Re: [PATCH v6 15/18] khwasan, arm64: add brk handler for inline instrumentation

[Dmitry Vyukov]

On Wed, Aug 29, 2018 at 1:35 PM, Andrey Konovalov <andreyknvl@xxxxxxxxxx> 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.
>
> 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.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/brk-imm.h |  2 +
>  arch/arm64/kernel/traps.c        | 69 +++++++++++++++++++++++++++++++-
>  2 files changed, 69 insertions(+), 2 deletions(-)
>
> 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 039e9ff379cc..fd70347d1ce7 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>
> @@ -269,10 +270,14 @@ 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.
> @@ -775,7 +780,7 @@ 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);
>         return DBG_HOOK_HANDLED;
>  }
>
> @@ -785,6 +790,59 @@ static struct break_hook bug_break_hook = {
>         .fn = bug_handler,
>  };
>
> +#ifdef CONFIG_KASAN_HW
> +
> +#define KHWASAN_ESR_RECOVER    0x20
> +#define KHWASAN_ESR_WRITE      0x10
> +#define KHWASAN_ESR_SIZE_MASK  0x0f
> +#define KHWASAN_ESR_SIZE(esr)  (1 << ((esr) & KHWASAN_ESR_SIZE_MASK))
> +
> +static int khwasan_handler(struct pt_regs *regs, unsigned int esr)
> +{
> +       bool recover = esr & KHWASAN_ESR_RECOVER;
> +       bool write = esr & KHWASAN_ESR_WRITE;
> +       size_t size = KHWASAN_ESR_SIZE(esr);
> +       u64 addr = regs->regs[0];
> +       u64 pc = regs->pc;
> +
> +       if (user_mode(regs))
> +               return DBG_HOOK_ERROR;
> +
> +       kasan_report(addr, size, write, pc);
> +
> +       /*
> +        * The instrumentation allows to control whether we can proceed after
> +        * a crash was detected. This is done by passing the -recover flag to
> +        * the compiler. Disabling recovery allows to generate more compact
> +        * code.
> +        *
> +        * Unfortunately disabling recovery doesn't work for the kernel right
> +        * now. KHWASAN reporting is disabled in some contexts (for example when
> +        * the allocator accesses slab object metadata; same is true for KASAN;
> +        * this is controlled by current->kasan_depth). All these accesses are
> +        * detected by the tool, even though the reports for them are not
> +        * printed.


I am not following this part.
Slab accesses metadata. OK.
This is detected as bad access. OK.
Report is not printed. OK.
We skip BRK and resume execution.
What is the problem?



> +        *
> +        * This is something that might be fixed at some point in the future.
> +        */
> +       if (!recover)
> +               die("Oops - KHWASAN", regs, 0);
> +
> +       /* If thread survives, skip over the brk instruction and continue: */
> +       __arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
> +       return DBG_HOOK_HANDLED;
> +}
> +
> +#define KHWASAN_ESR_VAL (0xf2000000 | KHWASAN_BRK_IMM)
> +#define KHWASAN_ESR_MASK 0xffffff00
> +
> +static struct break_hook khwasan_break_hook = {
> +       .esr_val = KHWASAN_ESR_VAL,
> +       .esr_mask = KHWASAN_ESR_MASK,
> +       .fn = khwasan_handler,
> +};
> +#endif
> +
>  /*
>   * Initial handler for AArch64 BRK exceptions
>   * This handler only used until debug_traps_init().
> @@ -792,6 +850,10 @@ static struct break_hook bug_break_hook = {
>  int __init early_brk64(unsigned long addr, unsigned int esr,
>                 struct pt_regs *regs)
>  {
> +#ifdef CONFIG_KASAN_HW
> +       if ((esr & KHWASAN_ESR_MASK) == KHWASAN_ESR_VAL)
> +               return khwasan_handler(regs, esr) != DBG_HOOK_HANDLED;
> +#endif
>         return bug_handler(regs, esr) != DBG_HOOK_HANDLED;
>  }
>
> @@ -799,4 +861,7 @@ int __init early_brk64(unsigned long addr, unsigned int esr,
>  void __init trap_init(void)
>  {
>         register_break_hook(&bug_break_hook);
> +#ifdef CONFIG_KASAN_HW
> +       register_break_hook(&khwasan_break_hook);
> +#endif
>  }
> --
> 2.19.0.rc0.228.g281dcd1b4d0-goog
>