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

[Vincenzo Frascino <vincenzo.frascino@arm.com>]

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.

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));
}

Let me know what do you think.

Thanks!
	
-- 
Regards,
Vincenzo