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

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

On Tue, Jan 19, 2021 at 08:35:49PM +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));
> > 
> 
> This seems not working on arm64 because according to virt_addr_valid 0 is a
> valid virtual address, in fact:
> 
> __is_lm_address(0) == true && pfn_valid(virt_to_pfn(0)) == true.

Ah, so __is_lm_address(0) is true. Maybe we should improve this since
virt_to_pfn(0) doesn't make much sense.

> An option could be to make an exception for virtual address 0 in
> addr_has_metadata() something like:
> 
> static inline bool addr_has_metadata(const void *addr)
> {
> 	if ((u64)addr == 0)
> 		return false;
> 
> 	return (is_vmalloc_addr(addr) || virt_addr_valid(addr));
> }

As Andrey replied, passing a non-zero small value would still be
incorrectly detected as valid.

-- 
Catalin

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