linux-kernel - Re: [PATCH v12 20/25] kasan, arm64: add brk handler for inline instrumentation

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Message-ID: <20181129180138.GB4318@arm.com>
Date:   Thu, 29 Nov 2018 18:01:38 +0000
From:   Will Deacon <will.deacon@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Dave Martin <dave.martin@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Ingo Molnar <mingo@...nel.org>,
        Paul Lawrence <paullawrence@...gle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        kasan-dev@...glegroups.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-sparse@...r.kernel.org, linux-mm@...ck.org,
        linux-kbuild@...r.kernel.org, Kostya Serebryany <kcc@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Lee Smith <Lee.Smith@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Jann Horn <jannh@...gle.com>,
        Mark Brand <markbrand@...gle.com>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Vishwath Mohan <vishwath@...gle.com>
Subject: Re: [PATCH v12 20/25] kasan, arm64: add brk handler for inline
 instrumentation

On Tue, Nov 27, 2018 at 05:55:38PM +0100, Andrey Konovalov wrote:
> Tag-based KASAN inline instrumentation mode (which embeds checks of shadow
> memory into the generated code, instead of inserting a callback) generates
> a brk instruction when a tag mismatch is detected.
> 
> This commit adds a tag-based KASAN specific brk handler, that decodes the
> immediate value passed to the brk instructions (to extract information
> about the memory access that triggered the mismatch), reads the register
> values (x0 contains the guilty address) and reports the bug.
> 
> Reviewed-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>
> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
> ---
>  arch/arm64/include/asm/brk-imm.h |  2 +
>  arch/arm64/kernel/traps.c        | 68 +++++++++++++++++++++++++++++++-
>  include/linux/kasan.h            |  3 ++
>  3 files changed, 71 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/brk-imm.h b/arch/arm64/include/asm/brk-imm.h
> index ed693c5bcec0..2945fe6cd863 100644
> --- a/arch/arm64/include/asm/brk-imm.h
> +++ b/arch/arm64/include/asm/brk-imm.h
> @@ -16,10 +16,12 @@
>   * 0x400: for dynamic BRK instruction
>   * 0x401: for compile time BRK instruction
>   * 0x800: kernel-mode BUG() and WARN() traps
> + * 0x9xx: tag-based KASAN trap (allowed values 0x900 - 0x9ff)
>   */
>  #define FAULT_BRK_IMM			0x100
>  #define KGDB_DYN_DBG_BRK_IMM		0x400
>  #define KGDB_COMPILED_DBG_BRK_IMM	0x401
>  #define BUG_BRK_IMM			0x800
> +#define KASAN_BRK_IMM			0x900
>  
>  #endif
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index 5f4d9acb32f5..04bdc53716ef 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -35,6 +35,7 @@
>  #include <linux/sizes.h>
>  #include <linux/syscalls.h>
>  #include <linux/mm_types.h>
> +#include <linux/kasan.h>
>  
>  #include <asm/atomic.h>
>  #include <asm/bug.h>
> @@ -284,10 +285,14 @@ void arm64_notify_die(const char *str, struct pt_regs *regs,
>  	}
>  }
>  
> -void arm64_skip_faulting_instruction(struct pt_regs *regs, unsigned long size)
> +void __arm64_skip_faulting_instruction(struct pt_regs *regs, unsigned long size)
>  {
>  	regs->pc += size;
> +}
>  
> +void arm64_skip_faulting_instruction(struct pt_regs *regs, unsigned long size)
> +{
> +	__arm64_skip_faulting_instruction(regs, size);
>  	/*
>  	 * If we were single stepping, we want to get the step exception after
>  	 * we return from the trap.
> @@ -959,7 +964,7 @@ static int bug_handler(struct pt_regs *regs, unsigned int esr)
>  	}
>  
>  	/* If thread survives, skip over the BUG instruction and continue: */
> -	arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
> +	__arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);

Why do you want to avoid the single-step logic here? Given that we're
skipping over the brk instruction, why wouldn't you want that to trigger
a step exception if single-step is enabled?

Will