Messages in this thread | | | Subject | Re: [PATCH] kasan: Add explicit preconditions to kasan_report() | From | Vincenzo Frascino <> | Date | Wed, 20 Jan 2021 16:16:02 +0000 |
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On 1/20/21 4:04 PM, Catalin Marinas wrote: > 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. >
How do you propose to improve it?
>> 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. >
I would like to remove the check completely and have virt_addr_valid(addr) to return the right thing if possible.
I admit, yesterday evening I did not thing it through completely before posting this code that had the sole purpose to open the discussion. I agree in principle on what Andrey said as well (addr < PAGE_SIZE).
-- Regards, Vincenzo
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