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arch: Introduce post-init read-only memory

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

This improves the security of the Linux kernel by marking formerly
read-write memory regions as read-only on a fully booted up system.

Based on work by PaX Team and Brad Spengler.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brad Spengler <spender@grsecurity.net>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: David Brown <david.brown@linaro.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Emese Revfy <re.emese@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathias Krause <minipli@googlemail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: PaX Team <pageexec@freemail.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel-hardening@lists.openwall.com
Cc: linux-arch <linux-arch@vger.kernel.org>
Link: http://lkml.kernel.org/r/1455748879-21872-5-git-send-email-keescook@chromium.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Kees Cook 2016-02-17 14:41:15 -08:00 committed by Ingo Molnar
parent 9ccaf77cf0
commit c74ba8b348
3 changed files with 18 additions and 0 deletions
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3
arch/parisc/include/asm/cache.h
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@ -22,6 +22,9 @@
#define __read_mostly __attribute__((__section__(".data..read_mostly")))
/* Read-only memory is marked before mark_rodata_ro() is called. */
#define __ro_after_init __read_mostly
void parisc_cache_init(void); /* initializes cache-flushing */
void disable_sr_hashing_asm(int); /* low level support for above */
void disable_sr_hashing(void); /* turns off space register hashing */

1
include/asm-generic/vmlinux.lds.h
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@ -256,6 +256,7 @@
.rodata : AT(ADDR(.rodata) - LOAD_OFFSET) { \
VMLINUX_SYMBOL(__start_rodata) = .; \
*(.rodata) *(.rodata.*) \
*(.data..ro_after_init) /* Read only after init */ \
*(__vermagic) /* Kernel version magic */ \
. = ALIGN(8); \
VMLINUX_SYMBOL(__start___tracepoints_ptrs) = .; \

14
include/linux/cache.h
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@ -12,10 +12,24 @@
#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
/*
* __ro_after_init 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").
*/
#ifndef __ro_after_init
#define __ro_after_init __attribute__((__section__(".data..ro_after_init")))
#endif
#ifndef ____cacheline_aligned
#define ____cacheline_aligned __attribute__((__aligned__(SMP_CACHE_BYTES)))
#endif