Re: [RFC PATCH 11/14] khwasan: add brk handler for inline instrumentation

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Mar 02, 2018 at 08:44:30PM +0100, Andrey Konovalov wrote:
> KHWASAN 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.

The compiler generates the BRK?

I'm a little worried about the ABI implications of that. So far we've
assumed that for hte kernel-side, the BRK space is completely under our
control.

How much does this save, compared to having a callback?

> This commit add a KHWASAN 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.
> ---
>  arch/arm64/include/asm/brk-imm.h |  2 ++
>  arch/arm64/kernel/traps.c        | 40 ++++++++++++++++++++++++++++++++
>  2 files changed, 42 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/brk-imm.h b/arch/arm64/include/asm/brk-imm.h
> index ed693c5bcec0..e4a7013321dc 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: KHWASAN 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 KHWASAN_BRK_IMM			0x900
>  
>  #endif
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index eb2d15147e8d..5df8cdf5af13 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>
> @@ -771,6 +772,38 @@ static struct break_hook bug_break_hook = {
>  	.fn = bug_handler,
>  };
>  
> +#ifdef CONFIG_KASAN_TAGS
> +static int khwasan_handler(struct pt_regs *regs, unsigned int esr)
> +{
> +	bool recover = esr & 0x20;
> +	bool write = esr & 0x10;

Can you please add mnemonics for these, e.g.

#define KHWASAN_ESR_RECOVER		0x20
#define KHWASAN_ESR_WRITE		0x10

> +	size_t size = 1 << (esr & 0xf);

#define KHWASAN_ESR_SIZE_MASK		0x4
#define KHWASAN_ESR_SIZE(esr)	(1 << (esr) & KHWASAN_ESR_SIZE_MASK)

> +	u64 addr = regs->regs[0];
> +	u64 pc = regs->pc;
> +
> +	if (user_mode(regs))
> +		return DBG_HOOK_ERROR;
> +
> +	khwasan_report(addr, size, write, pc);
> +
> +	if (!recover)
> +		die("Oops - KHWASAN", regs, 0);

Could you elaborate on what "recover" means, and why it's up the the
compiler to decide if the kernel should die()?

> +
> +	/* If thread survives, skip over the BUG instruction and continue: */
> +	arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);

This is for fast-forwarding user instruction streams, and isn't correct
to call for kernel faults (as it'll mess up the userspace single step
logic).

Thanks,
Mark.

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