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Date: Thu, 19 Dec 2019 11:21:51 +0100
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Jann Horn <jannh@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>,
"the arch/x86 maintainers" <x86@...nel.org>,
Andrey Ryabinin <aryabinin@...tuozzo.com>,
Alexander Potapenko <glider@...gle.com>,
kasan-dev <kasan-dev@...glegroups.com>,
LKML <linux-kernel@...r.kernel.org>,
Andrey Konovalov <andreyknvl@...gle.com>,
Andy Lutomirski <luto@...nel.org>,
Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v7 4/4] x86/kasan: Print original address on #GP
On Thu, Dec 19, 2019 at 12:12 AM Jann Horn <jannh@...gle.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
> general protection fault, probably for non-canonical address
> 0xe017577ddf75b7dd: 0000 [#1] PREEMPT SMP KASAN PTI
>
> into this:
>
> general protection fault, probably for non-canonical address
> 0xe017577ddf75b7dd: 0000 [#1] PREEMPT SMP KASAN PTI
> KASAN: maybe wild-memory-access in range
> [0x00badbeefbadbee8-0x00badbeefbadbeef]
>
> 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@...gle.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)
> - this version was "Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>"
> v3:
> - adjusted example output in commit message based on
> changes in preceding patch
> - ensure that KASAN output happens after bust_spinlocks(1)
> - moved hook in arch/x86/kernel/traps.c such that output
> appears after the first line of KASAN-independent error report
> v4:
> - adjust patch to changes in x86/traps patch
> v5:
> - adjust patch to changes in x86/traps patch
> - fix bug introduced in v3: remove die() call after oops_end()
> v6:
> - adjust sample output in commit message
> v7:
> - instead of open-coding __die_header()+__die_body() in traps.c,
> insert a hook call into die_body(), introduced in patch 3/4
> (Borislav)
>
> arch/x86/kernel/dumpstack.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/dumpstack.c b/arch/x86/kernel/dumpstack.c
> index 8995bf10c97c..ae64ec7f752f 100644
> --- a/arch/x86/kernel/dumpstack.c
> +++ b/arch/x86/kernel/dumpstack.c
> @@ -427,6 +427,8 @@ void die_addr(const char *str, struct pt_regs *regs, long err, long gp_addr)
> int sig = SIGSEGV;
>
> __die_header(str, regs, err);
> + if (gp_addr)
> + kasan_non_canonical_hook(gp_addr);
> if (__die_body(str, regs, err))
> sig = 0;
> oops_end(flags, regs, sig);
> diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
> index cf5bc37c90ac..763e71abc0fe 100644
> --- a/arch/x86/mm/kasan_init_64.c
> +++ b/arch/x86/mm/kasan_init_64.c
> @@ -288,23 +288,6 @@ static void __init kasan_shallow_populate_pgds(void *start, void *end)
> } while (pgd++, addr = next, addr != (unsigned long)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;
> @@ -341,10 +324,6 @@ 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 4f404c565db1..e0238af0388f 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -225,4 +225,10 @@ static inline void kasan_release_vmalloc(unsigned long start,
> unsigned long free_region_end) {}
> #endif
>
> +#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
Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>
Thanks!
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