Re: [PATCH v4 1/3] arm64: Improve kernel address detection of __is_lm_address()

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

On 1/25/21 5:56 PM, Catalin Marinas wrote:
> On Mon, Jan 25, 2021 at 04:09:57PM +0000, Vincenzo Frascino wrote:
>> On 1/25/21 2:59 PM, Catalin Marinas wrote:
>>> On Mon, Jan 25, 2021 at 02:36:34PM +0000, Vincenzo Frascino wrote:
>>>> On 1/25/21 1:02 PM, Mark Rutland wrote:
>>>>> On Fri, Jan 22, 2021 at 03:56:40PM +0000, Vincenzo Frascino wrote:
>>>>>> Currently, the __is_lm_address() check just masks out the top 12 bits
>>>>>> of the address, but if they are 0, it still yields a true result.
>>>>>> This has as a side effect that virt_addr_valid() returns true even for
>>>>>> invalid virtual addresses (e.g. 0x0).
>>>>>>
>>>>>> Improve the detection checking that it's actually a kernel address
>>>>>> starting at PAGE_OFFSET.
>>>>>>
>>>>>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>>>>>> Cc: Will Deacon <will@kernel.org>
>>>>>> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
>>>>>> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
>>>>>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
>>>>>
>>>>> Looking around, it seems that there are some existing uses of
>>>>> virt_addr_valid() that expect it to reject addresses outside of the
>>>>> TTBR1 range. For example, check_mem_type() in drivers/tee/optee/call.c.
>>>>>
>>>>> Given that, I think we need something that's easy to backport to stable.
>>>>>
>>>>
>>>> I agree, I started looking at it this morning and I found cases even in the main
>>>> allocators (slub and page_alloc) either then the one you mentioned.
>>>>
>>>>> This patch itself looks fine, but it's not going to backport very far,
>>>>> so I suspect we might need to write a preparatory patch that adds an
>>>>> explicit range check to virt_addr_valid() which can be trivially
>>>>> backported.
>>>>>
>>>>
>>>> I checked the old releases and I agree this is not back-portable as it stands.
>>>> I propose therefore to add a preparatory patch with the check below:
>>>>
>>>> #define __is_ttrb1_address(addr)	((u64)(addr) >= PAGE_OFFSET && \
>>>> 					(u64)(addr) < PAGE_END)
>>>>
>>>> If it works for you I am happy to take care of it and post a new version of my
>>>> patches.
>>>
>>> I'm not entirely sure we need a preparatory patch. IIUC (it needs
>>> checking), virt_addr_valid() was fine until 5.4, broken by commit
>>> 14c127c957c1 ("arm64: mm: Flip kernel VA space"). Will addressed the
>>> flip case in 68dd8ef32162 ("arm64: memory: Fix virt_addr_valid() using
>>> __is_lm_address()") but this broke the <PAGE_OFFSET case. So in 5.4 a
>>> NULL address is considered valid.
>>>
>>> Ard's commit f4693c2716b3 ("arm64: mm: extend linear region for 52-bit
>>> VA configurations") changed the test to no longer rely on va_bits but
>>> did not change the broken semantics.
>>>
>>> If Ard's change plus the fix proposed in this test works on 5.4, I'd say
>>> we just merge this patch with the corresponding Cc stable and Fixes tags
>>> and tweak it slightly when doing the backports as it wouldn't apply
>>> cleanly. IOW, I wouldn't add another check to virt_addr_valid() as we
>>> did not need one prior to 5.4.
>>
>> Thank you for the detailed analysis. I checked on 5.4 and it seems that Ard
>> patch (not a clean backport) plus my proposed fix works correctly and solves the
>> issue.
> 
> I didn't mean the backport of the whole commit f4693c2716b3 as it
> probably has other dependencies, just the __is_lm_address() change in
> that patch.
> 

Then call it preparatory patch ;)

>> Tomorrow I will post a new version of the series that includes what you are
>> suggesting.
> 
> Please post the __is_lm_address() fix separately from the kasan patches.
> I'll pick it up as a fix via the arm64 tree. The kasan change can go in
> 5.12 since it's not currently broken but I'll leave the decision with
> Andrey.
> 

Ok, will do.

-- 
Regards,
Vincenzo