Mar 2017, at 3:15 AM, Kees Cook <keescook@chromium.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@chromium.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()

Yeah, that's actually exactly what I did for the current tree:
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=kspp/write-rarely

-Kees

-- 
Kees Cook
Pixel Security