From: Kees Cook keescook@chromium.org
Commit c74ba8b3480da6ddaea17df2263ec09b869ac496 upstream.
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... Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Viresh Kumar viresh.kumar@linaro.org --- arch/parisc/include/asm/cache.h | 3 +++ include/asm-generic/vmlinux.lds.h | 1 + include/linux/cache.h | 14 ++++++++++++++ 3 files changed, 18 insertions(+)
diff --git a/arch/parisc/include/asm/cache.h b/arch/parisc/include/asm/cache.h index 3d0e17bcc8e9..df0f52bd18b4 100644 --- a/arch/parisc/include/asm/cache.h +++ b/arch/parisc/include/asm/cache.h @@ -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 */ diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h index a461b6604fd9..c63f92150eda 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..ro_after_init) /* 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..1be04f8c563a 100644 --- a/include/linux/cache.h +++ b/include/linux/cache.h @@ -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