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    SubjectRe: [PATCH] kasan: Add explicit preconditions to kasan_report()
    On Tue, Jan 19, 2021 at 07:00:57PM +0000, Vincenzo Frascino wrote:
    > On 1/19/21 6:52 PM, Catalin Marinas wrote:
    > > On Tue, Jan 19, 2021 at 07:27:43PM +0100, Andrey Konovalov wrote:
    > >> On Tue, Jan 19, 2021 at 6:26 PM Vincenzo Frascino
    > >> <vincenzo.frascino@arm.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@virtuozzo.com>
    > >>> Cc: Alexander Potapenko <glider@google.com>
    > >>> Cc: Dmitry Vyukov <dvyukov@google.com>
    > >>> Cc: Leon Romanovsky <leonro@mellanox.com>
    > >>> Cc: Andrey Konovalov <andreyknvl@google.com>
    > >>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.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?
    > >
    > > There isn't an architected way to probe whether a memory location has a
    > > VA->PA mapping. The tags are addressed by PA but you can't reach them if
    > > you get a page fault on the VA. So we either document the kasan_report()
    > > preconditions or, as you suggest, update addr_has_metadata() for the
    > > HW_TAGS case. Something like:
    > >
    > > return is_vmalloc_addr(virt) || virt_addr_valid(virt));
    >
    > Or we could have both ;)

    True. Documentation doesn't hurt (well, only when it's wrong ;)).

    --
    Catalin

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