linux-kernel - Re: [PATCH v4 1/3] arm64: Improve kernel address detection of __is_lm

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Message-ID: <4bd1c01b-613c-787f-4363-c55a071f14ae@arm.com>
Date:   Mon, 25 Jan 2021 16:09:57 +0000
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Leon Romanovsky <leonro@...lanox.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Will Deacon <will@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>
Subject: Re: [PATCH v4 1/3] arm64: Improve kernel address detection of
 __is_lm_address()



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@....com>
>>>> Cc: Will Deacon <will@...nel.org>
>>>> Suggested-by: Catalin Marinas <catalin.marinas@....com>
>>>> Reviewed-by: Catalin Marinas <catalin.marinas@....com>
>>>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....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.

Tomorrow I will post a new version of the series that includes what you are
suggesting.

-- 
Regards,
Vincenzo