Re: [PATCH v2 3/3] x86/kasan: Print original address on #GP

[Dmitry Vyukov <dvyukov@google.com>]

On Fri, Nov 15, 2019 at 8:17 PM Jann Horn <jannh@google.com> wrote:
>
> Make #GP exceptions caused by out-of-bounds KASAN shadow accesses easier
> to understand by computing the address of the original access and
> printing that. More details are in the comments in the patch.
>
> This turns an error like this:
>
>     kasan: CONFIG_KASAN_INLINE enabled
>     kasan: GPF could be caused by NULL-ptr deref or user memory access
>     traps: probably dereferencing non-canonical address 0xe017577ddf75b7dd
>     general protection fault: 0000 [#1] PREEMPT SMP KASAN PTI
>
> into this:
>
>     traps: dereferencing non-canonical address 0xe017577ddf75b7dd
>     traps: probably dereferencing non-canonical address 0xe017577ddf75b7dd
>     KASAN: maybe wild-memory-access in range
>             [0x00badbeefbadbee8-0x00badbeefbadbeef]
>     general protection fault: 0000 [#1] PREEMPT SMP KASAN PTI
>
> The hook is placed in architecture-independent code, but is currently
> only wired up to the X86 exception handler because I'm not sufficiently
> familiar with the address space layout and exception handling mechanisms
> on other architectures.
>
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
>
> Notes:
>     v2:
>      - move to mm/kasan/report.c (Dmitry)
>      - change hook name to be more generic
>      - use TASK_SIZE instead of TASK_SIZE_MAX for compiling on non-x86
>      - don't open-code KASAN_SHADOW_MASK (Dmitry)
>      - add "KASAN: " prefix, but not "BUG: " (Andrey, Dmitry)
>      - use same naming scheme as get_wild_bug_type (Andrey)
>
>  arch/x86/kernel/traps.c     |  2 ++
>  arch/x86/mm/kasan_init_64.c | 21 -------------------
>  include/linux/kasan.h       |  6 ++++++
>  mm/kasan/report.c           | 40 +++++++++++++++++++++++++++++++++++++
>  4 files changed, 48 insertions(+), 21 deletions(-)
>
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 12d42697a18e..87b52682a37a 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -37,6 +37,7 @@
>  #include <linux/mm.h>
>  #include <linux/smp.h>
>  #include <linux/io.h>
> +#include <linux/kasan.h>
>  #include <asm/stacktrace.h>
>  #include <asm/processor.h>
>  #include <asm/debugreg.h>
> @@ -540,6 +541,7 @@ static void print_kernel_gp_address(struct pt_regs *regs)
>
>         pr_alert("probably dereferencing non-canonical address 0x%016lx\n",
>                  addr_ref);
> +       kasan_non_canonical_hook(addr_ref);
>  #endif
>  }
>
> diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
> index 296da58f3013..69c437fb21cc 100644
> --- a/arch/x86/mm/kasan_init_64.c
> +++ b/arch/x86/mm/kasan_init_64.c
> @@ -245,23 +245,6 @@ static void __init kasan_map_early_shadow(pgd_t *pgd)
>         } while (pgd++, addr = next, addr != end);
>  }
>
> -#ifdef CONFIG_KASAN_INLINE
> -static int kasan_die_handler(struct notifier_block *self,
> -                            unsigned long val,
> -                            void *data)
> -{
> -       if (val == DIE_GPF) {
> -               pr_emerg("CONFIG_KASAN_INLINE enabled\n");
> -               pr_emerg("GPF could be caused by NULL-ptr deref or user memory access\n");
> -       }
> -       return NOTIFY_OK;
> -}
> -
> -static struct notifier_block kasan_die_notifier = {
> -       .notifier_call = kasan_die_handler,
> -};
> -#endif
> -
>  void __init kasan_early_init(void)
>  {
>         int i;
> @@ -298,10 +281,6 @@ void __init kasan_init(void)
>         int i;
>         void *shadow_cpu_entry_begin, *shadow_cpu_entry_end;
>
> -#ifdef CONFIG_KASAN_INLINE
> -       register_die_notifier(&kasan_die_notifier);
> -#endif
> -
>         memcpy(early_top_pgt, init_top_pgt, sizeof(early_top_pgt));
>
>         /*
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index cc8a03cc9674..7305024b44e3 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -194,4 +194,10 @@ static inline void *kasan_reset_tag(const void *addr)
>
>  #endif /* CONFIG_KASAN_SW_TAGS */
>
> +#ifdef CONFIG_KASAN_INLINE
> +void kasan_non_canonical_hook(unsigned long addr);
> +#else /* CONFIG_KASAN_INLINE */
> +static inline void kasan_non_canonical_hook(unsigned long addr) { }
> +#endif /* CONFIG_KASAN_INLINE */
> +
>  #endif /* LINUX_KASAN_H */
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 621782100eaa..5ef9f24f566b 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -512,3 +512,43 @@ void __kasan_report(unsigned long addr, size_t size, bool is_write, unsigned lon
>
>         end_report(&flags);
>  }
> +
> +#ifdef CONFIG_KASAN_INLINE
> +/*
> + * With CONFIG_KASAN_INLINE, accesses to bogus pointers (outside the high
> + * canonical half of the address space) cause out-of-bounds shadow memory reads
> + * before the actual access. For addresses in the low canonical half of the
> + * address space, as well as most non-canonical addresses, that out-of-bounds
> + * shadow memory access lands in the non-canonical part of the address space.
> + * Help the user figure out what the original bogus pointer was.
> + */
> +void kasan_non_canonical_hook(unsigned long addr)
> +{
> +       unsigned long orig_addr;
> +       const char *bug_type;
> +
> +       if (addr < KASAN_SHADOW_OFFSET)
> +               return;
> +
> +       orig_addr = (addr - KASAN_SHADOW_OFFSET) << KASAN_SHADOW_SCALE_SHIFT;
> +       /*
> +        * For faults near the shadow address for NULL, we can be fairly certain
> +        * that this is a KASAN shadow memory access.
> +        * For faults that correspond to shadow for low canonical addresses, we
> +        * can still be pretty sure - that shadow region is a fairly narrow
> +        * chunk of the non-canonical address space.
> +        * But faults that look like shadow for non-canonical addresses are a
> +        * really large chunk of the address space. In that case, we still
> +        * print the decoded address, but make it clear that this is not
> +        * necessarily what's actually going on.
> +        */
> +       if (orig_addr < PAGE_SIZE)
> +               bug_type = "null-ptr-deref";
> +       else if (orig_addr < TASK_SIZE)
> +               bug_type = "probably user-memory-access";
> +       else
> +               bug_type = "maybe wild-memory-access";
> +       pr_alert("KASAN: %s in range [0x%016lx-0x%016lx]\n", bug_type,
> +                orig_addr, orig_addr + KASAN_SHADOW_MASK);
> +}
> +#endif
> --
> 2.24.0.432.g9d3f5f5b63-goog


Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Thanks!