[PATCH 1/2] x86: introduce post-init read-only memory

Kees Cook
2015-11-24T21:39:35Z

One of the easiest ways to protect the kernel from attack is to reduce
the internal attack surface exposed when a "write" flaw is available. By
making as much of the kernel read-only as possible, we reduce the
attack surface.

Many things are written to only during __init, and never changed
again. These cannot be made "const" since the compiler will do the wrong
thing (we do actually need to write to them). Instead, move these items
into a memory region that will be made read-only during mark_rodata_ro()
which happens after all kernel __init code has finished.

This introduces __read_only as a way to mark such memory, and adds some
documentation about the existing __read_mostly marking.

Based on work by PaX Team and Brad Spengler.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
arch/x86/include/asm/cache.h | 1 +
include/asm-generic/vmlinux.lds.h | 1 +
include/linux/cache.h | 15 +++++++++++++++
3 files changed, 17 insertions(+)

diff --git a/arch/x86/include/asm/cache.h b/arch/x86/include/asm/cache.h
index 48f99f15452e..982b21c5eb1f 100644
--- a/arch/x86/include/asm/cache.h
+++ b/arch/x86/include/asm/cache.h
@@ -8,6 +8,7 @@
#define L1_CACHE_BYTES (1 << L1_CACHE_SHIFT)

#define __read_mostly __attribute__((__section__(".data..read_mostly")))
+#define __read_only __attribute__((__section__(".data..read_only")))

#define INTERNODE_CACHE_SHIFT CONFIG_X86_INTERNODE_CACHE_SHIFT
#define INTERNODE_CACHE_BYTES (1 << INTERNODE_CACHE_SHIFT)
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index c4bd0e2c173c..998a09d7731c 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -256,6 +256,7 @@
.rodata : AT(ADDR(.rodata) - LOAD_OFFSET) { \
VMLINUX_SYMBOL(__start_rodata) = .; \
*(.rodata) *(.rodata.*) \
+ *(.data..read_only) /* Read only after init */ \
*(__vermagic) /* Kernel version magic */ \
. = ALIGN(8); \
VMLINUX_SYMBOL(__start___tracepoints_ptrs) = .; \
diff --git a/include/linux/cache.h b/include/linux/cache.h
index 17e7e82d2aa7..b2967e711a75 100644
--- a/include/linux/cache.h
+++ b/include/linux/cache.h
@@ -12,10 +12,25 @@
#define SMP_CACHE_BYTES L1_CACHE_BYTES
#endif

+/*
+ * __read_mostly is used to keep rarely changing variables out of frequently
+ * updated cachelines. If an architecture doesn't support it, ignore the
+ * hint.
+ */
#ifndef __read_mostly
#define __read_mostly
#endif

+/*
+ * __read_only is used to mark things that are read-only after init (i.e.
+ * after mark_rodata_ro() has been called). These are effectively read-only,
+ * but may get written to during init, so can't live in .rodata (via "const").
+ * Hint to __read_mostly if the architecture hasn't wired this up.
+ */
+#ifndef __read_only
+#define __read_only __read_mostly
+#endif
+
#ifndef ____cacheline_aligned
#define ____cacheline_aligned __attribute__((__aligned__(SMP_CACHE_BYTES)))
#endif
--
1.9.1