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From: Kees Cook <keescook@chromium.org>
To: Ho-Eun Ryu <hoeun.ryu@gmail.com>
Cc: "kernel-hardening@lists.openwall.com" 
	<kernel-hardening@lists.openwall.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Andy Lutomirski <luto@kernel.org>,
	PaX Team <pageexec@freemail.hu>, Emese Revfy <re.emese@gmail.com>,
	Russell King <linux@armlinux.org.uk>,
	"x86@kernel.org" <x86@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC v2][PATCH 01/11] Introduce rare_write() infrastructure
Date: Fri, 7 Apr 2017 13:38:13 -0700
Message-ID: <CAGXu5jL_zp286sCynG0ARfF7TSJZUEqVsGS75dqH9MKfgRymNg@mail.gmail.com> (raw)
In-Reply-To: <FDCFFA43-5B6D-42CB-A65C-5FA485B64C7A@gmail.com>

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

  reply index

Thread overview: 63+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-29 18:15 [RFC v2] " Kees Cook
2017-03-29 18:15 ` [RFC v2][PATCH 01/11] " Kees Cook
2017-03-29 18:23   ` Kees Cook
2017-03-30  7:44     ` Ho-Eun Ryu
2017-03-30 17:02       ` Kees Cook
2017-04-07  8:09   ` Ho-Eun Ryu
2017-04-07 20:38     ` Kees Cook [this message]
2017-03-29 18:15 ` [RFC v2][PATCH 02/11] lkdtm: add test for " Kees Cook
2017-03-30  9:34   ` [kernel-hardening] " Ian Campbell
2017-03-30 16:16     ` Kees Cook
2017-03-29 18:15 ` [RFC v2][PATCH 03/11] net: switch sock_diag handlers to rare_write() Kees Cook
2017-03-29 18:15 ` [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap() Kees Cook
2017-03-29 22:38   ` Andy Lutomirski
2017-03-30  1:41     ` Kees Cook
2017-04-05 23:57       ` Andy Lutomirski
2017-04-06  0:14         ` Kees Cook
2017-04-06 15:59           ` Andy Lutomirski
2017-04-07  8:34             ` [kernel-hardening] " Mathias Krause
2017-04-07  9:46               ` Thomas Gleixner
2017-04-07 10:51                 ` Mathias Krause
2017-04-07 13:14                   ` Thomas Gleixner
2017-04-07 13:30                     ` Mathias Krause
2017-04-07 16:14                       ` Andy Lutomirski
2017-04-07 16:22                         ` Mark Rutland
2017-04-07 19:58                         ` PaX Team
2017-04-08  4:58                           ` Andy Lutomirski
2017-04-09 12:47                             ` PaX Team
2017-04-10  0:10                               ` Andy Lutomirski
2017-04-10 10:42                                 ` PaX Team
2017-04-10 16:01                                   ` Andy Lutomirski
2017-04-07 20:44                         ` Thomas Gleixner
2017-04-07 21:20                           ` Kees Cook
2017-04-08  4:12                             ` Daniel Micay
2017-04-08  4:13                               ` Daniel Micay
2017-04-08  4:21                         ` Daniel Micay
2017-04-08  5:07                           ` Andy Lutomirski
2017-04-08  7:33                             ` Daniel Micay
2017-04-08 15:20                               ` Andy Lutomirski
2017-04-09 10:53                                 ` Ingo Molnar
2017-04-10 10:22                                 ` Mark Rutland
2017-04-09 20:24                             ` PaX Team
2017-04-10  0:31                               ` Andy Lutomirski
2017-04-10 19:47                                 ` PaX Team
2017-04-10 20:27                                   ` Andy Lutomirski
2017-04-10 20:13                               ` Kees Cook
2017-04-10 20:17                                 ` Andy Lutomirski
2017-04-07 19:25                       ` Thomas Gleixner
2017-04-07 14:45                   ` Peter Zijlstra
2017-04-10 10:29                     ` Mark Rutland
2017-04-07 19:52                 ` PaX Team
2017-04-10  8:26                   ` Thomas Gleixner
2017-04-10 19:55                     ` PaX Team
2017-04-07  9:37   ` Peter Zijlstra
2017-03-29 18:15 ` [RFC v2][PATCH 05/11] ARM: mm: dump: Add domain to output Kees Cook
2017-03-29 18:15 ` [RFC v2][PATCH 06/11] ARM: domains: Extract common USER domain init Kees Cook
2017-03-29 18:15 ` [RFC v2][PATCH 07/11] ARM: mm: set DOMAIN_WR_RARE for rodata Kees Cook
2017-03-29 18:16 ` [RFC v2][PATCH 08/11] ARM: Implement __arch_rare_write_begin/end() Kees Cook
2017-04-07  9:36   ` Peter Zijlstra
2017-03-29 18:16 ` [RFC v2][PATCH 09/11] list: add rare_write() list helpers Kees Cook
2017-03-29 18:16 ` [RFC v2][PATCH 10/11] gcc-plugins: Add constify plugin Kees Cook
2017-03-29 18:16 ` [RFC v2][PATCH 11/11] cgroups: force all struct cftype const Kees Cook
2017-03-29 19:00 ` [RFC v2] Introduce rare_write() infrastructure Russell King - ARM Linux
2017-03-29 19:14   ` Kees Cook

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