linux-kernel - Re: [PATCH] kasan: Add explicit preconditions to kasan

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Date:   Tue, 19 Jan 2021 19:27:43 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Leon Romanovsky <leonro@...lanox.com>
Subject: Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

On Tue, Jan 19, 2021 at 6:26 PM Vincenzo Frascino
<vincenzo.frascino@....com> wrote:
>
> With the introduction of KASAN_HW_TAGS, kasan_report() dereferences
> the address passed as a parameter.
>
> Add a comment to make sure that the preconditions to the function are
> explicitly clarified.
>
> Note: An invalid address (e.g. NULL pointer address) passed to the
> function when, KASAN_HW_TAGS is enabled, leads to a kernel panic.
>
> Cc: Andrey Ryabinin <aryabinin@...tuozzo.com>
> Cc: Alexander Potapenko <glider@...gle.com>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Leon Romanovsky <leonro@...lanox.com>
> Cc: Andrey Konovalov <andreyknvl@...gle.com>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
> ---
>  mm/kasan/report.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index c0fb21797550..2485b585004d 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -403,6 +403,17 @@ static void __kasan_report(unsigned long addr, size_t size, bool is_write,
>         end_report(&flags);
>  }
>
> +/**
> + * kasan_report - report kasan fault details
> + * @addr: valid address of the allocation where the tag fault was detected
> + * @size: size of the allocation where the tag fault was detected
> + * @is_write: the instruction that caused the fault was a read or write?
> + * @ip: pointer to the instruction that cause the fault
> + *
> + * Note: When CONFIG_KASAN_HW_TAGS is enabled kasan_report() dereferences
> + * the address to access the tags, hence it must be valid at this point in
> + * order to not cause a kernel panic.
> + */

It doesn't dereference the address, it just checks the tags, right?

Ideally, kasan_report() should survive that with HW_TAGS like with the
other modes. The reason it doesn't is probably because of a blank
addr_has_metadata() definition for HW_TAGS in mm/kasan/kasan.h. I
guess we should somehow check that the memory comes from page_alloc or
kmalloc. Or otherwise make sure that it has tags. Maybe there's an arm
instruction to check whether the memory has tags?