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

[Will Deacon]

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@xxxxxxxxxxxxx>
> Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> ---
>  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