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Message-Id: <1488228186-110679-2-git-send-email-keescook@chromium.org>
Date: Mon, 27 Feb 2017 12:42:59 -0800
From: Kees Cook <keescook@...omium.org>
To: kernel-hardening@...ts.openwall.com
Cc: Kees Cook <keescook@...omium.org>,
Mark Rutland <mark.rutland@....com>,
Andy Lutomirski <luto@...nel.org>,
Hoeun Ryu <hoeun.ryu@...il.com>,
PaX Team <pageexec@...email.hu>,
Emese Revfy <re.emese@...il.com>,
Russell King <linux@...linux.org.uk>,
x86@...nel.org
Subject: [RFC][PATCH 1/8] Introduce rare_write() infrastructure
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 intended for data
that is either only rarely modified or especially security-sensitive. The
intent 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 the memory contents.
Variables declared __wr_rare will be made const when an architecture
supports HAVE_ARCH_WRITE_RARE. To change these variables, either the
rare_write() macro can be used, or multiple uses of __rare_write(),
wrapped in rare_write_enable()/rare_write_disable() macros. These macros
are handled by the arch-specific functions that perform the actions needed
to write to otherwise read-only memory.
The arch-specific helpers must be not allow non-current CPUs to write
the memory area, run non-preemptible to avoid accidentally leaving
memory writable, and defined as inline to avoid making them desirable
ROP targets for attackers.
Signed-off-by: Kees Cook <keescook@...omium.org>
---
arch/Kconfig | 15 +++++++++++++++
include/linux/compiler.h | 38 ++++++++++++++++++++++++++++++++++++++
include/linux/preempt.h | 6 ++++--
3 files changed, 57 insertions(+), 2 deletions(-)
diff --git a/arch/Kconfig b/arch/Kconfig
index 99839c23d453..2446de19f66d 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -781,4 +781,19 @@ config VMAP_STACK
the stack to map directly to the KASAN shadow map using a formula
that is incorrect if the stack is in vmalloc space.
+config HAVE_ARCH_RARE_WRITE
+ def_bool n
+ help
+ An arch should select this option if it has defined the functions
+ __arch_rare_write_map() and __arch_rare_write_unmap() 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).
+
source "kernel/gcov/Kconfig"
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index cf0fa5d86059..f95603a8ee72 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -325,6 +325,44 @@ 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_type(v) typeof(v)
+# define __rare_write_ptr(v) (&(v))
+# define __rare_write(__var, __val) ({ \
+ __var = __val; \
+ __var; \
+})
+# define rare_write_enable() do { } while (0)
+# define rare_write_disable() do { } while (0)
+#else
+# define __wr_rare __ro_after_init
+# define __wr_rare_type const
+# define __rare_write_type(v) typeof((typeof(v))0)
+# define __rare_write_ptr(v) ((__rare_write_type(v) *)&(v))
+# define __rare_write(__var, __val) ({ \
+ __rare_write_type(__var) *__rw_var; \
+ \
+ __rw_var = __rare_write_ptr(__var); \
+ *__rw_var = (__val); \
+ __var; \
+})
+# define rare_write_enable() __arch_rare_write_map()
+# define rare_write_disable() __arch_rare_write_unmap()
+#endif
+
+#define rare_write(__var, __val) ({ \
+ rare_write_enable(); \
+ __rare_write(__var, __val); \
+ rare_write_disable(); \
+ __var; \
+})
+
#endif /* __KERNEL__ */
#endif /* __ASSEMBLY__ */
diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index 7eeceac52dea..183c1d7a8594 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -237,10 +237,12 @@ 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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