linux-kernel - Re: [PATCH v2 1/2] arm64: Fix kernel address detection of __is_lm

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Message-ID: <5a389787-4f6a-7577-22fc-f5594409e1ae@arm.com>
Date:   Thu, 21 Jan 2021 16:02:14 +0000
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Leon Romanovsky <leonro@...lanox.com>,
        Alexander Potapenko <glider@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Will Deacon <will@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v2 1/2] arm64: Fix kernel address detection of
 __is_lm_address()



On 1/21/21 3:49 PM, Mark Rutland wrote:
> On Thu, Jan 21, 2021 at 03:30:51PM +0000, Vincenzo Frascino wrote:
>> On 1/21/21 3:12 PM, Mark Rutland wrote:
>>> On Thu, Jan 21, 2021 at 01:19:55PM +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).
>>>
>>> When it was added, __is_lm_address() was intended to distinguish valid
>>> kernel virtual addresses (i.e. those in the TTBR1 address range), and
>>> wasn't intended to do anything for addresses outside of this range. See
>>> commit:
>>>
>>>   ec6d06efb0bac6cd ("arm64: Add support for CONFIG_DEBUG_VIRTUAL")
>>>
>>> ... where it simply tests a bit.
>>>
>>> So I believe that it's working as intended (though this is poorly
>>> documented), but I think you're saying that usage isn't aligned with
>>> that intent. Given that, I'm not sure the fixes tag is right; I think it
>>> has never had the semantic you're after.
>>>
>> I did not do much thinking on the intended semantics. I based my interpretation
>> on what you are saying (the usage is not aligned with the intent). Based on what
>> you are are saying, I will change the patch description removing the "Fix" term.
> 
> Thanks! I assume that also means removing the fixes tag.
>

Obviously ;)

>>> I had thought the same was true for virt_addr_valid(), and that wasn't
>>> expected to be called for VAs outside of the kernel VA range. Is it
>>> actually safe to call that with NULL on other architectures?
>>
>> I am not sure on this, did not do any testing outside of arm64.
> 
> I think it'd be worth checking, if we're going to use this in common
> code.
> 

Ok, I will run some tests and let you know.

>>> I wonder if it's worth virt_addr_valid() having an explicit check for
>>> the kernel VA range, instead.
>>
>> I have no strong opinion either way even if personally I feel that modifying
>> __is_lm_address() is more clear. Feel free to propose something.
> 
> Sure; I'm happy for it to live within __is_lm_address() if that's
> simpler overall, given it doesn't look like it's making that more
> complex or expensive.
> 
>>>> Fix the detection checking that it's actually a kernel address starting
>>>> at PAGE_OFFSET.
>>>>
>>>> Fixes: f4693c2716b35 ("arm64: mm: extend linear region for 52-bit VA configurations")
>>>> Cc: Catalin Marinas <catalin.marinas@....com>
>>>> Cc: Will Deacon <will@...nel.org>
>>>> Suggested-by: Catalin Marinas <catalin.marinas@....com>
>>>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
>>>> ---
>>>>  arch/arm64/include/asm/memory.h | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
>>>> index 18fce223b67b..e04ac898ffe4 100644
>>>> --- a/arch/arm64/include/asm/memory.h
>>>> +++ b/arch/arm64/include/asm/memory.h
>>>> @@ -249,7 +249,7 @@ static inline const void *__tag_set(const void *addr, u8 tag)
>>>>  /*
>>>>   * The linear kernel range starts at the bottom of the virtual address space.
>>>>   */
>>>> -#define __is_lm_address(addr)	(((u64)(addr) & ~PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET))
>>>> +#define __is_lm_address(addr)	(((u64)(addr) ^ PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET))
>>>
>>> If we're going to make this stronger, can we please expand the comment
>>> with the intended semantic? Otherwise we're liable to break this in
>>> future.
>>
>> Based on your reply on the above matter, if you agree, I am happy to extend the
>> comment.
> 
> Works for me; how about:
> 
> /*
>  * Check whether an arbitrary address is within the linear map, which
>  * lives in the [PAGE_OFFSET, PAGE_END) interval at the bottom of the
>  * kernel's TTBR1 address range.
>  */
> 
> ... with "arbitrary" being the key word.
> 

Sounds good to me! I will post the new version after confirming the behavior of
virt_addr_valid() on the other architectures.

> Thanks,
> Mark.
> 

-- 
Regards,
Vincenzo