Re: [PATCH] kasan: Add explicit preconditions to kasan_report()

[Catalin Marinas <catalin.marinas@arm.com>]

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));

-- 
Catalin

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