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Date: Thu, 20 Oct 2016 10:30:50 +0200
From: Colin Vidal <colin@...dal.org>
To: kernel-hardening@...ts.openwall.com, Kees Cook <keescook@...omium.org>
Cc: "Reshetova, Elena" <elena.reshetova@...el.com>, David Windsor
<dave@...gbits.org>, Hans Liljestrand <ishkamiel@...il.com>, Mark Rutland
<mark.rutland@....com>
Subject: Re: Re: [RFC 2/2] arm: implementation for
HARDENED_ATOMIC
On Thu, 2016-10-20 at 14:58 +0900, AKASHI Takahiro wrote:
> On Wed, Oct 19, 2016 at 01:11:46PM -0700, Kees Cook wrote:
> >
> > On Wed, Oct 19, 2016 at 1:45 AM, Colin Vidal <colin@...dal.org> wrote:
> > >
> > > Hi Kees,
> > >
> > > >
> > > > >
> > > > > This adds arm-specific code in order to support HARDENED_ATOMIC
> > > > > feature. When overflow is detected in atomic_t, atomic64_t or
> > > > > atomic_long_t, an exception is raised and call
> > > > > hardened_atomic_overflow.
> > > >
> > > > Can you include some notes that this was originally in PaX/grsecurity,
> > > > and detail what is different from their implemention?
> > >
> > > Of course. I add it in the next version.
> > >
> > > >
> > > > >
> > > > > Signed-off-by: Colin Vidal <colin@...dal.org>
> > > > > ---
> > > > > arch/arm/Kconfig | 1 +
> > > > > arch/arm/include/asm/atomic.h | 434 +++++++++++++++++++++++++++++-------------
> > > > > arch/arm/mm/fault.c | 15 ++
> > > > > 3 files changed, 320 insertions(+), 130 deletions(-)
> > > > >
> > > > > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> > > > > index b5d529f..fcf4a64 100644
> > > > > --- a/arch/arm/Kconfig
> > > > > +++ b/arch/arm/Kconfig
> > > > > @@ -36,6 +36,7 @@ config ARM
> > > > > select HAVE_ARCH_AUDITSYSCALL if (AEABI && !OABI_COMPAT)
> > > > > select HAVE_ARCH_BITREVERSE if (CPU_32v7M || CPU_32v7) && !CPU_32v6
> > > > > select HAVE_ARCH_HARDENED_USERCOPY
> > > > > + select HAVE_ARCH_HARDENED_ATOMIC
> > > > > select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32 && MMU
> > > > > select HAVE_ARCH_KGDB if !CPU_ENDIAN_BE32 && MMU
> > > > > select HAVE_ARCH_MMAP_RND_BITS if MMU
> > > > > diff --git a/arch/arm/include/asm/atomic.h b/arch/arm/include/asm/atomic.h
> > > > > index 66d0e21..fdaee17 100644
> > > > > --- a/arch/arm/include/asm/atomic.h
> > > > > +++ b/arch/arm/include/asm/atomic.h
> > > > > @@ -17,18 +17,52 @@
> > > > > #include <linux/irqflags.h>
> > > > > #include <asm/barrier.h>
> > > > > #include <asm/cmpxchg.h>
> > > > > +#include <linux/bug.h>
> > > > >
> > > > > #define ATOMIC_INIT(i) { (i) }
> > > > >
> > > > > #ifdef __KERNEL__
> > > > >
> > > > > +#ifdef CONFIG_HARDENED_ATOMIC
> > > > > +#define HARDENED_ATOMIC_INSN "bkpt 0xf103"
> > > >
> > > > In PaX, I see a check for THUMB2 config:
> > > >
> > > > #ifdef CONFIG_THUMB2_KERNEL
> > > > #define REFCOUNT_TRAP_INSN "bkpt 0xf1"
> > > > #else
> > > > #define REFCOUNT_TRAP_INSN "bkpt 0xf103"
> > > > #endif
> > > >
> > > > That should probably stay unless I'm misunderstanding something. Also,
> > > > for your new ISNS define name, I'd leave "TRAP" in the name, since
> > > > that describes more clearly what it does.
> > >
> > > Oh yeah. I will add it. Actually I does not add it at first since I
> > > does not really understand why there is a special case for Thumbs2 (as
> > > far I understand, instructions size can also be 4 bytes). If ARM
> > > experts are around, I would appreciate pointers about it :-)
> >
> > Cool. Perhaps Takahiro or Mark (now on CC) can comment on it.
>
> "bkpt" is a 16-bit instruction with an 8-bit immediate value in thumb2.
Ok, I better understand why PaX uses this guard now.
Thanks!
Colin
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